jay/frontend/

j.rs

//! J frontend: lexer and the sentence parser, lowering to the shared IR.
//!
//! Word formation and sentence parsing follow the model published in the J
//! Dictionary. Words are formed left to right; a sentence is then executed
//! right to left by pushing words onto a stack and matching the leftmost four
//! stack slots against the parse table after every push.

use std::collections::{HashMap, HashSet};
use std::ops::Range;
use std::sync::Arc;

use crate::array::{Array, Data};
use crate::error::{Error, ErrorKind, Result, Span};
use crate::frontend::{Segment, SourceParts};
use crate::ir::{Branch, Control, ExplicitDef, Expr, Scope};
use crate::verb::{
    BoolDyad, DyadOp, Enclose, MonadOp, Power, Prim, ScalarDyad, ScalarMonad, Verb, WindowKind,
    RANK_INF,
};

/// Parse a J program (one sentence per line) into IR statements.
///
/// Sentences are parsed in order over a table of the names that have been
/// given verbs, because a name's part of speech decides how the sentence
/// around it parses. A sentence that names a verb records it and produces
/// no work; a later sentence that reads the name gets the verb substituted
/// into it. That is enough for the straight-line programs this frontend
/// compiles: there is no control flow for a definition to reach backwards
/// through, and reassigning the name simply rebinds it from there on.
pub fn parse(src: &SourceParts) -> Result<Vec<Expr>> {
    let mut scope = Names::default();
    let lines = lex(src)?;
    let mut out = Vec::new();
    let mut i = 0usize;
    while i < lines.len() {
        // A definition whose body is written on the lines below swallows
        // them, so the sentence that comes out may span several of them.
        let sentence = collect_definitions(&lines, &mut i, &mut scope, true)?;
        if sentence.is_empty() {
            continue;
        }
        out.push(scope.parse_sentence(sentence)?);
    }
    Ok(out)
}

/// What a name of modifier class stands for.
#[derive(Clone, Debug)]
enum Modifier {
    /// A primitive modifier, by the spelling it is written with.
    Prim(&'static str),
    /// An explicit adverb or conjunction, by the body it was written with.
    Explicit(Arc<ModSource>),
}

impl Modifier {
    /// How the modifier names itself in `explain` and in diagnostics.
    fn spelling(&self) -> String {
        match self {
            Modifier::Prim(g) => (*g).to_string(),
            Modifier::Explicit(src) => src.name.clone(),
        }
    }
}

/// The parts of speech a sentence is read against. A name's part of speech
/// decides how the sentence around it parses, so the table is carried from
/// sentence to sentence and into every definition body.
#[derive(Clone, Default)]
struct Names {
    verbs: HashMap<String, Verb>,
    /// Names given an adverb or a conjunction (`m =. /`), by what they
    /// stand for and whether it is a conjunction. A modifier is applied
    /// when a sentence is parsed, so the name has to be resolved then too,
    /// exactly as a verb name is.
    mods: HashMap<String, (bool, Modifier)>,
    /// Names that hold a value by the time a sentence is read. Only the
    /// diagnostics need this: a name that is neither a verb nor a value is
    /// an undefined name, not a sentence the parser has yet to learn.
    nouns: HashSet<String>,
    /// The value of a name given a literal, for the operands that have to
    /// be known while the sentence is read. A gerund is data, so
    /// `` g =. +`- `` and then `g@.1` needs what g holds; an assignment
    /// whose value is not settled at parse time takes the name back out.
    consts: HashMap<String, Array>,
}

impl Names {
    /// Parse one sentence against the table and note what it did to the
    /// names it mentions.
    fn parse_sentence(&mut self, mut sentence: Vec<Frag>) -> Result<Expr> {
        substitute_names(&mut sentence, &self.verbs, &self.mods);
        let whole = sentence_span(&sentence);
        let frag = reduce_to_fragment(sentence, self)?;
        // A sentence that names a modifier is settled here rather than in
        // the IR: what the name stands for is a parser object, and the
        // node the sentence lowers to carries only its spelling.
        if let Some(Frag::ModDef(name, conj, m, span)) = frag {
            let spelling = m.spelling();
            self.mods.insert(name.clone(), (conj, m));
            self.verbs.remove(&name);
            self.nouns.remove(&name);
            return Ok(Expr::ModDef { name, spelling, conjunction: conj, span });
        }
        let stmt = lower_sentence(frag, whole)?;
        self.record(&stmt);
        Ok(stmt)
    }

    /// Note what a parsed sentence did to the names it mentions.
    fn record(&mut self, stmt: &Expr) {
        match stmt {
            Expr::VerbDef { name, verb, .. } => {
                self.verbs.insert(name.clone(), verb.clone());
                self.mods.remove(name);
                self.nouns.remove(name);
            }
            // A name given a noun stops being a verb, at any depth: J lets
            // a name change part of speech, and the oracle agrees.
            other => {
                let mut assigned = Vec::new();
                assigned_names(other, &mut assigned);
                for name in assigned {
                    self.verbs.remove(&name);
                    self.mods.remove(&name);
                    self.nouns.insert(name.clone());
                    match literal_assigned(other, &name) {
                        Some(a) => self.consts.insert(name, a),
                        None => self.consts.remove(&name),
                    };
                }
            }
        }
    }
}

/// The literal a sentence gave a name, where the whole sentence is that one
/// assignment and its value is settled already.
fn literal_assigned(stmt: &Expr, name: &str) -> Option<Array> {
    match stmt {
        Expr::Assign { name: n, value, .. } if n == name => match &**value {
            Expr::Const(a, _) => Some(a.clone()),
            _ => None,
        },
        _ => None,
    }
}

// ------------------------------------------------------- explicit definitions

/// J's control words. `for_i.` and its relatives carry the name they bind,
/// which is why the suffix is kept apart from the word.
const CONTROL_WORDS: [&str; 18] = [
    "if.", "do.", "else.", "elseif.", "end.", "while.", "whilst.", "for.", "select.", "case.",
    "fcase.", "return.", "break.", "continue.", "try.", "catch.", "catcht.", "throw.",
];

/// A control word and, for `for_i.`, the name it binds.
fn control_word(word: &str) -> Option<(&'static str, Option<String>)> {
    if let Some(w) = CONTROL_WORDS.iter().copied().find(|&w| w == word) {
        return Some((w, None));
    }
    for (stem, w) in [("for_", "for."), ("goto_", "goto."), ("label_", "label.")] {
        if let Some(rest) = word.strip_prefix(stem) {
            let name = rest.strip_suffix('.')?;
            if !name.is_empty() && is_j_name(name) {
                return Some((w, Some(name.to_string())));
            }
        }
    }
    None
}

fn is_j_name(s: &str) -> bool {
    let mut cs = s.chars();
    cs.next().is_some_and(|c| c.is_ascii_alphabetic())
        && s.chars().all(|c| c.is_ascii_alphanumeric() || c == '_')
}

/// One piece of a definition body: a run of ordinary words, or a control
/// word. A line break ends a sentence, and so does every control word.
#[derive(Clone, Debug)]
enum Item {
    Sentence(Vec<Frag>),
    Word { word: &'static str, suffix: Option<String>, span: Span },
}

impl Item {
    fn word(&self) -> Option<&'static str> {
        match self {
            Item::Word { word, .. } => Some(word),
            Item::Sentence(_) => None,
        }
    }

    fn span(&self) -> Span {
        match self {
            Item::Word { span, .. } => *span,
            Item::Sentence(f) => sentence_span(f),
        }
    }
}

/// Collapse every explicit definition in one line into a verb fragment.
///
/// `lines[*i]` is the line to read; `*i` advances past it and past any lines
/// a definition took for its body. At the top level a control word is a
/// spelling error, which is what the reference calls it.
fn collect_definitions(
    lines: &[Vec<Frag>],
    i: &mut usize,
    scope: &mut Names,
    top_level: bool,
) -> Result<Vec<Frag>> {
    let mut sentence = lines[*i].clone();
    *i += 1;
    // `f =. 3 : '… f …'` calls itself by name, so the body has to be parsed
    // with `f` already a verb. The name is resolved when it is applied.
    let self_name = match (sentence.first(), sentence.get(1)) {
        (Some(Frag::Name(n, _)), Some(a)) if a.is_assign() => Some(n.clone()),
        _ => None,
    };
    loop {
        let Some(open) = sentence.iter().position(|f| matches!(f, Frag::DdOpen(..))) else {
            match find_colon_definition(&sentence) {
                Some(at) => {
                    take_colon_definition(&mut sentence, at, lines, i, scope, self_name.as_deref())?;
                    continue;
                }
                // Every definition on the line is now one verb fragment, so
                // a control word still standing is one nothing encloses.
                None => {
                    if top_level
                        && let Some(Frag::Control(_, _, span)) =
                            sentence.iter().find(|f| matches!(f, Frag::Control(..)))
                    {
                        return Err(Error::parse(
                            "control words are only meaningful inside an explicit definition",
                            *span,
                        ));
                    }
                    return Ok(sentence);
                }
            }
        };
        take_direct_definition(&mut sentence, open, lines, i, scope, self_name.as_deref())?;
    }
}

/// The index of the `:` of a `m : n` definition, if the line has one.
fn find_colon_definition(sentence: &[Frag]) -> Option<usize> {
    (1..sentence.len().saturating_sub(1)).find(|&k| {
        matches!(&sentence[k], Frag::Conj(Modifier::Prim(":"), _))
            && as_const(&sentence[k - 1]).is_some_and(|a| a.rank() == 0)
            && matches!(&sentence[k + 1], Frag::Noun(Expr::Const(..)))
    })
}

/// `m : n` — the definition whose body is a string, or the lines below when
/// the right operand is `0`.
fn take_colon_definition(
    sentence: &mut Vec<Frag>,
    at: usize,
    lines: &[Vec<Frag>],
    i: &mut usize,
    scope: &mut Names,
    self_name: Option<&str>,
) -> Result<()> {
    let span = Span::merge(sentence[at - 1].span(), sentence[at + 1].span());
    let valence = as_const(&sentence[at - 1])
        .and_then(Array::to_f64_vec)
        .and_then(|v| v.first().copied())
        .ok_or_else(|| Error::parse("an explicit definition starts with a number", span))?;
    let body_arr = as_const(&sentence[at + 1]).cloned().expect("checked by the finder");
    let body_span = sentence[at + 1].span();
    let (dyadic, modifier) = match valence {
        3.0 => (false, None),
        4.0 => (true, None),
        1.0 => (false, Some(false)),
        2.0 => (false, Some(true)),
        13.0 => return Err(Error::not_yet("tacit definitions (13 : '...')", span)),
        v => return Err(Error::domain(format!("{v} is not an explicit definition"), span)),
    };
    let body = match &body_arr.data {
        // `3 : 0`: the body is written on the lines below, ending with `)`.
        Data::I64(_)
        | Data::F64(_)
        | Data::Bool(_)
        | Data::Ext(_)
        | Data::Rat(_)
        | Data::Complex(_) => {
            if body_arr.to_f64_vec().as_deref() != Some(&[0.0]) {
                return Err(Error::parse("an explicit definition takes 0 or a string", body_span));
            }
            take_lines_until_paren(lines, i, body_span)?
        }
        Data::Char(chars) => {
            let text: String = chars.as_slice().iter().collect();
            let mut frags = Vec::new();
            // The body sits one character past the opening quote; a doubled
            // quote inside it shifts what follows by one column.
            lex_line(&text, body_span.start + 1, &mut frags)?;
            vec![frags]
        }
        Data::Symbol(_) | Data::Box(_) => {
            return Err(Error::parse("an explicit definition takes 0 or a string", body_span))
        }
    };
    if let Some(conjunction) = modifier {
        let name = if conjunction { "2 : '...'" } else { "1 : '...'" };
        let src = mod_source(name, conjunction, body, self_name);
        let frag = if conjunction {
            Frag::Conj(Modifier::Explicit(Arc::new(src)), span)
        } else {
            Frag::Adverb(Modifier::Explicit(Arc::new(src)), span)
        };
        sentence.splice(at - 1..at + 2, [frag]);
        return Ok(());
    }
    let name = if dyadic { "4 : '...'" } else { "3 : '...'" };
    let verb = build_definition(body, dyadic, name, scope, self_name)?;
    sentence.splice(at - 1..at + 2, [Frag::Verb(VerbFrag::V(verb), span)]);
    Ok(())
}

/// The lines of a `3 : 0` body: everything up to a line that is a lone `)`.
fn take_lines_until_paren(
    lines: &[Vec<Frag>],
    i: &mut usize,
    span: Span,
) -> Result<Vec<Vec<Frag>>> {
    let mut body = Vec::new();
    loop {
        let Some(line) = lines.get(*i) else {
            return Err(Error::parse("this definition's body has no closing `)`", span));
        };
        *i += 1;
        if line.len() == 1 && matches!(line[0], Frag::RParen(_)) {
            return Ok(body);
        }
        body.push(line.clone());
    }
}

/// `{{ … }}` — the body is the words between the braces, on this line or on
/// the lines below.
fn take_direct_definition(
    sentence: &mut Vec<Frag>,
    open: usize,
    lines: &[Vec<Frag>],
    i: &mut usize,
    scope: &mut Names,
    self_name: Option<&str>,
) -> Result<()> {
    let open_span = sentence[open].span();
    let Frag::DdOpen(marker, _) = sentence[open] else {
        return Err(Error::internal("expected a direct definition's opening brackets"));
    };
    let mut depth = 1usize;
    let mut body: Vec<Vec<Frag>> = Vec::new();
    let mut tail: Vec<Frag> = Vec::new();
    let mut close_span = open_span;
    let mut line: Vec<Frag> = sentence[open + 1..].to_vec();
    let mut cur: Vec<Frag> = Vec::new();
    loop {
        let mut closed = false;
        for (k, f) in line.iter().enumerate() {
            match f {
                Frag::DdOpen(..) => {
                    depth += 1;
                    cur.push(f.clone());
                }
                Frag::DdClose(s) => {
                    depth -= 1;
                    if depth == 0 {
                        close_span = *s;
                        tail = line[k + 1..].to_vec();
                        closed = true;
                        break;
                    }
                    cur.push(f.clone());
                }
                _ => cur.push(f.clone()),
            }
        }
        if !cur.is_empty() {
            body.push(std::mem::take(&mut cur));
        }
        if closed {
            break;
        }
        let Some(next) = lines.get(*i) else {
            return Err(Error::parse("this definition has no closing `}}`", open_span));
        };
        *i += 1;
        line = next.clone();
    }
    let span = Span::merge(open_span, close_span);
    // The body's own words decide the part of speech, as they do in the
    // reference: an operand name of the second position makes a
    // conjunction, one of the first an adverb, and neither a verb. A
    // `{{)a` marker says it outright instead.
    let part = match marker {
        None => {
            if mentions(&body, "v") || mentions(&body, "n") {
                Some(true)
            } else if mentions(&body, "u") || mentions(&body, "m") {
                Some(false)
            } else {
                None
            }
        }
        Some('a') => Some(false),
        Some('c') => Some(true),
        Some('v' | 'm' | 'd') => None,
        Some(other) => {
            return Err(Error::not_yet(
                format!("a direct definition marked `){other}`"),
                open_span,
            ))
        }
    };
    let frag = match part {
        Some(conjunction) => {
            let src = mod_source("{{ ... }}", conjunction, body, self_name);
            let m = Modifier::Explicit(Arc::new(src));
            if conjunction { Frag::Conj(m, span) } else { Frag::Adverb(m, span) }
        }
        None => {
            // `)d` and `)m` fix the valence; otherwise a body that names
            // `x` is a dyad and nothing else.
            let dyadic = match marker {
                Some('d') => true,
                Some('m') => false,
                _ => mentions(&body, "x"),
            };
            let verb = build_definition(body, dyadic, "{{ ... }}", scope, self_name)?;
            Frag::Verb(VerbFrag::V(verb), span)
        }
    };
    let mut head: Vec<Frag> = sentence[..open].to_vec();
    head.push(frag);
    head.extend(tail);
    *sentence = head;
    Ok(())
}

/// True where a definition's words include this name.
fn mentions(body: &[Vec<Frag>], name: &str) -> bool {
    body.iter().any(|l| l.iter().any(|f| matches!(f, Frag::Name(n, _) if n == name)))
}

/// Parse a definition's body and wrap it in a verb.
fn build_definition(
    body: Vec<Vec<Frag>>,
    dyadic: bool,
    name: &str,
    scope: &Names,
    self_name: Option<&str>,
) -> Result<Verb> {
    // The body reads the names the program has already given, and binds its
    // own arguments over them.
    let mut inner = scope.clone();
    inner.nouns.insert("y".to_string());
    inner.verbs.remove("y");
    inner.consts.remove("y");
    if dyadic {
        inner.nouns.insert("x".to_string());
        inner.verbs.remove("x");
        inner.consts.remove("x");
    }
    if let Some(n) = self_name {
        inner.nouns.remove(n);
        inner.consts.remove(n);
        inner.verbs.insert(n.to_string(), Verb::Named(n.to_string()));
    }
    // A body may hold definitions of its own, and one of them may run past
    // the end of its line, so the lines are collected before they are split
    // into sentences.
    let mut lines: Vec<Vec<Frag>> = Vec::new();
    let mut k = 0usize;
    while k < body.len() {
        let line = collect_definitions(&body, &mut k, &mut inner, false)?;
        if !line.is_empty() {
            lines.push(line);
        }
    }
    let items = split_items(&lines);
    let mut cursor = Cursor { items: &items, at: 0 };
    let stmts = parse_block(&mut cursor, &mut inner, &[])?;
    if let Some(item) = cursor.peek() {
        return Err(Error::parse(
            format!("`{}` has no matching opening word", item.word().unwrap_or("word")),
            item.span(),
        ));
    }
    let pure = stmts.iter().all(block_is_pure);
    Ok(Verb::Explicit(Arc::new(ExplicitDef {
        name: name.to_string(),
        left: dyadic.then(|| "x".to_string()),
        right: "y".to_string(),
        // J decides a definition's valence from its header (or, for a
        // `{{ }}`, from its words): one that takes `x` is a dyad only.
        dyad_only: dyadic,
        // A J explicit definition binds both arguments by name or refuses
        // the one it cannot bind, and nothing in J nests lexically.
        spare_left: false,
        enclosing: Vec::new(),
        id: 0,
        result: None,
        locals: Vec::new(),
        body: stmts,
        // A branch that runs nothing yields J's empty result, `i. 0 0`.
        labels: Vec::new(),
        empty: Some(crate::ir::empty_result()),
        pure,
    })))
}

// ------------------------------------------------------- explicit modifiers

/// An explicit adverb or conjunction: `1 : '…'`, `2 : '…'` and the `{{ … }}`
/// whose body names an operand.
///
/// The body is kept as words rather than as a parsed tree. Applying the
/// modifier substitutes the operands into those words and parses them
/// afresh, which is what J's own substitution rule says happens, and what
/// lets a body that mentions no argument yield a verb of its own.
#[derive(Debug)]
struct ModSource {
    /// How the definition names itself in diagnostics.
    name: String,
    /// Two operands rather than one.
    conjunction: bool,
    body: Vec<Vec<Frag>>,
    /// The body names `x` or `y`, so it is the body of the derived VERB and
    /// runs when that verb is applied. A body that names neither runs at
    /// derivation instead, and what it makes is what the modifier produced.
    deferred: bool,
    /// The body names `x`: the derived verb is a dyad only.
    dyadic: bool,
    /// The name this definition is being given, so that its body can
    /// mention it.
    self_name: Option<String>,
}

fn mod_source(
    name: &str,
    conjunction: bool,
    body: Vec<Vec<Frag>>,
    self_name: Option<&str>,
) -> ModSource {
    let dyadic = mentions(&body, "x");
    ModSource {
        name: name.to_string(),
        conjunction,
        deferred: dyadic || mentions(&body, "y"),
        dyadic,
        self_name: self_name.map(str::to_string),
        body,
    }
}

thread_local! {
    /// The explicit modifiers whose bodies are being parsed right now, by
    /// address. A body that derives the modifier it belongs to would parse
    /// for ever, so the nesting is what catches it.
    static DERIVING: std::cell::RefCell<Vec<usize>> = const { std::cell::RefCell::new(Vec::new()) };
}

/// Removes the innermost derivation from the in-progress list however it
/// ends.
struct Deriving;

impl Drop for Deriving {
    fn drop(&mut self) {
        DERIVING.with(|d| {
            d.borrow_mut().pop();
        });
    }
}

/// Apply an explicit modifier to its operands.
///
/// The operands are substituted into the body under the names J gives them
/// — `u` and `v` for verbs, `m` and `n` for nouns — and the body is parsed
/// with those substitutions in place. A body that names an argument becomes
/// the derived verb's body; one that does not is a sentence, and its value
/// (usually a tacit verb) is what the derivation produced.
fn derive_explicit(
    src: &Arc<ModSource>,
    u: Frag,
    v: Option<Frag>,
    scope: &Names,
    span: Span,
) -> Result<Frag> {
    let addr = Arc::as_ptr(src) as usize;
    let recursive = DERIVING.with(|d| {
        let mut d = d.borrow_mut();
        if d.contains(&addr) {
            return true;
        }
        d.push(addr);
        false
    });
    if recursive {
        return Err(Error::not_yet(
            "an explicit modifier whose body derives the modifier itself",
            span,
        ));
    }
    let _guard = Deriving;
    let mut body = src.body.clone();
    bind_operand(&mut body, "u", "m", &u);
    if let Some(v) = &v {
        bind_operand(&mut body, "v", "n", v);
    }
    // The body reads the names the program has given so far; the name this
    // definition is being given is one of them, and it stands for the
    // modifier rather than for a verb.
    let mut inner = scope.clone();
    if let Some(n) = &src.self_name {
        inner.verbs.remove(n);
        inner.nouns.remove(n);
        inner.consts.remove(n);
        inner.mods.insert(n.clone(), (src.conjunction, Modifier::Explicit(Arc::clone(src))));
    }
    if src.deferred {
        let verb = build_definition(body, src.dyadic, &src.name, &inner, None)?;
        return Ok(Frag::Verb(VerbFrag::V(verb), span));
    }
    // The derivation-time phase: the body is a sentence, parsed here and
    // now, and what it reduces to is what the modifier made.
    let mut lines: Vec<Vec<Frag>> = Vec::new();
    let mut k = 0usize;
    while k < body.len() {
        let line = collect_definitions(&body, &mut k, &mut inner, false)?;
        if !line.is_empty() {
            lines.push(line);
        }
    }
    if lines.len() != 1 {
        return Err(Error::not_yet(
            "an explicit modifier that names no argument and is more than one sentence",
            span,
        ));
    }
    let mut sentence = lines.pop().expect("checked length");
    substitute_names(&mut sentence, &inner.verbs, &inner.mods);
    match reduce_to_fragment(sentence, &inner)? {
        Some(f) if f.is_real_verb() || f.is_noun() => Ok(respan(f, span)),
        Some(f) => Err(Error::not_yet(
            format!("an explicit modifier that produces {}", part_of_speech(&f)),
            span,
        )),
        None => Err(Error::parse("syntax error", span)),
    }
}

/// What part of speech a fragment belongs to, for a diagnostic.
fn part_of_speech(f: &Frag) -> &'static str {
    match f {
        Frag::Adverb(..) => "an adverb",
        Frag::Conj(..) => "a conjunction",
        _ => "no value",
    }
}

/// Put an operand in the place of the name it arrives under. A verb operand
/// answers to `u` (or `v`), a noun one to `m` (or `n`); the other name is
/// left alone, so a body that reaches for it reports an undefined name, as
/// the reference does.
fn bind_operand(body: &mut [Vec<Frag>], verb_name: &str, noun_name: &str, operand: &Frag) {
    let wanted = if operand.is_real_verb() { verb_name } else { noun_name };
    for line in body.iter_mut() {
        for i in 0..line.len() {
            let Frag::Name(n, span) = &line[i] else { continue };
            if n != wanted {
                continue;
            }
            let span = *span;
            // An assignment to the name is a definition of it, not a use.
            if line.get(i + 1).is_some_and(Frag::is_assign) {
                continue;
            }
            line[i] = respan(operand.clone(), span);
        }
    }
}

/// True when nothing in this sentence can have an effect beyond its value.
fn block_is_pure(e: &Expr) -> bool {
    match e {
        Expr::Const(..) | Expr::Param(..) | Expr::Name(..) => true,
        Expr::Monad { verb, y, .. } => verb.is_pure() && block_is_pure(y),
        Expr::Dyad { verb, x, y, .. } => {
            verb.is_pure() && block_is_pure(x) && block_is_pure(y)
        }
        Expr::Assign { value, .. } => block_is_pure(value),
        Expr::Control(c, _) => control_is_pure(c),
        _ => false,
    }
}

fn control_is_pure(c: &Control) -> bool {
    let all = |b: &Vec<Expr>| b.iter().all(block_is_pure);
    match c {
        Control::Return | Control::Break | Control::Continue => true,
        // J has no branch; the variant only reaches this frontend through
        // the shared IR, and reading its target is as pure as any read.
        Control::Branch(target) => block_is_pure(target),
        Control::If { arms, otherwise } => {
            arms.iter().all(|a| {
                a.test.as_ref().is_none_or(all) && all(&a.body)
            }) && otherwise.as_ref().is_none_or(all)
        }
        // A dfn guard is APL's; the variant reaches this frontend only
        // through the shared IR.
        Control::While { test, body, .. } | Control::Guard { test, body } => {
            all(test) && all(body)
        }
        Control::For { source, body, .. } => block_is_pure(source) && all(body),
        Control::Select { subject, cases } => {
            block_is_pure(subject)
                && cases.iter().all(|c| c.test.as_ref().is_none_or(all) && all(&c.body))
        }
        Control::Try { body, catch } => all(body) && all(catch),
    }
}

/// Split a definition's lines into sentences and control words.
fn split_items(lines: &[Vec<Frag>]) -> Vec<Item> {
    let mut items = Vec::new();
    for line in lines {
        let mut run: Vec<Frag> = Vec::new();
        for f in line {
            match f {
                Frag::Control(word, suffix, span) => {
                    if !run.is_empty() {
                        items.push(Item::Sentence(std::mem::take(&mut run)));
                    }
                    items.push(Item::Word {
                        word,
                        suffix: suffix.clone(),
                        span: *span,
                    });
                }
                _ => run.push(f.clone()),
            }
        }
        if !run.is_empty() {
            items.push(Item::Sentence(run));
        }
    }
    items
}

struct Cursor<'a> {
    items: &'a [Item],
    at: usize,
}

impl<'a> Cursor<'a> {
    fn peek(&self) -> Option<&'a Item> {
        self.items.get(self.at)
    }

    fn peek_word(&self) -> Option<&'static str> {
        self.peek().and_then(Item::word)
    }

    fn next(&mut self) -> Option<&'a Item> {
        let it = self.items.get(self.at);
        if it.is_some() {
            self.at += 1;
        }
        it
    }

    fn last_span(&self) -> Span {
        self.items
            .get(self.at.saturating_sub(1))
            .map_or_else(|| Span::new(0, 0), Item::span)
    }

    /// Consume the word that must come next.
    fn expect(&mut self, want: &str) -> Result<Span> {
        match self.peek() {
            Some(Item::Word { word, span, .. }) if *word == want => {
                self.at += 1;
                Ok(*span)
            }
            Some(other) => {
                Err(Error::parse(format!("expected `{want}` here"), other.span()))
            }
            None => Err(Error::parse(format!("this block needs a `{want}`"), self.last_span())),
        }
    }
}

/// Parse sentences and control structures until one of `stop` is next.
fn parse_block(cur: &mut Cursor<'_>, scope: &mut Names, stop: &[&str]) -> Result<Vec<Expr>> {
    let mut out = Vec::new();
    loop {
        match cur.peek() {
            None => return Ok(out),
            Some(Item::Word { word, .. }) if stop.contains(word) => return Ok(out),
            Some(Item::Sentence(frags)) => {
                cur.at += 1;
                out.push(scope.parse_sentence(frags.clone())?);
            }
            Some(Item::Word { .. }) => out.push(parse_control(cur, scope)?),
        }
    }
}

fn parse_control(cur: &mut Cursor<'_>, scope: &mut Names) -> Result<Expr> {
    let Some(Item::Word { word, suffix, span }) = cur.next() else {
        return Err(Error::internal("expected a control word"));
    };
    let start = *span;
    let control = match *word {
        "if." => parse_if(cur, scope)?,
        "while." | "whilst." => {
            let body_first = *word == "whilst.";
            let test = parse_block(cur, scope, &["do."])?;
            cur.expect("do.")?;
            let body = parse_block(cur, scope, &["end."])?;
            cur.expect("end.")?;
            Control::While { test, body, body_first, until: false }
        }
        "for." => {
            if let Some(name) = suffix {
                scope.nouns.insert(name.clone());
                scope.nouns.insert(format!("{name}_index"));
                scope.verbs.remove(name);
                scope.consts.remove(name);
            }
            let source = parse_block(cur, scope, &["do."])?;
            cur.expect("do.")?;
            let body = parse_block(cur, scope, &["end."])?;
            let end = cur.expect("end.")?;
            let source = one_expr(source, Span::merge(start, end))?;
            Control::For { name: suffix.clone(), source: Box::new(source), body }
        }
        "select." => parse_select(cur, scope, start)?,
        "try." => {
            let body = parse_block(cur, scope, &["catch.", "catcht.", "end."])?;
            if cur.peek_word() == Some("catcht.") {
                return Err(Error::not_yet("throw. and catcht.", cur.last_span()));
            }
            let catch = if cur.peek_word() == Some("catch.") {
                cur.expect("catch.")?;
                parse_block(cur, scope, &["end."])?
            } else {
                Vec::new()
            };
            cur.expect("end.")?;
            Control::Try { body, catch }
        }
        "return." => Control::Return,
        "break." => Control::Break,
        "continue." => Control::Continue,
        "throw." | "catcht." => return Err(Error::not_yet("throw. and catcht.", start)),
        "goto." | "label." => {
            return Err(Error::not_yet("goto_name. and label_name.", start))
        }
        other => {
            return Err(Error::parse(
                format!("`{other}` has no matching opening word"),
                start,
            ))
        }
    };
    let span = Span::merge(start, cur.last_span());
    Ok(Expr::Control(Box::new(control), span))
}

fn parse_if(cur: &mut Cursor<'_>, scope: &mut Names) -> Result<Control> {
    let mut arms = Vec::new();
    let mut otherwise = None;
    loop {
        let test = parse_block(cur, scope, &["do."])?;
        cur.expect("do.")?;
        let body = parse_block(cur, scope, &["elseif.", "else.", "end."])?;
        arms.push(Branch { test: Some(test), body, fall_through: false });
        match cur.peek_word() {
            Some("elseif.") => {
                cur.at += 1;
            }
            Some("else.") => {
                cur.at += 1;
                otherwise = Some(parse_block(cur, scope, &["end."])?);
                cur.expect("end.")?;
                break;
            }
            _ => {
                cur.expect("end.")?;
                break;
            }
        }
    }
    // `elseif. do.` with no test is the reference's other spelling of
    // `else.`: a final arm that always runs.
    if let Some(last) = arms.last_mut() && last.test.as_ref().is_some_and(Vec::is_empty) {
        last.test = None;
    }
    Ok(Control::If { arms, otherwise })
}

fn parse_select(cur: &mut Cursor<'_>, scope: &mut Names, start: Span) -> Result<Control> {
    let subject = parse_block(cur, scope, &["case.", "fcase.", "end."])?;
    let subject = one_expr(subject, start)?;
    let mut cases = Vec::new();
    loop {
        let fall_through = match cur.peek_word() {
            Some("case.") => false,
            Some("fcase.") => true,
            _ => {
                cur.expect("end.")?;
                break;
            }
        };
        cur.at += 1;
        let test = parse_block(cur, scope, &["do."])?;
        cur.expect("do.")?;
        let body = parse_block(cur, scope, &["case.", "fcase.", "end."])?;
        // `case. do.` with no test is the default arm.
        let test = (!test.is_empty()).then_some(test);
        cases.push(Branch { test, body, fall_through });
    }
    Ok(Control::Select { subject: Box::new(subject), cases })
}

/// A block that has to be one sentence — a `for.` source, a `select.`
/// subject. The value is the last sentence's, so the rest run for effect.
fn one_expr(mut stmts: Vec<Expr>, span: Span) -> Result<Expr> {
    match stmts.pop() {
        Some(e) if stmts.is_empty() => Ok(e),
        Some(_) => Err(Error::not_yet("several sentences where one value is needed", span)),
        None => Err(Error::parse("this control word needs a value", span)),
    }
}

/// Replace every name known to be a verb or a modifier by what it stands
/// for, except where the name is the target of an assignment, which is a
/// definition of the name rather than a use of it.
fn substitute_names(
    sentence: &mut [Frag],
    verbs: &HashMap<String, Verb>,
    mods: &HashMap<String, (bool, Modifier)>,
) {
    for i in 0..sentence.len() {
        let Frag::Name(name, span) = &sentence[i] else { continue };
        let (name, span) = (name.clone(), *span);
        if sentence.get(i + 1).is_some_and(Frag::is_assign) {
            continue;
        }
        if let Some(v) = verbs.get(&name) {
            sentence[i] = Frag::Verb(VerbFrag::V(v.clone()), span);
        } else if let Some((conj, m)) = mods.get(&name) {
            sentence[i] = if *conj {
                Frag::Conj(m.clone(), span)
            } else {
                Frag::Adverb(m.clone(), span)
            };
        }
    }
}

/// Every name this sentence assigns a value to, inline assignments included.
fn assigned_names(e: &Expr, out: &mut Vec<String>) {
    match e {
        Expr::Assign { name, value, .. } => {
            out.push(name.clone());
            assigned_names(value, out);
        }
        Expr::Monad { y, .. } => assigned_names(y, out),
        Expr::Dyad { x, y, .. } => {
            assigned_names(x, out);
            assigned_names(y, out);
        }
        Expr::PrintPass { value, .. } => assigned_names(value, out),
        _ => {}
    }
}

// ---------------------------------------------------------------- fragments

/// A stack fragment. The lexer emits these directly: a token and a parser
/// fragment are the same thing in J, which is why the parse table can be
/// stated over four adjacent stack slots.
#[derive(Clone, Debug)]
enum Frag {
    /// Left edge of the sentence.
    Mark,
    Noun(Expr),
    /// A name used as a value, or an assignment target.
    Name(String, Span),
    Verb(VerbFrag, Span),
    Adverb(Modifier, Span),
    Conj(Modifier, Span),
    LParen(Span),
    RParen(Span),
    AssignLocal(Span),
    AssignGlobal(Span),
    /// A finished verb definition: `mean =. +/ % #`. It belongs to no part
    /// of speech, so no rule reaches it and it can only end a sentence.
    VerbDef(String, Verb, Span),
    /// A finished modifier definition: `m =. /`. Like `VerbDef`, it belongs
    /// to no part of speech and can only end a sentence. The flag says
    /// whether it is a conjunction.
    ModDef(String, bool, Modifier, Span),
    /// A control word, with the name `for_i.` binds when it has one. Only a
    /// definition's body may hold one.
    Control(&'static str, Option<String>, Span),
    /// `{{` and `}}`, the direct definition's brackets. The opening one
    /// carries the letter of a `{{)a` marker where the source wrote one.
    DdOpen(Option<char>, Span),
    DdClose(Span),
}

/// `[:` has the verb category but no verb of its own: it is only meaningful
/// as the left tine of a fork, where it caps the fork into an atop.
#[derive(Clone, Debug)]
enum VerbFrag {
    V(Verb),
    Cap,
}

impl Frag {
    fn span(&self) -> Span {
        match self {
            Frag::Mark => Span::new(0, 0),
            Frag::Noun(e) => e.span(),
            Frag::Name(_, s)
            | Frag::Verb(_, s)
            | Frag::Adverb(_, s)
            | Frag::Conj(_, s)
            | Frag::LParen(s)
            | Frag::RParen(s)
            | Frag::AssignLocal(s)
            | Frag::AssignGlobal(s)
            | Frag::DdClose(s)
            | Frag::VerbDef(_, _, s)
            | Frag::ModDef(_, _, _, s) => *s,
            Frag::DdOpen(_, s) => *s,
            Frag::Control(_, _, s) => *s,
        }
    }

    fn is_edge(&self) -> bool {
        matches!(self, Frag::Mark | Frag::AssignLocal(_) | Frag::AssignGlobal(_) | Frag::LParen(_))
    }

    /// Verb category, `[:` included.
    fn is_verb(&self) -> bool {
        matches!(self, Frag::Verb(..))
    }

    /// A verb that can actually be applied or bound to a modifier.
    fn is_real_verb(&self) -> bool {
        matches!(self, Frag::Verb(VerbFrag::V(_), _))
    }

    /// Names are nouns in this subset; only assignment treats them apart.
    fn is_noun(&self) -> bool {
        matches!(self, Frag::Noun(_) | Frag::Name(..))
    }

    fn is_adverb(&self) -> bool {
        matches!(self, Frag::Adverb(..))
    }

    fn is_conj(&self) -> bool {
        matches!(self, Frag::Conj(..))
    }

    fn is_avn(&self) -> bool {
        self.is_adverb() || self.is_verb() || self.is_noun()
    }

    fn is_cavn(&self) -> bool {
        self.is_conj() || self.is_avn()
    }

    fn is_assign(&self) -> bool {
        matches!(self, Frag::AssignLocal(_) | Frag::AssignGlobal(_))
    }
}

// -------------------------------------------------------------- primitives

const fn prim(name: &'static str, monad: MonadOp, dyad: DyadOp, ranks: [i64; 3]) -> Prim {
    Prim { name, monad, dyad, ranks }
}

/// The primitive verbs this frontend knows, by their J spelling. Verbs whose
/// meaning exists in J but not here carry `NotYet` so the diagnostic arrives
/// at evaluation, pointing at the verb.
fn primitive(word: &str) -> Option<Prim> {
    use DyadOp as D;
    use MonadOp as M;
    use ScalarDyad as SD;
    use ScalarMonad as SM;
    const INF: i64 = RANK_INF;
    Some(match word {
        "+" => prim("+", M::Scalar(SM::Conj), D::Scalar(SD::Add), [0, 0, 0]),
        "-" => prim("-", M::Scalar(SM::Neg), D::Scalar(SD::Sub), [0, 0, 0]),
        "*" => prim("*", M::Scalar(SM::Signum), D::Scalar(SD::Mul), [0, 0, 0]),
        "%" => prim("%", M::Scalar(SM::Recip), D::Scalar(SD::DivJ), [0, 0, 0]),
        "^" => prim("^", M::Scalar(SM::Exp), D::Scalar(SD::Pow), [0, 0, 0]),
        "%:" => prim("%:", M::Scalar(SM::Sqrt), D::Scalar(SD::Root), [0, 0, 0]),
        "^." => prim("^.", M::Scalar(SM::Ln), D::Scalar(SD::Log), [0, 0, 0]),
        "|" => prim("|", M::Scalar(SM::Abs), D::Scalar(SD::Residue), [0, 0, 0]),
        "<." => prim("<.", M::Scalar(SM::Floor), D::Scalar(SD::Min), [0, 0, 0]),
        ">." => prim(">.", M::Scalar(SM::Ceil), D::Scalar(SD::Max), [0, 0, 0]),
        "=" => prim("=", M::SelfClassify, D::Scalar(SD::Eq), [INF, 0, 0]),
        "<" => prim("<", M::Enclose(Enclose::Always), D::Scalar(SD::Lt), [INF, 0, 0]),
        ">" => prim(">", M::Open, D::Scalar(SD::Gt), [0, 0, 0]),
        "<:" => prim("<:", M::Scalar(SM::Dec), D::Scalar(SD::Le), [0, 0, 0]),
        ">:" => prim(">:", M::Scalar(SM::Inc), D::Scalar(SD::Ge), [0, 0, 0]),
        "+:" => prim("+:", M::Scalar(SM::Double), D::Boolean(BoolDyad::Nor), [0, 0, 0]),
        "*:" => prim("*:", M::Scalar(SM::Square), D::Boolean(BoolDyad::Nand), [0, 0, 0]),
        "-:" => prim("-:", M::Scalar(SM::Halve), D::Match, [0, INF, INF]),
        "-." => prim("-.", M::Scalar(SM::OneMinus), D::Less, [0, INF, INF]),
        "*." => prim("*.", M::ComplexParts { polar: true }, D::Scalar(SD::Lcm), [0, 0, 0]),
        "+." => prim("+.", M::ComplexParts { polar: false }, D::Scalar(SD::Gcd), [0, 0, 0]),
        "~:" => prim("~:", M::NubSieve, D::Scalar(SD::Ne), [INF, 0, 0]),
        "~." => prim("~.", M::Nub, D::None, [INF, INF, INF]),
        "$" => prim("$", M::ShapeOf, D::Reshape, [INF, 1, INF]),
        "," => prim(",", M::Ravel, D::AppendLeading, [INF, INF, INF]),
        // `,.` is J's `,"_1` dyadically. Its monad is not `,"_1`: that
        // would leave `,. 5` a one-item list, where J answers a 1-by-1
        // table, so the monad ravels the items itself at infinite rank.
        ",." => prim(",.", M::RavelItems, D::AppendLeading, [INF, -1, -1]),
        ",:" => prim(",:", M::Itemize, D::Laminate, [INF, INF, INF]),
        "#" => prim("#", M::Tally, D::Copy, [INF, 1, INF]),
        "#." => prim("#.", M::DecodeBits, D::Decode, [1, 1, 1]),
        // The width of `#: y` comes from the largest value in the whole
        // argument, which is why the monad has infinite rank.
        "#:" => prim("#:", M::EncodeBits, D::Encode, [INF, 1, 0]),
        "!" => prim("!", M::Scalar(SM::Factorial), D::Scalar(SD::Binomial), [0, 0, 0]),
        "\":" => {
            prim("\":", M::Format, D::FormatSpecJ, [INF, 1, INF])
        }
        "o." => prim("o.", M::Scalar(SM::Pi), D::Scalar(SD::Circle), [0, 0, 0]),
        "j." => prim("j.", M::Scalar(SM::Imaginary), D::Scalar(SD::MakeComplex), [0, 0, 0]),
        "r." => prim("r.", M::Scalar(SM::Polar), D::Scalar(SD::PolarBy), [0, 0, 0]),
        "{" => prim("{", M::Catalogue, D::From, [INF, 0, INF]),
        "{." => prim("{.", M::Head, D::Take, [INF, 1, INF]),
        "}." => prim("}.", M::Behead, D::Drop, [INF, 1, INF]),
        "{:" => prim("{:", M::Tail, D::None, [INF, INF, INF]),
        "}:" => prim("}:", M::Curtail, D::None, [INF, INF, INF]),
        "|." => prim("|.", M::Reverse, D::Rotate, [INF, 1, INF]),
        "|:" => prim("|:", M::TransposeAxes, D::TransposeJ, [INF, 1, INF]),
        "i." => prim("i.", M::IotaJ, D::IndexOf { origin: 0, vector_left: false }, [1, INF, INF]),
        "i:" => prim("i:", M::Steps, D::IndexOfLast { origin: 0 }, [0, INF, INF]),
        "I." => prim(
            "I.",
            M::Indices { origin: 0, boxed_coords: false },
            D::IntervalIndex { offset: 0, closed: false },
            [1, 1, INF],
        ),
        // The dyad reads the whole argument: `2 x: y` gives every value a
        // numerator and a denominator, which becomes a trailing axis.
        "x:" => prim("x:", M::ToExact, D::ExactForm, [INF, 0, INF]),
        "p:" => prim("p:", M::NthPrime, D::PrimeMeta, [0, 0, 0]),
        // The coefficients are one vector and the point one atom, so the
        // rank machinery evaluates a whole array of points at once.
        "p." => prim("p.", M::PolyRoots, D::PolyEval, [1, 1, 0]),
        "p.." => prim("p..", M::PolyDeriv, D::PolyIntegral, [1, 0, 1]),
        "$." => prim("$.", M::Sparse, D::SparseForm, [INF, INF, INF]),
        "q:" => prim("q:", M::PrimeFactors, D::PrimeExponents, [0, 0, 0]),
        "%." => prim("%.", M::MatrixInverse, D::MatrixDivide, [2, INF, 2]),
        // The monad takes the whole argument: one invocation is one run of
        // the generator, consumed in ravel order.
        "?" => prim(
            "?",
            M::Roll { origin: 0, fixed: false, float_at_zero: true },
            D::Deal { origin: 0, fixed: false },
            [INF, 0, 0],
        ),
        "?." => prim(
            "?.",
            M::Roll { origin: 0, fixed: true, float_at_zero: true },
            D::Deal { origin: 0, fixed: true },
            [INF, 0, 0],
        ),
        "{::" => prim("{::", M::MapPaths, D::Fetch, [INF, INF, INF]),
        "e." => prim("e.", M::RazeIn, D::MemberJ, [INF, INF, INF]),
        "/:" => prim(
            "/:",
            M::GradeUp { origin: 0 },
            D::GradeSelect { down: false },
            [INF, INF, INF],
        ),
        "\\:" => prim(
            "\\:",
            M::GradeDown { origin: 0 },
            D::GradeSelect { down: true },
            [INF, INF, INF],
        ),
        ";" => prim(";", M::Raze, D::Link, [INF, INF, INF]),
        ";:" => prim(
            ";:",
            M::Words,
            D::SequentialMachine,
            [INF, INF, INF],
        ),
        "L." => prim("L.", M::LevelOf, D::None, [INF, INF, INF]),
        "\"." => prim(
            "\".",
            M::Execute { apl: false },
            D::ParseNumbers,
            [1, INF, 1],
        ),
        "A." => prim("A.", M::AnagramIndex, D::AnagramFrom, [1, 0, INF]),
        "C." => prim("C.", M::CycleForm, D::Permute, [1, 1, INF]),
        "E." => prim("E.", M::None, D::FindSeq, [INF, INF, INF]),
        "u:" => prim("u:", M::Unicode { pass_chars: true }, D::UnicodeForm, [INF, 0, INF]),
        // The monad reads the whole argument: one delimiter governs the
        // whole list. The dyad takes the form as an atom.
        "s:" => prim("s:", M::Symbols, D::SymbolForm, [INF, 0, INF]),
        "]" => prim("]", M::Same, D::Right, [INF, INF, INF]),
        "[" => prim("[", M::Same, D::Left, [INF, INF, INF]),
        "echo" => prim("echo", M::Echo, D::None, [INF, INF, INF]),
        _ => return None,
    })
}

/// The constant nouns J spells as inflected words. `a.` is the 256
/// characters of J's alphabet in codepoint order; `a:` is the ace, the box
/// holding an empty numeric list; `_.` is the indeterminate value, which is
/// a NaN and prints as itself.
fn noun_word(word: &str) -> Option<Array> {
    match word {
        "a." => Some(Array::from_chars(
            (0u32..256).map(|c| char::from_u32(c).expect("a Latin-1 codepoint")).collect(),
        )),
        "a:" => Some(Array::boxed(Array::empty(crate::dtype::DType::I64))),
        "_." => Some(Array::scalar_f64(f64::NAN)),
        _ => None,
    }
}

/// The verb a word denotes: a bare primitive, whose own ranks carry the
/// rank a word like `,.` is defined at.
fn verb_for(word: &str) -> Option<Verb> {
    Some(Verb::Prim(primitive(word)?))
}

/// A constant verb: the noun itself, whatever the arguments are. `3:` and
/// the noun operand of `::` both need one.
fn constant_verb(n: Array) -> Verb {
    // `n [ (x ] y)` is n whatever the arguments are, and the noun fork has
    // both valences, which a bond does not.
    Verb::NounFork(
        n,
        Box::new(verb_for("[").expect("`[` is a primitive")),
        Box::new(verb_for("]").expect("`]` is a primitive")),
    )
}

/// The spelling of a constant verb: `_9:` … `9:`, and `_:` for infinity.
/// The word must be complete — `3::` is the adverse conjunction after a
/// number, not a constant verb.
fn constant_verb_word(cs: &[(usize, char)], i: usize) -> Option<(usize, Array)> {
    let at = |k: usize| cs.get(k).map(|&(_, c)| c);
    let (digits, value) = match (at(i), at(i + 1), at(i + 2)) {
        (Some('_'), Some(':'), _) => (2, f64::INFINITY),
        (Some('_'), Some(d), Some(':')) if d.is_ascii_digit() => {
            (3, -((d as u8 - b'0') as f64))
        }
        (Some(d), Some(':'), _) if d.is_ascii_digit() => (2, (d as u8 - b'0') as f64),
        _ => return None,
    };
    if at(i + digits) == Some(':') {
        return None;
    }
    let arr = if value.is_infinite() {
        Array::scalar_f64(value)
    } else {
        Array::scalar_i64(value as i64)
    };
    Some((digits, arr))
}

/// The verb one J spelling denotes, for the parts of the evaluator that
/// need to name a verb rather than parse one — the obverse table above all.
pub(crate) fn verb_named(word: &str) -> Option<Verb> {
    verb_for(word)
}

const ADVERBS: [&str; 9] = ["/", "\\", "/.", "\\.", "~", "}", "f.", "M.", "b."];

/// Conjunction spellings. The ones without a meaning here are recognised so
/// that their diagnostic names the conjunction rather than the word.
const CONJUNCTIONS: [&str; 24] = [
    "\"", "@", "@.", "@:", "&", "&.", "&.:", "&:", "^:", ";.", "!.", "!:", "`", "`:", ".", ":",
    ":.", "::", "L:", "S:", "H.", "T.", "t.", "t:",
];

fn adverb(word: &str) -> Option<&'static str> {
    ADVERBS.iter().copied().find(|&g| g == word)
}

fn conjunction(word: &str) -> Option<&'static str> {
    CONJUNCTIONS.iter().copied().find(|&g| g == word)
}

// ------------------------------------------------------------------- lexer

/// Split the source into sentences of fragments. Text segments are lexed;
/// each interpolation hole becomes a noun fragment holding its parameter.
fn lex(src: &SourceParts) -> Result<Vec<Vec<Frag>>> {
    let mut sentences: Vec<Vec<Frag>> = Vec::new();
    let mut cur: Vec<Frag> = Vec::new();
    for seg in &src.segments {
        match seg {
            Segment::Text { text, offset } => {
                let mut pos = 0usize;
                for (n, line) in text.split('\n').enumerate() {
                    if n > 0 && !cur.is_empty() {
                        sentences.push(std::mem::take(&mut cur));
                    }
                    lex_line(line, offset + pos, &mut cur)?;
                    pos += line.len() + 1;
                }
            }
            Segment::Param { index, offset, len } => {
                let span = Span::new(*offset, *offset + *len);
                cur.push(Frag::Noun(Expr::Param(*index, span)));
            }
        }
    }
    if !cur.is_empty() {
        sentences.push(cur);
    }
    Ok(sentences)
}

/// A numeric word's value. Kept apart from `Array` so that a list of words
/// can pick one element type for the whole vector.
#[derive(Clone, Debug)]
enum Num {
    I(i64),
    F(f64),
    /// An extended-precision integer: `123x`.
    X(crate::exact::Ext),
    /// A rational: `1r3`.
    R(crate::exact::Rat),
    C(crate::complex::Cx),
}

fn lex_line(text: &str, base: usize, out: &mut Vec<Frag>) -> Result<()> {
    let cs: Vec<(usize, char)> = text.char_indices().collect();
    let at = |i: usize| cs.get(i).map(|&(_, c)| c);
    let off = |i: usize| cs.get(i).map(|&(o, _)| o).unwrap_or(text.len());
    let span = |a: usize, b: usize| Span::new(base + off(a), base + off(b));
    let mut i = 0usize;
    while i < cs.len() {
        let c = cs[i].1;
        if c.is_whitespace() {
            i += 1;
            continue;
        }
        // `NB.` is only a comment at the start of a word, which is where
        // this loop always stands.
        if c == 'N' && at(i + 1) == Some('B') && at(i + 2) == Some('.') {
            break;
        }
        if c == '\'' {
            let start = i;
            i += 1;
            let mut chars: Vec<char> = Vec::new();
            loop {
                match at(i) {
                    None => {
                        return Err(Error::parse(
                            "unterminated string literal",
                            span(start, cs.len()),
                        ));
                    }
                    Some('\'') if at(i + 1) == Some('\'') => {
                        chars.push('\'');
                        i += 2;
                    }
                    Some('\'') => {
                        i += 1;
                        break;
                    }
                    Some(ch) => {
                        chars.push(ch);
                        i += 1;
                    }
                }
            }
            // One character is an atom; anything else is a vector.
            let shape = if chars.len() == 1 { vec![] } else { vec![chars.len()] };
            let arr = Array::new(shape, Data::Char(chars.into()));
            out.push(Frag::Noun(Expr::Const(arr, span(start, i))));
            continue;
        }
        if let Some((len, n)) = constant_verb_word(&cs, i) {
            out.push(Frag::Verb(VerbFrag::V(constant_verb(n)), span(i, i + len)));
            i += len;
            continue;
        }
        if starts_number(&cs, i) {
            // Numeric words separated only by blanks form one vector.
            let start = i;
            let mut nums: Vec<Num> = Vec::new();
            let mut end;
            loop {
                let ws = i;
                while at(i).is_some_and(|c| c.is_ascii_alphanumeric() || c == '.' || c == '_') {
                    i += 1;
                }
                nums.push(parse_number(&text[off(ws)..off(i)], span(ws, i))?);
                end = i;
                let mut k = i;
                while at(k).is_some_and(char::is_whitespace) {
                    k += 1;
                }
                // A constant verb (`3:`) ends the numeric word rather than
                // joining it: `2 3: 4` is 2, the verb `3:`, and 4.
                if k < cs.len()
                    && starts_number(&cs, k)
                    && constant_verb_word(&cs, k).is_none()
                {
                    i = k;
                } else {
                    break;
                }
            }
            out.push(Frag::Noun(Expr::Const(num_array(&nums), span(start, end))));
            continue;
        }
        if c.is_ascii_alphabetic() {
            let start = i;
            i += 1;
            while at(i).is_some_and(|c| c.is_ascii_alphanumeric() || c == '_') {
                i += 1;
            }
            // An alphabetic word may be inflected into a primitive (`i.`,
            // `p..`), a modifier (`f.`, `L:`) or a control word (`if.`,
            // `for_i.`). The longer inflection wins where it names
            // something: `p..` is one word, not `p.` and the dot.
            let mut inflected = None;
            if matches!(at(i), Some('.') | Some(':')) {
                let most = if matches!(at(i + 1), Some('.') | Some(':')) { 2 } else { 1 };
                for n in (1..=most).rev() {
                    let word = &text[off(start)..off(i + n)];
                    let sp = span(start, i + n);
                    let frag = if let Some(v) = verb_for(word) {
                        Frag::Verb(VerbFrag::V(v), sp)
                    } else if let Some(value) = noun_word(word) {
                        Frag::Noun(Expr::Const(value, sp))
                    } else if let Some(g) = adverb(word) {
                        Frag::Adverb(Modifier::Prim(g), sp)
                    } else if let Some(g) = conjunction(word) {
                        Frag::Conj(Modifier::Prim(g), sp)
                    } else if let Some((cw, suffix)) = control_word(word) {
                        Frag::Control(cw, suffix, sp)
                    } else {
                        continue;
                    };
                    inflected = Some((frag, n));
                    break;
                }
            }
            if let Some((frag, n)) = inflected {
                i += n;
                out.push(frag);
                continue;
            }
            let word = &text[off(start)..off(i)];
            match verb_for(word) {
                Some(v) => out.push(Frag::Verb(VerbFrag::V(v), span(start, i))),
                None => out.push(Frag::Name(word.to_string(), span(start, i))),
            }
            continue;
        }
        // `{{` and `}}` bracket J's direct definition; neither is two words.
        if c == '{' && at(i + 1) == Some('{') {
            // `{{)a` and its relatives state the definition's part of
            // speech instead of leaving it to the words of the body. The
            // reference takes the marker only where nothing follows it on
            // the line, and reads `{{)a u y }}` as a domain error.
            let marker = match (at(i + 2), at(i + 3)) {
                (Some(')'), Some(m)) if m.is_ascii_alphabetic() => Some(m),
                _ => None,
            };
            if let Some(m) = marker {
                if cs[i + 4..].iter().any(|&(_, c)| !c.is_whitespace()) {
                    return Err(Error::parse(
                        format!("`)`{m} names the part of speech of a direct definition, \
                                 and has to be the last thing on its line"),
                        span(i, i + 4),
                    ));
                }
                out.push(Frag::DdOpen(Some(m), span(i, i + 4)));
                i += 4;
                continue;
            }
            out.push(Frag::DdOpen(None, span(i, i + 2)));
            i += 2;
            continue;
        }
        if c == '}' && at(i + 1) == Some('}') {
            out.push(Frag::DdClose(span(i, i + 2)));
            i += 2;
            continue;
        }
        // A symbol word is one character plus a trailing inflection, which
        // always binds: `~:` is one word, never `~` followed by `:`. The
        // parentheses are the exception; they are never inflected.
        let inflectable = c != '(' && c != ')';
        let mut len =
            if inflectable && matches!(at(i + 1), Some('.') | Some(':')) { 2 } else { 1 };
        // A doubly inflected word (`&.:`) exists only where the table says
        // it does; everything else stops at one inflection.
        if len == 2 && at(i + 2) == Some(':') {
            let w = &text[off(i)..off(i + 3)];
            if conjunction(w).is_some() || verb_for(w).is_some() {
                len = 3;
            }
        }
        let word = &text[off(i)..off(i + len)];
        match symbol_frag(word, span(i, i + len)) {
            Some(frag) => {
                out.push(frag);
                i += len;
            }
            None => {
                return Err(Error::parse(format!("unknown word: {word}"), span(i, i + len)));
            }
        }
    }
    Ok(())
}

fn symbol_frag(word: &str, span: Span) -> Option<Frag> {
    Some(match word {
        "(" => Frag::LParen(span),
        ")" => Frag::RParen(span),
        "=." => Frag::AssignLocal(span),
        "=:" => Frag::AssignGlobal(span),
        "[:" => Frag::Verb(VerbFrag::Cap, span),
        // `$:` stands for the explicit definition it is written in.
        "$:" => Frag::Verb(VerbFrag::V(Verb::SelfRef), span),
        // An inflected verb wins over the adverb its stem spells: `~.` is
        // the nub, never `~` followed by an inflection.
        _ => {
            if let Some(v) = verb_for(word) {
                Frag::Verb(VerbFrag::V(v), span)
            } else if let Some(g) = adverb(word) {
                Frag::Adverb(Modifier::Prim(g), span)
            } else {
                Frag::Conj(Modifier::Prim(conjunction(word)?), span)
            }
        }
    })
}

/// A numeric word starts with a digit, or with `_` used as a negative sign
/// or as infinity (`_`, `__`) — but not as the start of a name.
fn starts_number(cs: &[(usize, char)], i: usize) -> bool {
    let c = cs[i].1;
    if c.is_ascii_digit() {
        return true;
    }
    if c != '_' {
        return false;
    }
    match cs.get(i + 1).map(|&(_, c)| c) {
        None => true,
        // `j` is the one letter an infinity can be followed by: `_j_` and
        // `_j1` are the rectangular form with an infinite real part, and a
        // J name never begins with `_`, so nothing else claims the word.
        Some(d) => d.is_ascii_digit() || d == '.' || d == 'j' || !d.is_alphanumeric(),
    }
}

fn parse_number(word: &str, span: Span) -> Result<Num> {
    // `_.` is the indeterminate value, not a number with a decimal point.
    if word == "_." {
        return Ok(Num::F(f64::NAN));
    }
    // `1x` is an extended-precision integer; `1x1` is a multiple of e, and
    // `1p1` a multiple of π. The letter is the separator in both, and it
    // binds LOOSEST: `1ar1p1` is the polar value `1ar1` scaled by π.
    if let Some(k) = word.find(['p', 'x']) {
        if word[k + 1..].is_empty() {
            // A trailing `x` is the extended-precision suffix, and only a
            // whole decimal number carries it: `1.5x` and `1e10x` are
            // ill-formed, as they are in the reference.
            if word.as_bytes()[k] == b'x' {
                return extended_literal(&word[..k], word, span);
            }
            return Err(Error::parse(format!("invalid number: {word}"), span));
        }
        let base =
            if word.as_bytes()[k] == b'p' { std::f64::consts::PI } else { std::f64::consts::E };
        let mantissa = plain_number(&word[..k], word, span)?;
        let exponent = plain_number(&word[k + 1..], word, span)?;
        return Ok(scale(mantissa, base, exponent));
    }
    // `3j4` is the rectangular form. A `b` earlier in the word makes the
    // `j` a base-literal digit instead (`36bj` is 19).
    if let Some(k) = word.find('j') && !word[..k].contains('b') {
        let re = as_f64(plain_number(&word[..k], word, span)?);
        let im = as_f64(plain_number(&word[k + 1..], word, span)?);
        return Ok(Num::C([re, im]));
    }
    // `1ad45` and `1ar1` are the polar forms: a magnitude, then the angle
    // in degrees or in radians.
    if let Some(k) = word.find("ad").or_else(|| word.find("ar")) && !word[..k].contains('b') {
        let magnitude = as_f64(plain_number(&word[..k], word, span)?);
        let angle = as_f64(plain_number(&word[k + 2..], word, span)?);
        return Ok(Num::C(if word.as_bytes()[k + 1] == b'd' {
            crate::complex::from_degrees(magnitude, angle)
        } else {
            crate::complex::from_radians(magnitude, angle)
        }));
    }
    // `3r4` is a rational, and `1r_2` spells its negative denominator with
    // J's own negative sign. A `b` earlier in the word makes the `r` a
    // base-literal digit instead.
    if let Some(k) = word.find('r') && !word[..k].contains('b') {
        return rational_literal(&word[..k], &word[k + 1..], word, span);
    }
    if let Some(k) = word.find('b') {
        return base_literal(&word[..k], &word[k + 1..], word, span);
    }
    plain_number(word, word, span)
}

/// `123x`: the digits as an extended-precision integer. The value is exact
/// however many digits it has, which is the whole point of the suffix.
fn extended_literal(digits: &str, word: &str, span: Span) -> Result<Num> {
    Ok(Num::X(whole_digits(digits, word, span)?))
}

/// `3r4`: a rational. A zero denominator is J's infinity rather than a
/// number — the only spelling that leaves the exact types on sight.
fn rational_literal(num: &str, den: &str, word: &str, span: Span) -> Result<Num> {
    use num_traits::Zero;
    let num = whole_digits(num, word, span)?;
    let den = whole_digits(den, word, span)?;
    if den.is_zero() {
        if num.is_zero() {
            return Ok(Num::I(0));
        }
        return Ok(Num::F(if num.sign() == num_bigint::Sign::Minus {
            f64::NEG_INFINITY
        } else {
            f64::INFINITY
        }));
    }
    Ok(Num::R(
        crate::exact::Rat::new(num, den).ok_or_else(|| Error::internal("a zero denominator"))?,
    ))
}

/// One run of decimal digits, with J's `_` as the negative sign.
fn whole_digits(word: &str, whole: &str, span: Span) -> Result<crate::exact::Ext> {
    let invalid = || Error::parse(format!("invalid number: {whole}"), span);
    let (digits, negative) = match word.strip_prefix('_') {
        Some(rest) => (rest, true),
        None => (word, false),
    };
    if digits.is_empty() || !digits.bytes().all(|b| b.is_ascii_digit()) {
        return Err(invalid());
    }
    let v: crate::exact::Ext = digits.parse().map_err(|_| invalid())?;
    Ok(if negative { -v } else { v })
}

/// A mantissa scaled by a power of π or e. Either half may be complex —
/// `1p1j1` is π to the power `1j1`.
fn scale(mantissa: Num, base: f64, exponent: Num) -> Num {
    if matches!(mantissa, Num::C(_)) || matches!(exponent, Num::C(_)) {
        let m = as_cx(mantissa);
        let f = crate::complex::pow([base, 0.0], as_cx(exponent));
        return Num::C(crate::complex::mul(m, f));
    }
    Num::F(as_f64(mantissa) * base.powf(as_f64(exponent)))
}

fn as_cx(n: Num) -> crate::complex::Cx {
    match n {
        Num::C(z) => z,
        other => [as_f64(other), 0.0],
    }
}

fn as_f64(n: Num) -> f64 {
    match n {
        Num::I(v) => v as f64,
        Num::F(v) => v,
        Num::X(v) => crate::exact::ext_to_f64(&v),
        Num::R(v) => v.to_f64(),
        // A complex part is itself written as a plain number, so this is
        // never reached from a well-formed literal.
        Num::C(z) => z[0],
    }
}

/// `mBd…`: the digits `d…` read in base `m`. Digits run `0`–`9` then `a`–`z`,
/// and a `_` in front of them negates the value, as the reference does.
fn base_literal(base: &str, digits: &str, word: &str, span: Span) -> Result<Num> {
    let invalid = || Error::parse(format!("invalid number: {word}"), span);
    let base = as_f64(plain_number(base, word, span)?);
    let (digits, negative) = match digits.strip_prefix('_') {
        Some(rest) => (rest, true),
        None => (digits, false),
    };
    if digits.is_empty() {
        return Err(invalid());
    }
    let mut value = 0.0f64;
    for ch in digits.chars() {
        let d = match ch {
            '0'..='9' => ch as u32 - '0' as u32,
            'a'..='z' => ch as u32 - 'a' as u32 + 10,
            _ => return Err(invalid()),
        };
        value = value * base + f64::from(d);
    }
    if negative {
        value = -value;
    }
    // An exact whole number stays an integer, as the reference prints it.
    if value.fract() == 0.0 && value.abs() < 9.007_199_254_740_992e15 {
        return Ok(Num::I(value as i64));
    }
    Ok(Num::F(value))
}

/// One constituent of a literal — a whole one, a mantissa, an exponent, or
/// half of a complex or polar form. Every part is itself a number in the
/// same grammar, which is what makes `1ar1p1` and `1p1j1` read.
fn plain_number(word: &str, whole: &str, span: Span) -> Result<Num> {
    if word.is_empty() {
        return Err(Error::parse(format!("invalid number: {whole}"), span));
    }
    if word.contains(['j', 'p', 'x', 'b', 'r']) || word.contains("ad") || word.contains("ar") {
        return parse_number(word, span);
    }
    parse_plain(word, span)
}

fn parse_plain(word: &str, span: Span) -> Result<Num> {
    if word == "_" {
        return Ok(Num::F(f64::INFINITY));
    }
    if word == "__" {
        return Ok(Num::F(f64::NEG_INFINITY));
    }
    // As a component of a complex literal: `_.j_` and `_j_.` are both
    // words the reference reads.
    if word == "_." {
        return Ok(Num::F(f64::NAN));
    }
    let invalid = || Error::parse(format!("invalid number: {word}"), span);
    // `_` is J's negative sign, in the mantissa and after `e`.
    let mut norm = String::with_capacity(word.len());
    for (k, ch) in word.char_indices() {
        if ch == '_' {
            if k != 0 && !word[..k].ends_with('e') {
                return Err(invalid());
            }
            norm.push('-');
        } else {
            norm.push(ch);
        }
    }
    // Exponent notation yields a float, as a fractional part does.
    if norm.contains('.') || norm.contains('e') {
        return norm.parse::<f64>().map(Num::F).map_err(|_| invalid());
    }
    // Digits that overflow a machine word are a float, as they are in J;
    // the `x` suffix is what asks for an exact value instead.
    match norm.parse::<i64>() {
        Ok(v) => Ok(Num::I(v)),
        Err(_) => norm.parse::<f64>().map(Num::F).map_err(|_| invalid()),
    }
}

/// One numeric word list as an array. The widest type any word reached
/// carries the whole vector: `1 2 3x` is extended throughout, and one
/// rational or float among the words pulls its neighbours up with it.
/// J `x ". y`: the numbers one line of text spells.
///
/// The line is split at blanks and every word read as a J numeric literal;
/// a word that is not one takes the value `fallback` instead, which is what
/// separates this from `".` the monad — a line that does not parse is
/// answered rather than refused. One word gives a scalar, as reading that
/// line as a noun would; several give a vector of that many.
///
/// None where the fallback is not a number: there is nothing to stand in
/// with.
pub(crate) fn numbers_from_text(line: &str, fallback: &Array) -> Option<Array> {
    let stand_in = match &fallback.data {
        Data::Bool(v) => Num::I(i64::from(*v.as_slice().first()?)),
        Data::I64(v) => Num::I(*v.as_slice().first()?),
        Data::F64(v) => Num::F(*v.as_slice().first()?),
        Data::Ext(v) => Num::X(v.as_slice().first()?.clone()),
        Data::Rat(v) => Num::R(v.as_slice().first()?.clone()),
        Data::Complex(v) => Num::C(*v.as_slice().first()?),
        Data::Char(_) | Data::Symbol(_) | Data::Box(_) => return None,
    };
    let nums: Vec<Num> = line
        .split_whitespace()
        .map(|w| parse_number(w, Span::new(0, 0)).unwrap_or_else(|_| stand_in.clone()))
        .collect();
    Some(num_array(&nums))
}

fn num_array(nums: &[Num]) -> Array {
    use crate::exact::{Ext, Rat};
    let shape = if nums.len() == 1 { vec![] } else { vec![nums.len()] };
    let has = |f: fn(&Num) -> bool| nums.iter().any(f);
    if has(|n| matches!(n, Num::C(_))) {
        let data = nums.iter().map(|n| as_cx(n.clone())).collect();
        return Array::new(shape, Data::Complex(data));
    }
    if has(|n| matches!(n, Num::F(_))) {
        let data = nums.iter().map(|n| as_f64(n.clone())).collect();
        return Array::new(shape, Data::F64(data));
    }
    if has(|n| matches!(n, Num::R(_))) {
        let data = nums
            .iter()
            .map(|n| match n {
                Num::I(v) => Rat::from_int(Ext::from(*v)),
                Num::X(v) => Rat::from_int(v.clone()),
                Num::R(v) => v.clone(),
                Num::F(_) | Num::C(_) => Rat::zero(),
            })
            .collect();
        return Array::new(shape, Data::Rat(data));
    }
    if has(|n| matches!(n, Num::X(_))) {
        let data = nums
            .iter()
            .map(|n| match n {
                Num::I(v) => Ext::from(*v),
                Num::X(v) => v.clone(),
                _ => Ext::default(),
            })
            .collect();
        return Array::new(shape, Data::Ext(data));
    }
    let data = nums
        .iter()
        .map(|n| match n {
            Num::I(v) => *v,
            _ => 0,
        })
        .collect();
    Array::new(shape, Data::I64(data))
}

// ------------------------------------------------------------------ parser

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum Rule {
    Monad1,
    Monad2,
    Dyad3,
    Adverb4,
    Conj5,
    Fork6,
    Bident7,
    Assign8,
    Paren9,
}

/// Run the parse table over a sentence's words. The result is the one
/// fragment left standing, or None where the sentence did not reduce to
/// one — which is the reference's syntax error.
fn reduce_to_fragment(tokens: Vec<Frag>, scope: &Names) -> Result<Option<Frag>> {
    check_parens(&tokens)?;
    let mut stack: Vec<Frag> = Vec::new();
    for frag in tokens.into_iter().rev() {
        stack.insert(0, frag);
        reduce(&mut stack, scope)?;
    }
    stack.insert(0, Frag::Mark);
    reduce(&mut stack, scope)?;
    if stack.len() == 2 {
        return Ok(Some(stack.pop().expect("checked length")));
    }
    Ok(None)
}

/// The IR statement a finished sentence stands for. `whole` is the span of
/// the sentence, for the complaint that it has no reading at all.
fn lower_sentence(frag: Option<Frag>, whole: Span) -> Result<Expr> {
    match frag {
        Some(f @ (Frag::Noun(_) | Frag::Name(..))) => as_noun(f),
        Some(Frag::VerbDef(name, verb, span)) => Ok(Expr::VerbDef { name, verb, span }),
        Some(Frag::ModDef(name, conjunction, m, span)) => {
            Ok(Expr::ModDef { name, spelling: m.spelling(), conjunction, span })
        }
        Some(Frag::Verb(VerbFrag::V(_), span)) => {
            Err(Error::not_yet("tacit verb definitions (a sentence that is a verb)", span))
        }
        Some(Frag::Adverb(_, span) | Frag::Conj(_, span)) => Err(Error::not_yet(
            "displaying a modifier (a sentence that is an adverb or a conjunction)",
            span,
        )),
        _ => Err(Error::parse("syntax error", whole)),
    }
}

/// Report an unbalanced parenthesis at the parenthesis itself, before the
/// sentence is reduced: the reduction would otherwise blame whatever
/// fragments the stray one left stranded beside each other.
fn check_parens(tokens: &[Frag]) -> Result<()> {
    let mut open: Vec<Span> = Vec::new();
    for frag in tokens {
        match frag {
            Frag::LParen(s) => open.push(*s),
            Frag::RParen(s) => {
                if open.pop().is_none() {
                    return Err(Error::parse("this `)` has no opening `(`", *s));
                }
            }
            _ => {}
        }
    }
    match open.pop() {
        None => Ok(()),
        Some(s) => Err(Error::parse("this `(` has no closing `)`", s)),
    }
}

fn sentence_span(tokens: &[Frag]) -> Span {
    tokens
        .iter()
        .map(Frag::span)
        .reduce(Span::merge)
        .unwrap_or_else(|| Span::new(0, 0))
}

fn reduce(stack: &mut Vec<Frag>, scope: &Names) -> Result<()> {
    while apply(stack, scope)? {}
    Ok(())
}

/// The parse table: the first matching row wins, and matching restarts after
/// every reduction. Slot 0 is the leftmost (most recently pushed) fragment.
fn match_rule(s: &[Frag]) -> Option<Rule> {
    let is = |i: usize, f: fn(&Frag) -> bool| s.get(i).is_some_and(f);
    // Slot 0 is only ever context: an edge, or a fragment that keeps the
    // reduction from reaching further left than it should.
    let ctx = |i: usize| s.get(i).is_some_and(|f| f.is_edge() || f.is_avn());
    let verb_or_noun =
        |i: usize| s.get(i).is_some_and(|f| f.is_real_verb() || f.is_noun());
    if is(0, Frag::is_edge) && is(1, Frag::is_real_verb) && is(2, Frag::is_noun) {
        return Some(Rule::Monad1);
    }
    if ctx(0) && is(1, Frag::is_verb) && is(2, Frag::is_real_verb) && is(3, Frag::is_noun) {
        return Some(Rule::Monad2);
    }
    if ctx(0) && is(1, Frag::is_noun) && is(2, Frag::is_real_verb) && is(3, Frag::is_noun) {
        return Some(Rule::Dyad3);
    }
    if ctx(0) && verb_or_noun(1) && is(2, Frag::is_adverb) {
        return Some(Rule::Adverb4);
    }
    if ctx(0) && verb_or_noun(1) && is(2, Frag::is_conj) && verb_or_noun(3) {
        return Some(Rule::Conj5);
    }
    if ctx(0)
        && s.get(1).is_some_and(|f| f.is_verb() || f.is_noun())
        && is(2, Frag::is_real_verb)
        && is(3, Frag::is_real_verb)
    {
        return Some(Rule::Fork6);
    }
    if is(0, Frag::is_edge) && is(1, Frag::is_cavn) && is(2, Frag::is_cavn) {
        return Some(Rule::Bident7);
    }
    if is(0, Frag::is_noun) && is(1, Frag::is_assign) && is(2, Frag::is_cavn) {
        return Some(Rule::Assign8);
    }
    if matches!(s.first(), Some(Frag::LParen(_)))
        && is(1, Frag::is_cavn)
        && matches!(s.get(2), Some(Frag::RParen(_)))
    {
        return Some(Rule::Paren9);
    }
    None
}

fn take(stack: &mut Vec<Frag>, range: Range<usize>) -> Vec<Frag> {
    stack.drain(range).collect()
}

/// The fragment, pointing at `to` instead of at its own words. Removing a
/// pair of parentheses uses it so that the fragment left behind still
/// covers the brackets it was written in.
fn respan(f: Frag, to: Span) -> Frag {
    match f {
        Frag::Noun(mut e) => {
            e.set_span(to);
            Frag::Noun(e)
        }
        Frag::Name(n, _) => Frag::Name(n, to),
        Frag::Verb(v, _) => Frag::Verb(v, to),
        Frag::Adverb(a, _) => Frag::Adverb(a, to),
        Frag::Conj(c, _) => Frag::Conj(c, to),
        other => other,
    }
}

fn apply(stack: &mut Vec<Frag>, scope: &Names) -> Result<bool> {
    let Some(rule) = match_rule(stack) else {
        return Ok(false);
    };
    match rule {
        Rule::Monad1 => {
            let mut t = take(stack, 1..3);
            let y = t.pop().expect("two slots");
            let v = t.pop().expect("two slots");
            let frag = monad(v, y)?;
            stack.insert(1, frag);
        }
        Rule::Monad2 => {
            let mut t = take(stack, 2..4);
            let y = t.pop().expect("two slots");
            let v = t.pop().expect("two slots");
            let frag = monad(v, y)?;
            stack.insert(2, frag);
        }
        Rule::Dyad3 => {
            let mut t = take(stack, 1..4);
            let y = t.pop().expect("three slots");
            let v = t.pop().expect("three slots");
            let x = t.pop().expect("three slots");
            let frag = dyad(x, v, y)?;
            stack.insert(1, frag);
        }
        Rule::Adverb4 => {
            let mut t = take(stack, 1..3);
            let a = t.pop().expect("two slots");
            let u = t.pop().expect("two slots");
            let frag = apply_adverb(u, a, scope)?;
            stack.insert(1, frag);
        }
        Rule::Conj5 => {
            let mut t = take(stack, 1..4);
            let v = t.pop().expect("three slots");
            let c = t.pop().expect("three slots");
            let u = t.pop().expect("three slots");
            let frag = apply_conj(u, c, v, scope)?;
            stack.insert(1, frag);
        }
        Rule::Fork6 => {
            let mut t = take(stack, 1..4);
            let h = t.pop().expect("three slots");
            let g = t.pop().expect("three slots");
            let f = t.pop().expect("three slots");
            let frag = apply_fork(f, g, h)?;
            stack.insert(1, frag);
        }
        Rule::Bident7 => {
            let mut t = take(stack, 1..3);
            let b = t.pop().expect("two slots");
            let a = t.pop().expect("two slots");
            let frag = apply_bident(a, b, &scope.nouns)?;
            stack.insert(1, frag);
        }
        Rule::Assign8 => {
            let mut t = take(stack, 0..3);
            let value = t.pop().expect("three slots");
            let assign = t.pop().expect("three slots");
            let target = t.pop().expect("three slots");
            let scope = match assign {
                Frag::AssignGlobal(_) => Scope::Global,
                _ => Scope::Local,
            };
            let frag = apply_assign(target, value, scope)?;
            stack.insert(0, frag);
        }
        Rule::Paren9 => {
            let mut t = take(stack, 0..3);
            let close = t.pop().expect("three slots");
            let inner = t.pop().expect("three slots");
            let open = t.pop().expect("three slots");
            let outer = Span::merge(open.span(), close.span());
            stack.insert(0, respan(inner, outer));
        }
    }
    Ok(true)
}

// --------------------------------------------------------------- lowering

fn as_noun(f: Frag) -> Result<Expr> {
    match f {
        Frag::Noun(e) => Ok(e),
        Frag::Name(n, s) => Ok(Expr::Name(n, s)),
        other => Err(Error::internal(format!("expected a noun fragment, got {other:?}"))),
    }
}

fn as_verb(f: Frag) -> Result<(Verb, Span)> {
    match f {
        Frag::Verb(VerbFrag::V(v), s) => Ok((v, s)),
        other => Err(Error::internal(format!("expected a verb fragment, got {other:?}"))),
    }
}

/// The literal array behind a noun fragment, if it is one. Derived verbs that
/// capture a noun (rank specifications, noun forks) need the value now.
fn as_const(f: &Frag) -> Option<&Array> {
    match f {
        Frag::Noun(Expr::Const(a, _)) => Some(a),
        _ => None,
    }
}

/// A noun fragment's value, where it is a literal or an expression over
/// literals that settles at compile time. An index specification such as
/// `(<a:;1)` is written out rather than typed in, so a modifier capturing
/// one has to fold it.
fn noun_value(f: &Frag) -> Option<Array> {
    if let Some(a) = as_const(f) {
        return Some(a.clone());
    }
    let Frag::Noun(e) = f else { return None };
    let cfg = crate::verb::EvalCfg {
        agreement: crate::verb::Agreement::LeadingPrefix,
        fmt: crate::fmt::FmtOpts::J,
        tol: crate::verb::Tol::J,
        rules: crate::frontend::Rules::default(),
    };
    crate::ir::fold_const(e, cfg)
}

fn monad(v: Frag, y: Frag) -> Result<Frag> {
    let (verb, vspan) = as_verb(v)?;
    let y = as_noun(y)?;
    let span = Span::merge(vspan, y.span());
    Ok(Frag::Noun(Expr::Monad { verb, y: Box::new(y), span }))
}

fn dyad(x: Frag, v: Frag, y: Frag) -> Result<Frag> {
    let x = as_noun(x)?;
    let (verb, vspan) = as_verb(v)?;
    let y = as_noun(y)?;
    let span = Span::merge(Span::merge(x.span(), vspan), y.span());
    Ok(Frag::Noun(Expr::Dyad { verb, x: Box::new(x), y: Box::new(y), span }))
}

fn apply_adverb(u: Frag, a: Frag, scope: &Names) -> Result<Frag> {
    let Frag::Adverb(m, aspan) = a else {
        return Err(Error::internal("expected an adverb fragment"));
    };
    let span = Span::merge(u.span(), aspan);
    let glyph = match m {
        Modifier::Prim(g) => g,
        Modifier::Explicit(src) => return derive_explicit(&src, u, None, scope, span),
    };
    // `}` takes either operand: `m}` amends at the indices m, and `u}`
    // computes them from the arguments instead.
    if glyph == "}" {
        if !u.is_real_verb() {
            let m = noun_value(&u)
                .ok_or_else(|| Error::not_yet("amend over a computed index", span))?;
            return Ok(Frag::Verb(VerbFrag::V(Verb::Amend(m)), span));
        }
        let (v, _) = as_verb(u)?;
        return Ok(Frag::Verb(VerbFrag::V(Verb::AmendVerb(Box::new(v))), span));
    }
    // `b.` takes either operand too: a noun names one of the thirty-two
    // boolean functions, a verb asks after the verb's own characteristics.
    if glyph == "b." && !u.is_real_verb() {
        let m = as_const(&u)
            .and_then(Array::to_i64_vec)
            .and_then(|v| v.first().copied())
            .filter(|&m| (0..32).contains(&m))
            .ok_or_else(|| {
                Error::not_yet("a boolean function outside `0 b.` … `31 b.`", span)
            })?;
        let p = crate::verb::Prim {
            name: "b.",
            monad: MonadOp::None,
            dyad: DyadOp::TruthTable(m as u8),
            ranks: [crate::verb::RANK_INF, 0, 0],
        };
        return Ok(Frag::Verb(VerbFrag::V(Verb::Prim(p)), span));
    }
    if !u.is_real_verb() {
        return Err(Error::not_yet("noun-operand adverbs", span));
    }
    let (v, _) = as_verb(u)?;
    let derived = match glyph {
        "/" => Verb::Reduce(Box::new(v)),
        "\\" => Verb::Windowed(Box::new(v), WindowKind::Prefix),
        "\\." => Verb::Windowed(Box::new(v), WindowKind::Suffix),
        "~" => Verb::Commute(Box::new(v)),
        "/." => Verb::Key(Box::new(v)),
        // Names are already substituted where they were used, so a fixed
        // verb is the verb itself.
        "f." => v,
        "M." => Verb::Memo(Box::new(v), Default::default()),
        "b." => Verb::Characteristics(Box::new(v)),
        _ => return Err(Error::not_yet(format!("adverb ({glyph})"), span)),
    };
    Ok(Frag::Verb(VerbFrag::V(derived), span))
}

fn apply_conj(u: Frag, c: Frag, v: Frag, scope: &Names) -> Result<Frag> {
    let Frag::Conj(m, cspan) = c else {
        return Err(Error::internal("expected a conjunction fragment"));
    };
    let span = Span::merge(Span::merge(u.span(), cspan), v.span());
    let glyph = match m {
        Modifier::Prim(g) => g,
        Modifier::Explicit(src) => return derive_explicit(&src, u, Some(v), scope, span),
    };
    match glyph {
        "\"" => {
            let f = verb_operand(u, span)?;
            if v.is_verb() {
                return Err(Error::not_yet("verb rank (u\"v)", span));
            }
            let ranks = rank_spec(&v, span)?;
            Ok(Frag::Verb(VerbFrag::V(Verb::Rank(Box::new(f), ranks)), span))
        }
        "@:" => {
            let f = verb_operand(u, span)?;
            let g = verb_operand(v, span)?;
            Ok(Frag::Verb(VerbFrag::V(Verb::Atop(Box::new(f), Box::new(g))), span))
        }
        // `u@v` is `u@:v` applied at v's own ranks: one v-cell at a time,
        // with u run on each result. That difference in rank is all that
        // separates the two spellings.
        "@" => {
            let f = verb_operand(u, span)?;
            let g = verb_operand(v, span)?;
            let ranks = g.ranks();
            let atop = Verb::Atop(Box::new(f), Box::new(g));
            Ok(Frag::Verb(VerbFrag::V(Verb::Rank(Box::new(atop), ranks)), span))
        }
        "&" => compose(u, v, false, span),
        "&:" => compose(u, v, true, span),
        // `u&.>` is the one under that is not built out of an inverse:
        // opening each box and boxing the result again is J's each.
        "&." if is_open(&v) => {
            let f = verb_operand(u, span)?;
            Ok(Frag::Verb(VerbFrag::V(Verb::Each(Box::new(f), Enclose::Always)), span))
        }
        // `u&.,` is the other one. `,` has no obverse — a ravel says
        // nothing about the shape it came from — but the shape is in hand
        // here, so u runs over the ravel and the shape goes back on.
        "&." if is_ravel(&v) => {
            let f = verb_operand(u, span)?;
            Ok(Frag::Verb(VerbFrag::V(Verb::UnderRavel(Box::new(f))), span))
        }
        // `u&.v` is `v^:_1 @: u &: v`: v prepares both arguments, u runs on
        // what it made, and v's obverse puts the answer back. `&.` does it
        // at v's monadic rank, `&.:` on the arguments whole — the same
        // difference `&` and `&:` have.
        "&." | "&.:" => {
            let f = verb_operand(u, span)?;
            let g = verb_operand(v, span)?;
            let back = obverse_of(&g, span)?;
            let composed = Verb::Compose(Box::new(f), Box::new(g.clone()));
            let under = Verb::Atop(Box::new(back), Box::new(composed));
            if glyph == "&.:" {
                return Ok(Frag::Verb(VerbFrag::V(under), span));
            }
            let rank = g.ranks()[0];
            Ok(Frag::Verb(VerbFrag::V(Verb::Rank(Box::new(under), [rank; 3])), span))
        }
        "^:" => {
            let f = verb_operand(u, span)?;
            if v.is_verb() {
                // `u^:v` asks v for the number of applications; the while
                // loop is that verb under `^:_`.
                let g = verb_operand(v, span)?;
                let p = Verb::PowerV(Box::new(f), Box::new(g));
                return Ok(Frag::Verb(VerbFrag::V(p), span));
            }
            // A negative power runs the obverse that many times, which is
            // what makes `u^:_1` the inverse.
            // A negative count runs the obverse that many times, whether it
            // was written plainly or in a box (`u^:(<_3)`).
            let negative = noun_value(&v).is_some_and(|a| {
                let inner = match a.as_boxes() {
                    Some([b]) => b.clone(),
                    _ => a,
                };
                inner.to_f64_vec().is_some_and(|n| n.len() == 1 && n[0] < 0.0)
            });
            let p = power_spec(&v, span)?;
            let f = if negative { obverse_of(&f, span)? } else { f };
            Ok(Frag::Verb(VerbFrag::V(Verb::PowerN(Box::new(f), p)), span))
        }
        ";." => {
            let f = verb_operand(u, span)?;
            let n = one_atom(&v, "cut", span)?;
            if n.fract() != 0.0 || !matches!(n as i64, -3..=3) {
                return Err(Error::not_yet(format!("cut (u;.{n})"), span));
            }
            Ok(Frag::Verb(VerbFrag::V(Verb::Cut(Box::new(f), n as i64)), span))
        }
        // `u!.n` is the tolerance for the verbs whose meaning uses one; on
        // any other verb J's `!.` specifies a fill, which is its own
        // feature and not this one.
        "!." => {
            let f = verb_operand(u, span)?;
            // `|.!.f` is the fill shift: the fit specifies what the places
            // an item left behind are filled with, not a tolerance.
            if matches!(&f, Verb::Prim(p) if p.name == "|.") {
                let fill = as_const(&v)
                    .cloned()
                    .ok_or_else(|| Error::not_yet("a computed fill (|.!.n)", span))?;
                return Ok(Frag::Verb(VerbFrag::V(Verb::ShiftFill(fill)), span));
            }
            let n = one_atom(&v, "fit", span)?;
            if !f.uses_tolerance() {
                return Err(Error::not_yet(
                    format!("fill specification ({}!.n)", f.name()),
                    span,
                ));
            }
            // J refuses a tolerance above 2^-34, and so does libjay.
            if !(0.0..=LARGEST_TOLERANCE).contains(&n) {
                return Err(Error::domain(
                    format!("a comparison tolerance must be between 0 and {LARGEST_TOLERANCE}"),
                    span,
                ));
            }
            Ok(Frag::Verb(VerbFrag::V(Verb::Fit(Box::new(f), n)), span))
        }
        // `u :. v` declares v to be u's obverse; it changes nothing about
        // how u applies, only what `^:_1` and `&.` may then do with it.
        ":." => {
            let f = verb_operand(u, span)?;
            let g = verb_operand(v, span)?;
            Ok(Frag::Verb(
                VerbFrag::V(Verb::WithObverse(Box::new(f), Box::new(g))),
                span,
            ))
        }
        // `u@.v` picks one verb of the gerund u by v's value at the
        // arguments; a noun on the right picks one now and for good.
        "@." => {
            let vs = gerund_verbs(&u, scope, span)?;
            if v.is_verb() {
                let w = verb_operand(v, span)?;
                return Ok(Frag::Verb(VerbFrag::V(Verb::Agenda(vs, Box::new(w))), span));
            }
            let at = near_whole(one_atom(&v, "agenda", span)?);
            if at.fract() != 0.0 {
                return Err(Error::parse("an agenda index must be a whole number", span));
            }
            let picked = crate::verb::pick_gerund(&vs, at as i64, span)?;
            Ok(Frag::Verb(VerbFrag::V(picked), span))
        }
        // `u`v` ties two entities into a gerund, which is ordinary boxed
        // data: one box per atomic representation, catenated.
        "`" => {
            let left = tie_side(&u, scope, span)?;
            let right = tie_side(&v, scope, span)?;
            let tied = crate::verb::catenate(&left, &right, true, true, span)?;
            Ok(Frag::Noun(Expr::Const(tied, span)))
        }
        // `u :: v` answers a refusal of u by running v instead. A noun on
        // the right is the constant verb yielding it, as J reads it.
        "::" => {
            let f = verb_operand(u, span)?;
            let g = if v.is_noun() {
                constant_verb(bond_noun(&v, span)?)
            } else {
                verb_operand(v, span)?
            };
            Ok(Frag::Verb(VerbFrag::V(Verb::Adverse(Box::new(f), Box::new(g))), span))
        }
        // `u L: n` and `u S: n` apply u at a boxing level: `L:` puts each
        // answer back in the box its operand came from, `S:` spreads them
        // into one array.
        "L:" | "S:" => {
            let f = verb_operand(u, span)?;
            let n = one_atom(&v, "level", span)?;
            if n.fract() != 0.0 || !n.is_finite() {
                return Err(Error::not_yet(format!("a level of {n} ({glyph})"), span));
            }
            let level = Verb::Level {
                u: Box::new(f),
                level: n as i64,
                spread: glyph == "S:",
            };
            Ok(Frag::Verb(VerbFrag::V(level), span))
        }
        // `` m`:n ``: 0 applies every verb of the gerund to the arguments
        // and frames the answers, 3 inserts them between the items of y,
        // and 6 is the train the gerund spells, which is built here.
        "`:" => {
            if u.is_verb() {
                return Err(Error::domain(
                    "`: reads a gerund, which is boxed data, not a verb",
                    span,
                ));
            }
            let vs = gerund_verbs(&u, scope, span)?;
            let n = one_atom(&v, "evoke gerund", span)?;
            if vs.is_empty() {
                return Err(Error::domain("an evoked gerund is empty", span));
            }
            match n {
                0.0 | 3.0 => {
                    Ok(Frag::Verb(VerbFrag::V(Verb::Evoke(vs, n as i64)), span))
                }
                6.0 => train_of(vs, span),
                _ => Err(Error::domain(
                    format!("`:{n} is not one of the evoke forms 0, 3 and 6"),
                    span,
                )),
            }
        }
        // `m H. n`: the generalised hypergeometric function, m the
        // numerator parameters and n the denominator ones. Both are nouns,
        // and an empty list on either side is the ordinary case of none.
        "H." => {
            let num = series_parameters(&u, span)?;
            let den = series_parameters(&v, span)?;
            Ok(Frag::Verb(VerbFrag::V(Verb::Hypergeometric { num, den }), span))
        }
        // Threads reach outside the expression, which the sandbox closes;
        // libjay's own parallelism is not something a sentence asks for.
        // That is a property of libjay, not a queue position.
        "T." => Err(Error::sandbox(
            "T. starts J's own threads, which libjay does not open",
            span,
        )),
        // `u t. n` schedules u in one of J's thread pools and answers with
        // a pyx — a task, not a value. The sandbox does not open those
        // threads, which is libjay's own policy and not a queue position.
        // The reference rejects `t:` outright — an invalid inflection, as
        // it does `d.`, `D.` and `D:`. There is nothing here to implement.
        "t:" => Err(Error::new(
            ErrorKind::Language,
            "t: is not a J inflection; the reference rejects the spelling",
            Some(span),
        )),
        "t." => Err(Error::sandbox(
            "t. runs a verb in one of J's thread pools, which libjay does not open",
            span,
        )),
        // `u . v`: the inner product, of which `+/ . *` is the matrix
        // product and `-/ . *` the determinant.
        "." => {
            let f = verb_operand(u, span)?;
            let g = verb_operand(v, span)?;
            Ok(Frag::Verb(VerbFrag::V(Verb::InnerProduct {
                u: Box::new(f),
                v: Box::new(g),
                apl: false,
            }), span))
        }
        "!:" => foreign(&u, &v, span),
        // `u : v` is J's monad/dyad conjunction. The explicit definitions
        // spelled `3 : '…'` and `4 : '…'` are read by the lexer and never
        // reach here.
        ":" => Err(Error::not_yet("the monad-dyad conjunction (u : v)", span)),
        _ => Err(Error::not_yet(format!("the conjunction {glyph}"), span)),
    }
}

/// `u&v` and `u&:v`, in all three shapes the conjunction takes.
///
/// With two verbs it composes: monadically `u v y`, dyadically
/// `(v x) u (v y)` — and `&` runs that at v's monadic rank on both sides
/// while `&:` runs it on the arguments whole. With a noun on either side it
/// bonds that noun into the dyad, giving a verb with a monadic valence only;
/// `&:` takes no noun at all.
fn compose(u: Frag, v: Frag, infinite: bool, span: Span) -> Result<Frag> {
    let verb = |v: Verb| Ok(Frag::Verb(VerbFrag::V(v), span));
    if infinite || (!u.is_noun() && !v.is_noun()) {
        let f = verb_operand(u, span)?;
        let g = verb_operand(v, span)?;
        let monadic_rank = g.ranks()[0];
        let composed = Verb::Compose(Box::new(f), Box::new(g));
        if infinite {
            return verb(composed);
        }
        return verb(Verb::Rank(Box::new(composed), [monadic_rank; 3]));
    }
    if u.is_noun() && v.is_noun() {
        return Err(Error::not_yet("noun-operand conjunctions", span));
    }
    // A bond applies its verb dyadically to the WHOLE argument: `m&v y` is
    // `m v y`, and its rank is infinite whatever v's is — `1 2&+ b. 0`
    // reports `_ _ _`, and `1 2&+ i. 2 2` agrees row by row rather than
    // pairing the noun with every atom.
    if u.is_noun() {
        let m = bond_noun(&u, span)?;
        let g = as_verb(v)?.0;
        return verb(Verb::BondLeft(m, Box::new(g)));
    }
    let f = as_verb(u)?.0;
    let n = bond_noun(&v, span)?;
    verb(Verb::BondRight(Box::new(f), n))
}

/// The largest comparison tolerance `!.` accepts, as J's does: 2^-34.
const LARGEST_TOLERANCE: f64 = 5.820_766_091_346_741e-11;

/// A conjunction's single numeric noun operand.
/// One side's parameter list for `m H. n`: a numeric list, known now.
fn series_parameters(f: &Frag, span: Span) -> Result<Vec<crate::complex::Cx>> {
    let Some(arr) = as_const(f) else {
        return Err(Error::not_yet("computed hypergeometric parameters (m H. n)", span));
    };
    if arr.count() == 0 {
        return Ok(Vec::new());
    }
    if arr.rank() > 1 {
        return Err(Error::parse("a hypergeometric parameter list is a vector", span));
    }
    match arr.data.cast(crate::dtype::DType::Complex) {
        Some(Data::Complex(v)) => Ok(v.as_slice().to_vec()),
        _ => Err(Error::parse("hypergeometric parameters are numbers", span)),
    }
}

/// `m !: n`: J's foreigns, the family that reaches outside the language.
///
/// Three of them are libjay's. `1!:1` reads a line from the input source
/// and `1!:2` writes one to the output sink — the two halves of the stdio
/// the sandbox opens — and `3!:0` names an element type, which computes
/// and touches nothing.
///
/// The rest divide in two, and the division is the whole point of the
/// dispatcher. A foreign that would reach a file, a directory, the host or
/// a script is closed by the sandbox and no release will open it; one that
/// only computes is a queue position, and names itself as one.
fn foreign(u: &Frag, v: &Frag, span: Span) -> Result<Frag> {
    let family = foreign_number(u, span)?;
    let member = foreign_number(v, span)?;
    let prim = |name, monad, dyad| {
        Ok(Frag::Verb(
            VerbFrag::V(Verb::Prim(Prim { name, monad, dyad, ranks: [RANK_INF; 3] })),
            span,
        ))
    };
    let closed = |what: &str| {
        Err(Error::sandbox(format!("{family}!:{member} {what}, which is outside the program"), span))
    };
    match (family, member) {
        (1, 1) => prim("1!:1", MonadOp::ReadStream, DyadOp::None),
        (1, 2) => prim("1!:2", MonadOp::None, DyadOp::WriteStream),
        (3, 0) => prim("3!:0", MonadOp::TypeCode, DyadOp::None),
        // `5!:1 <'name'` is the atomic representation of what the name
        // stands for — the same boxed data a gerund is made of.
        (5, 1) => prim("5!:1", MonadOp::AtomicRep, DyadOp::None),
        (0, _) => closed("runs a script file"),
        // The rest of the file family: stdin and stdout are the streams the
        // sandbox opens, and every other member of it is the filesystem.
        (1, _) => closed("reaches the filesystem"),
        (2, _) => closed("reaches the host — its environment, its shell, its processes"),
        (6, _) => closed("reads the clock"),
        (15, _) => closed("calls into a shared library"),
        _ => Err(Error::not_yet(format!("the foreign {family}!:{member}"), span)),
    }
}

/// One side of `m !: n`: a whole number, known now. A foreign is chosen by
/// its two numbers, so neither may be computed.
fn foreign_number(f: &Frag, span: Span) -> Result<i64> {
    if f.is_verb() {
        return Err(Error::parse("a foreign is spelled m!:n, with two numbers", span));
    }
    let Some(arr) = as_const(f) else {
        return Err(Error::not_yet("a computed foreign number (m!:n)", span));
    };
    match arr.to_i64_vec().as_deref() {
        Some([n]) if *n >= 0 => Ok(*n),
        _ => Err(Error::parse("a foreign is spelled m!:n, with two whole numbers", span)),
    }
}

fn one_atom(f: &Frag, what: &str, span: Span) -> Result<f64> {
    let Some(arr) = as_const(f) else {
        return Err(Error::not_yet(format!("a computed {what} specification"), span));
    };
    let Some(vals) = arr.to_f64_vec() else {
        return Err(Error::parse(format!("{what} takes a numeric operand"), span));
    };
    match vals[..] {
        [n] => Ok(n),
        _ => Err(Error::parse(format!("{what} takes one atom"), span)),
    }
}

/// The array a bonded noun operand holds; it has to be known now.
fn bond_noun(f: &Frag, span: Span) -> Result<Array> {
    as_const(f)
        .cloned()
        .ok_or_else(|| Error::not_yet("bonds over a non-literal noun", span))
}

/// True for the fragment holding the primitive `>`, the only right operand
/// `&.` accepts.
fn is_open(f: &Frag) -> bool {
    matches!(f, Frag::Verb(VerbFrag::V(Verb::Prim(p)), _) if p.monad == MonadOp::Open)
}

fn is_ravel(f: &Frag) -> bool {
    matches!(f, Frag::Verb(VerbFrag::V(Verb::Prim(p)), _) if p.monad == MonadOp::Ravel)
}

fn verb_operand(f: Frag, span: Span) -> Result<Verb> {
    if f.is_noun() {
        return Err(Error::not_yet("noun-operand conjunctions", span));
    }
    Ok(as_verb(f)?.0)
}

/// `u"n`: 1 atom applies to every valence, 2 atoms are `left right` with the
/// monadic rank taken from the right, 3 atoms are given in full.
fn rank_spec(f: &Frag, span: Span) -> Result<[i64; 3]> {
    let Some(arr) = as_const(f) else {
        return Err(Error::not_yet("computed rank specifications", span));
    };
    let Some(vals) = arr.to_f64_vec() else {
        return Err(Error::parse("rank must be numeric", span));
    };
    if vals.is_empty() || vals.len() > 3 {
        return Err(Error::parse("rank takes 1 to 3 atoms", span));
    }
    let mut r = Vec::with_capacity(vals.len());
    for x in vals {
        if x == f64::INFINITY {
            r.push(RANK_INF);
        } else if x == f64::NEG_INFINITY {
            r.push(-RANK_INF);
        } else if x.fract() != 0.0 {
            return Err(Error::parse("rank must be an integer", span));
        } else {
            r.push(x as i64);
        }
    }
    Ok(match r.len() {
        1 => [r[0], r[0], r[0]],
        2 => [r[1], r[0], r[1]],
        _ => [r[0], r[1], r[2]],
    })
}

/// `u^:n`: one nonnegative integer atom, or `_` for "iterate until the
/// result stops changing".
/// The whole number a count operand stands for, or the value unchanged.
///
/// A noun operand read at compile time makes the same near-integer
/// admission a count read at run time makes: jconsole answers
/// `(>: ^: (2 + 1e_13)) 3` with 5 and `(+`- @. (1 + 1e_14)) 5` with _5.
fn near_whole(n: f64) -> f64 {
    crate::array::NearInt::J.round(n).map_or(n, |k| k as f64)
}

fn power_spec(f: &Frag, span: Span) -> Result<Power> {
    let Some(arr) = noun_value(f) else {
        return Err(Error::not_yet("computed power (u^:n)", span));
    };
    let arr = &arr;
    // A boxed count traces the applications rather than taking one of
    // them: `u^:(<n)` is `u^:(i.n)`, and `u^:a:` traces to convergence.
    if let Some(boxes) = arr.as_boxes() {
        let [inner] = boxes else {
            return Err(Error::parse("a boxed power takes one box", span));
        };
        if inner.count() == 0 {
            return Ok(Power::ConvergeTrace);
        }
        let Some(vals) = inner.to_f64_vec() else {
            return Err(Error::parse("power must be numeric", span));
        };
        let vals: Vec<f64> = vals.into_iter().map(near_whole).collect();
        let [n] = vals[..] else {
            return Err(Error::not_yet("a boxed list of power counts (u^:(<n))", span));
        };
        if n.fract() != 0.0 || n.abs() > 1e6 {
            return Err(Error::parse("a boxed power must be a whole count", span));
        }
        // `u^:(<n)` is `u^:(i.n)`: n counts, downwards where n is negative.
        if n == 0.0 {
            return Err(Error::domain("a boxed power traces at least one application", span));
        }
        // A negative n counts the same way with the obverse, which the
        // caller has already put in the verb's place.
        return Ok(Power::Each((0..n.abs() as u64).collect()));
    }
    let Some(vals) = arr.to_f64_vec() else {
        return Err(Error::parse("power must be numeric", span));
    };
    let vals: Vec<f64> = vals.into_iter().map(near_whole).collect();
    if vals.len() > 1 {
        // A list of counts gives one answer each, framed.
        let mut counts = Vec::with_capacity(vals.len());
        for n in &vals {
            if n.fract() != 0.0 || *n < 0.0 || *n > 1e6 {
                return Err(Error::not_yet("a power count outside 0 … 1e6", span));
            }
            counts.push(*n as u64);
        }
        return Ok(Power::Each(counts));
    }
    let [n] = vals[..] else {
        return Err(Error::not_yet("power over a list of counts (u^:n)", span));
    };
    if n == f64::INFINITY {
        return Ok(Power::Converge);
    }
    if n.fract() != 0.0 {
        return Err(Error::parse("power must be a whole number", span));
    }
    if n < 0.0 {
        // A negative power is the obverse applied that many times; the
        // caller substitutes the obverse for the verb.
        return Ok(Power::Times((-n) as u64));
    }
    Ok(Power::Times(n as u64))
}

/// The obverse of a verb, or the diagnostic naming the verb that has none.
pub(crate) fn obverse_of(v: &Verb, span: Span) -> Result<Verb> {
    crate::verb::obverse(v).ok_or_else(|| {
        Error::not_yet(format!("the obverse of {} (no inverse is known)", v.name()), span)
    })
}

/// One side of `` u`v ``: a verb becomes the box holding its atomic
/// representation, a noun stands for itself. Catenating the two is the tie,
/// which is why `` u`v`w `` builds up left to right with no special case.
fn tie_side(f: &Frag, scope: &Names, span: Span) -> Result<Array> {
    if f.is_real_verb() {
        let (v, _) = as_verb(f.clone())?;
        return Ok(Array::boxed(verb_ar(&v, span)?.to_array()));
    }
    noun_in_scope(f, scope)
        .ok_or_else(|| Error::not_yet("a tie over a computed noun", span))
}

/// A verb's atomic representation, with the diagnostic for the verbs libjay
/// has no J spelling to give.
fn verb_ar(v: &Verb, span: Span) -> Result<crate::gerund::Ar> {
    crate::gerund::verb_ar(v).ok_or_else(|| {
        Error::not_yet(format!("the atomic representation of {}", v.name()), span)
    })
}

/// A noun fragment's value, a name that holds a literal included. A gerund
/// is data, so `` g =. +`- `` and then `g@.1` has to find what g holds.
fn noun_in_scope(f: &Frag, scope: &Names) -> Option<Array> {
    if let Frag::Name(n, _) = f {
        return scope.consts.get(n).cloned();
    }
    noun_value(f)
}

/// The verbs a gerund holds. A lone verb is a gerund of one, and boxed data
/// is read as the atomic representations it is.
fn gerund_verbs(f: &Frag, scope: &Names, span: Span) -> Result<Vec<Verb>> {
    if f.is_real_verb() {
        return Ok(vec![as_verb(f.clone())?.0]);
    }
    let arr = noun_in_scope(f, scope)
        .ok_or_else(|| Error::not_yet("a gerund computed at run time", span))?;
    let Some(items) = arr.as_boxes() else {
        return Err(Error::domain("a gerund is boxed data", span));
    };
    items.iter().map(|a| ar_verb(a, scope, span)).collect()
}

/// One atomic representation as the verb it stands for.
fn ar_verb(a: &Array, scope: &Names, span: Span) -> Result<Verb> {
    let ar = crate::gerund::Ar::from_array(a)
        .ok_or_else(|| Error::domain("this is not an atomic representation", span))?;
    let (v, _) = as_verb(ar_frag(&ar, scope, span)?)?;
    Ok(v)
}

/// One atomic representation as the fragment it stands for: a verb, or the
/// noun a modifier takes as an operand.
fn ar_frag(ar: &crate::gerund::Ar, scope: &Names, span: Span) -> Result<Frag> {
    use crate::gerund::Ar;
    match ar {
        Ar::Noun(a) => Ok(Frag::Noun(Expr::Const(a.clone(), span))),
        Ar::Prim(word) => {
            if word == "[:" {
                return Ok(Frag::Verb(VerbFrag::Cap, span));
            }
            match verb_for(word) {
                Some(v) => Ok(Frag::Verb(VerbFrag::V(v), span)),
                None => Err(Error::domain(
                    format!("`{word}` is not a verb an atomic representation may name"),
                    span,
                )),
            }
        }
        Ar::Train(parts) => {
            let frags: Result<Vec<Frag>> =
                parts.iter().map(|p| ar_frag(p, scope, span)).collect();
            let mut frags = frags?;
            match frags.len() {
                2 => {
                    let b = frags.pop().expect("two parts");
                    let a = frags.pop().expect("two parts");
                    apply_bident(a, b, &scope.nouns)
                }
                3 => {
                    let h = frags.pop().expect("three parts");
                    let g = frags.pop().expect("three parts");
                    let f = frags.pop().expect("three parts");
                    apply_fork(f, g, h)
                }
                _ => Err(Error::domain("a train is two or three parts", span)),
            }
        }
        Ar::Derived(word, ops) => {
            let frags: Result<Vec<Frag>> = ops.iter().map(|p| ar_frag(p, scope, span)).collect();
            let mut frags = frags?;
            if let Some(glyph) = adverb(word) {
                if frags.len() != 1 {
                    return Err(Error::domain(format!("{glyph} takes one operand"), span));
                }
                let u = frags.pop().expect("one operand");
                return apply_adverb(u, Frag::Adverb(Modifier::Prim(glyph), span), scope);
            }
            if let Some(glyph) = conjunction(word) {
                if frags.len() != 2 {
                    return Err(Error::domain(format!("{glyph} takes two operands"), span));
                }
                let v = frags.pop().expect("two operands");
                let u = frags.pop().expect("two operands");
                return apply_conj(u, Frag::Conj(Modifier::Prim(glyph), span), v, scope);
            }
            Err(Error::domain(
                format!("`{word}` is not a modifier an atomic representation may name"),
                span,
            ))
        }
    }
}

/// The train a gerund spells: `` `:6 `` groups the verbs from the right,
/// three at a time, which is how J reads a train written out.
fn train_of(vs: Vec<Verb>, span: Span) -> Result<Frag> {
    let mut frags: Vec<Frag> =
        vs.into_iter().map(|v| Frag::Verb(VerbFrag::V(v), span)).collect();
    while frags.len() > 3 {
        let h = frags.pop().expect("three or more");
        let g = frags.pop().expect("three or more");
        let f = frags.pop().expect("three or more");
        frags.push(apply_fork(f, g, h)?);
    }
    match frags.len() {
        1 => Ok(frags.pop().expect("one")),
        2 => {
            let b = frags.pop().expect("two");
            let a = frags.pop().expect("two");
            apply_bident(a, b, &HashSet::new())
        }
        _ => {
            let h = frags.pop().expect("three");
            let g = frags.pop().expect("three");
            let f = frags.pop().expect("three");
            apply_fork(f, g, h)
        }
    }
}

fn apply_fork(f: Frag, g: Frag, h: Frag) -> Result<Frag> {
    let span = Span::merge(Span::merge(f.span(), g.span()), h.span());
    let (gv, _) = as_verb(g)?;
    let (hv, _) = as_verb(h)?;
    match f {
        // `[: g h` is g atop h: the left tine produces nothing to fork over.
        Frag::Verb(VerbFrag::Cap, _) => {
            Ok(Frag::Verb(VerbFrag::V(Verb::Atop(Box::new(gv), Box::new(hv))), span))
        }
        Frag::Verb(VerbFrag::V(fv), _) => Ok(Frag::Verb(
            VerbFrag::V(Verb::Fork(Box::new(fv), Box::new(gv), Box::new(hv))),
            span,
        )),
        noun => {
            let Some(arr) = as_const(&noun) else {
                return Err(Error::not_yet("noun forks over a non-literal noun", span));
            };
            Ok(Frag::Verb(
                VerbFrag::V(Verb::NounFork(arr.clone(), Box::new(gv), Box::new(hv))),
                span,
            ))
        }
    }
}

fn apply_bident(a: Frag, b: Frag, nouns: &HashSet<String>) -> Result<Frag> {
    let span = Span::merge(a.span(), b.span());
    // A name here is not a verb, or it would have been substituted; if it
    // is not a value either, that is what is wrong with the sentence, and
    // it is what the reference reports.
    if let Frag::Name(n, nspan) = &a && !nouns.contains(n) {
        return Err(Error::new(
            ErrorKind::Value,
            format!("undefined name: {n}"),
            Some(*nspan),
        ));
    }
    if a.is_real_verb() && b.is_real_verb() {
        let (f, _) = as_verb(a)?;
        let (g, _) = as_verb(b)?;
        return Ok(Frag::Verb(VerbFrag::V(Verb::Hook(Box::new(f), Box::new(g))), span));
    }
    // Two verbs are the only pair J makes a train of. Anything else here —
    // a noun beside a noun, a noun beside a verb, a leftover modifier — is
    // a sentence the language does not have a reading for, which is what
    // the reference calls a syntax error. It is not a queue position.
    if matches!(a, Frag::Verb(VerbFrag::Cap, _)) {
        return Err(Error::parse("`[:` caps a fork; it has no verb of its own", span));
    }
    Err(Error::parse("syntax error", span))
}

fn apply_assign(target: Frag, value: Frag, scope: Scope) -> Result<Frag> {
    let span = Span::merge(target.span(), value.span());
    match target {
        // `=.` names a local and `=:` a global; the two differ only inside
        // an explicit definition, which is the only thing with a local
        // frame to name.
        Frag::Name(name, _) => match value {
            // Naming a verb is settled here, at parse time: `parse` records
            // the name and substitutes the verb into later sentences.
            Frag::Verb(VerbFrag::V(verb), _) => Ok(Frag::VerbDef(name, verb, span)),
            Frag::Verb(VerbFrag::Cap, _) => Err(Error::not_yet("assigning [: on its own", span)),
            // Naming a modifier is settled at parse time too: the name
            // stands for the spelling wherever a later sentence writes it.
            Frag::Adverb(m, _) => Ok(Frag::ModDef(name, false, m, span)),
            Frag::Conj(m, _) => Ok(Frag::ModDef(name, true, m, span)),
            v if v.is_noun() => {
                let value = as_noun(v)?;
                Ok(Frag::Noun(Expr::Assign { name, value: Box::new(value), scope, span }))
            }
            other => Err(Error::internal(format!("cannot assign {other:?}"))),
        },
        Frag::Noun(_) => Err(Error::not_yet("multiple assignment", span)),
        other => Err(Error::internal(format!("expected an assignment target, got {other:?}"))),
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::dtype::DType;
    use crate::error::ErrorKind;
    use rstest::rstest;

    fn parse_str(src: &str) -> Result<Vec<Expr>> {
        parse(&SourceParts::from_source(src).expect("source parts"))
    }

    /// Parse literal text with no interpolation. `{. ` and `}.` are J words
    /// that `from_source` would read as a hole, so those tests take the
    /// pre-split path instead.
    fn one_literal(src: &str) -> Expr {
        let sp = SourceParts::from_parts(&[src], &[]);
        let mut s = parse(&sp).unwrap_or_else(|e| panic!("parse of {src:?} failed: {e}"));
        assert_eq!(s.len(), 1, "expected one sentence in {src:?}");
        s.pop().expect("one sentence")
    }

    fn stmts(src: &str) -> Vec<Expr> {
        parse_str(src).unwrap_or_else(|e| panic!("parse of {src:?} failed: {e}"))
    }

    /// The single statement of a one-sentence program.
    fn one(src: &str) -> Expr {
        let mut s = stmts(src);
        assert_eq!(s.len(), 1, "expected one sentence in {src:?}");
        s.pop().expect("one sentence")
    }

    fn err(src: &str) -> Error {
        match parse_str(src) {
            Ok(v) => panic!("expected an error for {src:?}, got {v:?}"),
            Err(e) => e,
        }
    }

    // The shape inspectors return owned copies so that a test can inspect
    // the result of `one(...)` in one expression.

    fn konst(e: &Expr) -> Array {
        match e {
            Expr::Const(a, _) => a.clone(),
            other => panic!("expected a constant, got {other:?}"),
        }
    }

    fn ints(e: &Expr) -> Vec<i64> {
        konst(e).as_i64_slice().expect("integer data").to_vec()
    }

    fn prim_of(v: &Verb) -> Prim {
        match v {
            Verb::Prim(p) => *p,
            other => panic!("expected a primitive, got {other:?}"),
        }
    }

    fn monad_of(e: &Expr) -> (Verb, Expr) {
        match e {
            Expr::Monad { verb, y, .. } => (verb.clone(), (**y).clone()),
            other => panic!("expected a monad, got {other:?}"),
        }
    }

    fn dyad_of(e: &Expr) -> (Verb, Expr, Expr) {
        match e {
            Expr::Dyad { verb, x, y, .. } => (verb.clone(), (**x).clone(), (**y).clone()),
            other => panic!("expected a dyad, got {other:?}"),
        }
    }

    // ------------------------------------------------------------- literals

    #[test]
    fn single_number_is_an_atom() {
        let e = one("5");
        assert_eq!(konst(&e).shape, Vec::<usize>::new());
        assert_eq!(ints(&e), vec![5]);
        assert_eq!(e.span(), Span::new(0, 1));
    }

    #[test]
    fn adjacent_numbers_merge_into_one_vector() {
        let e = one("1 2 3");
        assert_eq!(konst(&e).shape, vec![3]);
        assert_eq!(ints(&e), vec![1, 2, 3]);
        assert_eq!(e.span(), Span::new(0, 5));
    }

    #[test]
    fn a_float_makes_the_whole_vector_float() {
        let a = konst(&one("1 2.5 3"));
        assert_eq!(a.dtype(), DType::F64);
        assert_eq!(a.as_f64_slice(), Some(&[1.0, 2.5, 3.0][..]));
    }

    #[test]
    fn negatives_and_infinities() {
        let a = konst(&one("_3 1.5 _ __"));
        assert_eq!(a.shape, vec![4]);
        let v = a.as_f64_slice().expect("float vector");
        assert_eq!(v[0], -3.0);
        assert_eq!(v[1], 1.5);
        assert!(v[2].is_infinite() && v[2] > 0.0);
        assert!(v[3].is_infinite() && v[3] < 0.0);
    }

    #[test]
    fn negative_integers_stay_integers() {
        let a = konst(&one("_3 _4"));
        assert_eq!(a.dtype(), DType::I64);
        assert_eq!(a.as_i64_slice(), Some(&[-3i64, -4][..]));
    }

    #[rstest]
    #[case("1e3", 1000.0)]
    #[case("1e_3", 0.001)]
    #[case("2.5e2", 250.0)]
    #[case("_1.5", -1.5)]
    fn exponent_and_sign_forms(#[case] src: &str, #[case] want: f64) {
        let a = konst(&one(src));
        assert_eq!(a.dtype(), DType::F64);
        assert_eq!(a.to_f64_vec().expect("numeric"), vec![want]);
    }

    #[test]
    fn adjacent_numbers_stop_at_a_non_number() {
        // `i.` after a vector is a separate word, not numeric characters.
        let (_, x, y) = dyad_of(&one("2 3 i. 4"));
        assert_eq!(konst(&x).shape, vec![2]);
        assert_eq!(konst(&y).shape, Vec::<usize>::new());
    }

    #[test]
    fn string_of_several_characters_is_a_vector() {
        let e = one("'abc'");
        let a = konst(&e);
        assert_eq!(a.shape, vec![3]);
        assert_eq!(a.data, Data::Char(vec!['a', 'b', 'c'].into()));
        assert_eq!(e.span(), Span::new(0, 5));
    }

    #[test]
    fn one_character_string_is_an_atom() {
        let a = konst(&one("'a'"));
        assert_eq!(a.shape, Vec::<usize>::new());
        assert_eq!(a.data, Data::Char(vec!['a'].into()));
    }

    #[test]
    fn empty_string_is_an_empty_vector() {
        let a = konst(&one("''"));
        assert_eq!(a.shape, vec![0]);
        assert_eq!(a.dtype(), DType::Char);
    }

    #[test]
    fn doubled_quote_is_an_escaped_quote() {
        let a = konst(&one("'it''s'"));
        assert_eq!(a.shape, vec![4]);
        assert_eq!(a.data, Data::Char(vec!['i', 't', '\'', 's'].into()));
    }

    #[test]
    fn unterminated_string_is_a_parse_error() {
        let e = err("'abc");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert!(e.msg.contains("unterminated"), "{}", e.msg);
        assert_eq!(e.span, Some(Span::new(0, 4)));
    }

    // ------------------------------------------------------------- comments

    #[test]
    fn comment_runs_to_end_of_line() {
        let e = one("1 2 NB. and the rest + - ' is ignored");
        assert_eq!(konst(&e).shape, vec![2]);
    }

    #[test]
    fn comment_only_line_yields_no_sentence() {
        assert!(stmts("NB. nothing here").is_empty());
        let s = stmts("NB. header\n5");
        assert_eq!(s.len(), 1);
        assert_eq!(ints(&s[0]), vec![5]);
    }

    #[test]
    fn nb_inside_a_name_is_not_a_comment() {
        // `aNB` is a name; only a whole word `NB.` starts a comment.
        match one("aNB") {
            Expr::Name(n, _) => assert_eq!(n, "aNB"),
            other => panic!("expected a name, got {other:?}"),
        }
    }

    // -------------------------------------------------------------- parsing

    #[test]
    fn empty_program_has_no_sentences() {
        assert!(stmts("").is_empty());
        assert!(stmts("\n\n").is_empty());
    }

    #[test]
    fn trains_of_dyads_are_right_associative() {
        let e = one("1 + 2 + 3");
        let (v, x, y) = dyad_of(&e);
        assert_eq!(prim_of(&v).name, "+");
        assert_eq!(ints(&x), vec![1]);
        let (v2, x2, y2) = dyad_of(&y);
        assert_eq!(prim_of(&v2).name, "+");
        assert_eq!(ints(&x2), vec![2]);
        assert_eq!(ints(&y2), vec![3]);
        assert_eq!(e.span(), Span::new(0, 9));
    }

    #[test]
    fn a_verb_with_no_left_argument_is_a_monad() {
        let e = one("- 5");
        let (v, y) = monad_of(&e);
        assert_eq!(prim_of(&v).monad, MonadOp::Scalar(ScalarMonad::Neg));
        assert_eq!(ints(&y), vec![5]);
        assert_eq!(e.span(), Span::new(0, 3));
    }

    #[test]
    fn a_verb_with_a_left_argument_is_a_dyad() {
        let (v, _, _) = dyad_of(&one("1 - 5"));
        assert_eq!(prim_of(&v).dyad, DyadOp::Scalar(ScalarDyad::Sub));
    }

    #[test]
    fn a_monad_binds_to_the_right_inside_a_dyad() {
        let (v, x, y) = dyad_of(&one("2 * - 3"));
        assert_eq!(prim_of(&v).name, "*");
        assert_eq!(ints(&x), vec![2]);
        let (mv, my) = monad_of(&y);
        assert_eq!(prim_of(&mv).name, "-");
        assert_eq!(ints(&my), vec![3]);
    }

    #[test]
    fn parentheses_group_the_left_argument() {
        let (v, x, y) = dyad_of(&one("(1 + 2) * 3"));
        assert_eq!(prim_of(&v).name, "*");
        let (iv, _, _) = dyad_of(&x);
        assert_eq!(prim_of(&iv).name, "+");
        // The parentheses are dropped, but the span still covers them, so
        // that a caret under the group underlines something balanced.
        assert_eq!(x.span(), Span::new(0, 7));
        assert_eq!(ints(&y), vec![3]);
    }

    #[test]
    fn names_are_nouns() {
        match one("x") {
            Expr::Name(n, s) => {
                assert_eq!(n, "x");
                assert_eq!(s, Span::new(0, 1));
            }
            other => panic!("expected a name, got {other:?}"),
        }
        let (_, x, y) = dyad_of(&one("x + y"));
        assert!(matches!(x, Expr::Name(..)));
        assert!(matches!(y, Expr::Name(..)));
    }

    #[test]
    fn echo_is_a_verb() {
        let (v, y) = monad_of(&one("echo 5"));
        assert_eq!(prim_of(&v).monad, MonadOp::Echo);
        assert_eq!(ints(&y), vec![5]);
    }

    #[test]
    fn inflected_letter_words_are_primitives() {
        let (v, _) = monad_of(&one("i. 3"));
        let p = prim_of(&v);
        assert_eq!(p.monad, MonadOp::IotaJ);
        assert_eq!(p.ranks, [1, RANK_INF, RANK_INF]);
    }

    #[rstest]
    #[case("|: 1 2 3", MonadOp::TransposeAxes)]
    #[case("$ 1 2 3", MonadOp::ShapeOf)]
    #[case("# 1 2 3", MonadOp::Tally)]
    #[case(", 1 2 3", MonadOp::Ravel)]
    #[case("%: 1 2 3", MonadOp::Scalar(ScalarMonad::Sqrt))]
    #[case("<. 1.5", MonadOp::Scalar(ScalarMonad::Floor))]
    fn inflected_symbol_words(#[case] src: &str, #[case] want: MonadOp) {
        let (v, _) = monad_of(&one(src));
        assert_eq!(prim_of(&v).monad, want);
    }

    #[rstest]
    #[case("{. 1 2 3", MonadOp::Head, DyadOp::Take)]
    #[case("}. 1 2 3", MonadOp::Behead, DyadOp::Drop)]
    fn brace_words(#[case] src: &str, #[case] monad: MonadOp, #[case] dyad: DyadOp) {
        let (v, _) = monad_of(&one_literal(src));
        let p = prim_of(&v);
        assert_eq!(p.monad, monad);
        assert_eq!(p.dyad, dyad);
        assert_eq!(p.ranks, [RANK_INF, 1, RANK_INF]);
    }

    #[test]
    fn a_brace_word_takes_a_left_argument() {
        let (v, x, y) = dyad_of(&one_literal("2 {. 1 2 3"));
        assert_eq!(prim_of(&v).dyad, DyadOp::Take);
        assert_eq!(ints(&x), vec![2]);
        assert_eq!(konst(&y).shape, vec![3]);
    }

    #[rstest]
    #[case("2 $ 1 2 3", DyadOp::Reshape)]
    #[case("2 [ 3", DyadOp::Left)]
    #[case("2 ] 3", DyadOp::Right)]
    #[case("2 <. 3", DyadOp::Scalar(ScalarDyad::Min))]
    #[case("2 >: 3", DyadOp::Scalar(ScalarDyad::Ge))]
    fn dyadic_primitives(#[case] src: &str, #[case] want: DyadOp) {
        let (v, _, _) = dyad_of(&one(src));
        assert_eq!(prim_of(&v).dyad, want);
    }

    #[test]
    fn unimplemented_meanings_reach_the_verb_not_the_parser() {
        // Both dyads take a numbered form as their left argument, and the
        // numbers with no meaning here are refused inside the verb — where
        // the number is known — rather than at the parser.
        let (v, _, _) = dyad_of(&one("2 s: 'a b'"));
        assert_eq!(prim_of(&v).dyad, DyadOp::SymbolForm);
        let (v, _, _) = dyad_of(&one("6 $. 'a b'"));
        assert_eq!(prim_of(&v).dyad, DyadOp::SparseForm);
    }

    #[test]
    fn multiple_sentences_become_multiple_statements() {
        let s = stmts("a =. 1 2\n+/ a\n");
        assert_eq!(s.len(), 2);
        assert!(matches!(s[0], Expr::Assign { .. }));
        assert!(matches!(s[1], Expr::Monad { .. }));
    }

    // ------------------------------------------------------------ modifiers

    #[test]
    fn an_adverb_binds_before_the_verb_is_applied() {
        let e = one("+/ 1 2 3");
        let (v, y) = monad_of(&e);
        match &v {
            Verb::Reduce(inner) => assert_eq!(prim_of(inner).name, "+"),
            other => panic!("expected a reduction, got {other:?}"),
        }
        assert_eq!(konst(&y).shape, vec![3]);
        assert_eq!(e.span(), Span::new(0, 8));
    }

    #[test]
    fn rank_applies_to_the_derived_verb() {
        let (v, _) = monad_of(&one("+/\"1 m"));
        match &v {
            Verb::Rank(inner, ranks) => {
                assert_eq!(*ranks, [1, 1, 1]);
                assert!(matches!(**inner, Verb::Reduce(_)), "got {inner:?}");
            }
            other => panic!("expected a ranked verb, got {other:?}"),
        }
    }

    #[rstest]
    #[case("+\"1 m", [1, 1, 1])]
    #[case("+\"1 2 m", [2, 1, 2])]
    #[case("+\"0 1 2 m", [0, 1, 2])]
    #[case("+\"_ m", [RANK_INF, RANK_INF, RANK_INF])]
    #[case("+\"_1 m", [-1, -1, -1])]
    #[case("+\"2.0 m", [2, 2, 2])]
    fn rank_specifications(#[case] src: &str, #[case] want: [i64; 3]) {
        let (v, _) = monad_of(&one(src));
        assert_eq!(v.ranks(), want);
    }

    #[test]
    fn rank_must_be_one_to_three_integer_atoms() {
        let e = err("+\"1 2 3 4 m");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert!(e.msg.contains("1 to 3 atoms"), "{}", e.msg);
        let e = err("+\"1.5 m");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert!(e.msg.contains("integer"), "{}", e.msg);
        let e = err("+\"'a' m");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert!(e.msg.contains("numeric"), "{}", e.msg);
    }

    #[test]
    fn verb_rank_is_not_supported_yet() {
        let e = err("+\"- m");
        assert_eq!(e.kind, ErrorKind::NotYet);
        assert!(e.msg.contains("verb rank"), "{}", e.msg);
    }

    #[test]
    fn computed_rank_is_not_supported_yet() {
        let e = err("+\"{r} m");
        assert_eq!(e.kind, ErrorKind::NotYet);
        assert!(e.msg.contains("computed rank"), "{}", e.msg);
    }

    #[test]
    fn atop_conjunction() {
        let (v, _) = monad_of(&one("+/ @: , y"));
        match &v {
            Verb::Atop(f, g) => {
                assert!(matches!(**f, Verb::Reduce(_)), "got {f:?}");
                assert_eq!(prim_of(g).name, ",");
            }
            other => panic!("expected an atop, got {other:?}"),
        }
    }

    #[rstest]
    #[case("+ ^: {n} y", "computed power")]
    #[case("(+/ % #) ^: _1 y", "the obverse of")]
    // `&.,` needs no obverse — the shape is put back instead — so the
    // verb whose obverse is missing has to be the one on the RIGHT.
    #[case("+: &. (+/ % #) y", "the obverse of")]
    #[case("(1 + 2) & , y", "bonds over a non-literal noun")]
        fn other_conjunctions_are_not_supported_yet(#[case] src: &str, #[case] msg: &str) {
        let e = err(src);
        assert_eq!(e.kind, ErrorKind::NotYet);
        assert!(e.msg.contains(msg), "{}", e.msg);
    }

    #[test]
    fn atop_at_rank_and_compose() {
        // `u@v` is `u@:v` at v's ranks; `u&v` is the composition at v's
        // monadic rank; `u&:v` is that composition on the arguments whole.
        let (v, _) = monad_of(&one("+/ @ (,\"1) y"));
        match &v {
            Verb::Rank(inner, ranks) => {
                assert_eq!(*ranks, [1, 1, 1]);
                assert!(matches!(**inner, Verb::Atop(..)), "got {inner:?}");
            }
            other => panic!("expected a ranked atop, got {other:?}"),
        }
        let (v, _) = monad_of(&one("+ & (*:\"0) y"));
        match &v {
            Verb::Rank(inner, ranks) => {
                assert_eq!(*ranks, [0, 0, 0]);
                assert!(matches!(**inner, Verb::Compose(..)), "got {inner:?}");
            }
            other => panic!("expected a ranked composition, got {other:?}"),
        }
        let (v, _) = monad_of(&one("+ &: *: y"));
        assert!(matches!(v, Verb::Compose(..)), "got {v:?}");
    }

    #[test]
    fn a_noun_operand_bonds_the_conjunction() {
        // `m&v y` is `m v y` whole. The bond's own rank is infinite — J's
        // `1 2&+ b. 0` reports `_ _ _` — and the verb inside it applies its
        // own ranks to the pair.
        let (v, _) = monad_of(&one("1 & + y"));
        match &v {
            Verb::BondLeft(a, g) => {
                assert_eq!(a.as_i64_slice(), Some(&[1i64][..]));
                assert_eq!(prim_of(g).name, "+");
            }
            other => panic!("expected a left bond, got {other:?}"),
        }
        assert_eq!(v.ranks(), [crate::verb::RANK_INF; 3]);
        let (v, _) = monad_of(&one("{. & 2 y"));
        assert!(matches!(v, Verb::BondRight(..)), "got {v:?}");
        assert_eq!(v.ranks(), [crate::verb::RANK_INF; 3]);
    }

    #[test]
    fn window_scan_and_commute_adverbs() {
        let (v, _) = monad_of(&one("+/\\ 1 2 3"));
        match &v {
            Verb::Windowed(u, WindowKind::Prefix) => assert!(matches!(**u, Verb::Reduce(_))),
            other => panic!("expected a prefix application, got {other:?}"),
        }
        // The window size is the left argument, so the derived verb has both
        // valences and its left cell is an atom.
        assert_eq!(v.ranks(), [RANK_INF, 0, RANK_INF]);
        let (v, _, _) = dyad_of(&one("2 +/\\ 1 2 3"));
        assert!(matches!(v, Verb::Windowed(_, WindowKind::Prefix)));
        let (v, _) = monad_of(&one("+/\\. 1 2 3"));
        assert!(matches!(v, Verb::Windowed(_, WindowKind::Suffix)));
        let (v, _) = monad_of(&one("+~ 1 2 3"));
        match &v {
            Verb::Commute(u) => assert_eq!(prim_of(u).name, "+"),
            other => panic!("expected a commute, got {other:?}"),
        }
        let (v, _) = monad_of(&one("+:^:3 (1)"));
        assert!(matches!(v, Verb::PowerN(_, Power::Times(3))));
        let (v, _) = monad_of(&one("%:^:_ (100)"));
        assert!(matches!(v, Verb::PowerN(_, Power::Converge)));
    }

    #[test]
    fn the_key_adverb_derives_a_verb() {
        match one("+/. 1 2 3") {
            Expr::Monad { verb: Verb::Key(_), .. } => {}
            other => panic!("expected a key, got {other:?}"),
        }
    }

    #[test]
    fn noun_operand_adverbs_are_not_supported_yet() {
        let e = err("1/ 2");
        assert_eq!(e.kind, ErrorKind::NotYet);
        assert!(e.msg.contains("noun-operand adverbs"), "{}", e.msg);
    }

    #[test]
    fn noun_operand_conjunctions_are_not_supported_yet() {
        let e = err("1 @: + y");
        assert_eq!(e.kind, ErrorKind::NotYet);
        assert!(e.msg.contains("noun-operand conjunctions"), "{}", e.msg);
    }

    // --------------------------------------------------------------- trains

    #[test]
    fn three_verbs_in_parentheses_are_a_fork() {
        let (v, y) = monad_of(&one("(+/ % #) 1 2 3"));
        match &v {
            Verb::Fork(f, g, h) => {
                assert!(matches!(**f, Verb::Reduce(_)), "got {f:?}");
                assert_eq!(prim_of(g).name, "%");
                assert_eq!(prim_of(h).name, "#");
            }
            other => panic!("expected a fork, got {other:?}"),
        }
        assert_eq!(konst(&y).shape, vec![3]);
    }

    #[test]
    fn a_noun_left_tine_is_a_noun_fork() {
        let (v, _) = monad_of(&one("(2 + #) 1 2 3"));
        match &v {
            Verb::NounFork(a, g, h) => {
                assert_eq!(a.as_i64_slice(), Some(&[2i64][..]));
                assert_eq!(prim_of(g).name, "+");
                assert_eq!(prim_of(h).name, "#");
            }
            other => panic!("expected a noun fork, got {other:?}"),
        }
    }

    #[test]
    fn two_verbs_in_parentheses_are_a_hook() {
        let (v, _) = monad_of(&one("(+ #) 1 2 3"));
        match &v {
            Verb::Hook(f, g) => {
                assert_eq!(prim_of(f).name, "+");
                assert_eq!(prim_of(g).name, "#");
            }
            other => panic!("expected a hook, got {other:?}"),
        }
    }

    #[test]
    fn cap_makes_a_fork_an_atop() {
        let (v, _) = monad_of(&one("([: +/ ,) 1 2 3"));
        match &v {
            Verb::Atop(f, g) => {
                assert!(matches!(**f, Verb::Reduce(_)), "got {f:?}");
                assert_eq!(prim_of(g).name, ",");
            }
            other => panic!("expected an atop, got {other:?}"),
        }
    }

    #[test]
    fn a_five_verb_train_folds_from_the_right() {
        // (a b c d e) is a fork whose right tine is the fork (c d e).
        let (v, _) = monad_of(&one("(] , [ , ]) 1 2 3"));
        match &v {
            Verb::Fork(f, g, h) => {
                assert_eq!(prim_of(f).name, "]");
                assert_eq!(prim_of(g).name, ",");
                assert!(matches!(**h, Verb::Fork(..)), "got {h:?}");
            }
            other => panic!("expected a fork, got {other:?}"),
        }
    }

    #[test]
    fn a_noun_fork_needs_a_literal_noun() {
        let e = err("({n} + #) 1 2 3");
        assert_eq!(e.kind, ErrorKind::NotYet);
        assert!(e.msg.contains("noun forks"), "{}", e.msg);
    }

    #[test]
    fn cap_is_never_applied_as_a_verb() {
        // `[:` has no meaning of its own; it only caps a fork. Here it is
        // left over beside the result of `# 1 2 3`.
        let e = err("[: # 1 2 3");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert!(e.msg.contains("caps a fork"), "{}", e.msg);
    }

    #[test]
    fn two_nouns_side_by_side_are_a_syntax_error() {
        // The reference reads no train here, and neither does libjay.
        let e = err("'ab' 'cd'");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert_eq!(e.msg, "syntax error");
    }

    #[test]
    fn a_sentence_that_is_a_verb_is_not_supported_yet() {
        let e = err("+/ % #");
        assert_eq!(e.kind, ErrorKind::NotYet);
        assert!(e.msg.contains("tacit"), "{}", e.msg);
    }

    // ----------------------------------------------------------- assignment

    #[rstest]
    #[case("x =. 5", Scope::Local)]
    #[case("x =: 5", Scope::Global)]
    fn assignment_yields_an_assign_node(#[case] src: &str, #[case] want: Scope) {
        match one(src) {
            Expr::Assign { name, value, scope, span } => {
                assert_eq!(name, "x");
                assert_eq!(ints(&value), vec![5]);
                assert_eq!(scope, want);
                assert_eq!(span, Span::new(0, 6));
            }
            other => panic!("expected an assignment, got {other:?}"),
        }
    }

    #[test]
    fn assignment_in_expression_position() {
        let (v, x, y) = dyad_of(&one("y + x =. 3"));
        assert_eq!(prim_of(&v).name, "+");
        assert!(matches!(x, Expr::Name(..)));
        match y {
            Expr::Assign { name, span, .. } => {
                assert_eq!(name, "x");
                assert_eq!(span, Span::new(4, 10));
            }
            other => panic!("expected an assignment, got {other:?}"),
        }
    }

    #[test]
    fn assignment_takes_the_whole_right_hand_sentence() {
        match one("x =. 1 + 2") {
            Expr::Assign { value, .. } => {
                let (v, _, _) = dyad_of(&value);
                assert_eq!(prim_of(&v).name, "+");
            }
            other => panic!("expected an assignment, got {other:?}"),
        }
    }

    // ---------------------------------------------------- naming a verb

    #[test]
    fn assigning_a_verb_names_it_and_runs_nothing() {
        let s = stmts("mean =. +/ % #");
        assert_eq!(s.len(), 1);
        match &s[0] {
            Expr::VerbDef { name, verb, span } => {
                assert_eq!(name, "mean");
                assert!(matches!(verb, Verb::Fork(..)), "got {verb:?}");
                assert_eq!(*span, Span::new(0, 14));
            }
            other => panic!("expected a verb definition, got {other:?}"),
        }
    }

    #[test]
    fn a_named_verb_applies_in_a_later_sentence() {
        let s = stmts("mean =. +/ % #\nmean 1 2 3 4");
        assert_eq!(s.len(), 2);
        let (v, y) = monad_of(&s[1]);
        assert!(matches!(v, Verb::Fork(..)), "got {v:?}");
        assert_eq!(konst(&y).shape, vec![4]);
    }

    #[test]
    fn a_named_verb_is_a_verb_inside_a_train_and_under_a_conjunction() {
        let (v, _) = monad_of(&stmts("mean =. +/ % #\n(mean - {.) 1 2 3 4").pop().expect("two"));
        match &v {
            Verb::Fork(f, g, h) => {
                assert!(matches!(**f, Verb::Fork(..)), "got {f:?}");
                assert_eq!(prim_of(g).name, "-");
                assert_eq!(prim_of(h).name, "{.");
            }
            other => panic!("expected a fork, got {other:?}"),
        }
        let (v, _) = monad_of(&stmts("mean =. +/ % #\nmean\"1 m").pop().expect("two"));
        match &v {
            Verb::Rank(inner, r) => {
                assert_eq!(*r, [1, 1, 1]);
                assert!(matches!(**inner, Verb::Fork(..)), "got {inner:?}");
            }
            other => panic!("expected a ranked verb, got {other:?}"),
        }
    }

    #[test]
    fn redefinition_rebinds_from_that_sentence_on() {
        let s = stmts("f =. +/\nf 1 2 3\nf =. #\nf 1 2 3");
        assert_eq!(s.len(), 4);
        assert!(matches!(monad_of(&s[1]).0, Verb::Reduce(_)));
        assert_eq!(prim_of(&monad_of(&s[3]).0).name, "#");
    }

    #[test]
    fn a_name_may_change_part_of_speech_in_either_direction() {
        // The oracle accepts both; the last assignment decides.
        let s = stmts("a =. 1 2 3\na =. +/\na 1 2 3");
        assert!(matches!(s[0], Expr::Assign { .. }));
        assert!(matches!(s[1], Expr::VerbDef { .. }));
        assert!(matches!(monad_of(&s[2]).0, Verb::Reduce(_)));
        let s = stmts("f =. +/\nf =. 10 20\nf");
        assert!(matches!(s[0], Expr::VerbDef { .. }));
        assert!(matches!(s[1], Expr::Assign { .. }));
        assert!(matches!(s[2], Expr::Name(..)));
    }

    #[test]
    fn an_undefined_name_applied_as_a_verb_is_a_value_error() {
        // The reference says `value error: zz`, pointing at the name.
        let e = err("zz 1 2 3");
        assert_eq!(e.kind, ErrorKind::Value);
        assert_eq!(e.msg, "undefined name: zz");
        assert_eq!(e.span, Some(Span::new(0, 2)));
        // A name that does hold a value is a different complaint: two
        // nouns side by side, which the reference calls a syntax error.
        let e = err("a =. 5\na 1 2 3");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert_eq!(e.msg, "syntax error");
    }

    #[test]
    fn assignment_names_an_adverb_or_a_conjunction() {
        match one("insert =. /") {
            Expr::ModDef { name, spelling, conjunction, .. } => {
                assert_eq!(name, "insert");
                assert_eq!(spelling, "/");
                assert!(!conjunction);
            }
            other => panic!("expected a modifier definition, got {other:?}"),
        }
        match one("atop =. @") {
            Expr::ModDef { spelling, conjunction, .. } => {
                assert_eq!(spelling, "@");
                assert!(conjunction);
            }
            other => panic!("expected a modifier definition, got {other:?}"),
        }
        // The name is a modifier from there on, so the sentence that uses
        // it parses around it as the glyph would.
        let s = stmts("insert =. /\n+ insert 1 2 3");
        assert!(matches!(s[1], Expr::Monad { verb: Verb::Reduce(_), .. }), "{:?}", s[1]);
    }

    #[test]
    fn a_sentence_that_is_a_modifier_is_a_named_gap() {
        let e = err("insert =. /\ninsert");
        assert_eq!(e.kind, ErrorKind::NotYet);
        assert!(e.msg.contains("displaying a modifier"), "{}", e.msg);
    }

    #[rstest]
    #[case("f =. 3 : 'y + 1'", None)]
    #[case("f =. 4 : 'x + y'", Some("x"))]
    #[case("f =. {{ y + 1 }}", None)]
    #[case("f =. {{ x + y }}", Some("x"))]
    fn an_explicit_definition_names_a_verb(#[case] src: &str, #[case] left: Option<&str>) {
        match one(src) {
            Expr::VerbDef { name, verb: Verb::Explicit(d), .. } => {
                assert_eq!(name, "f");
                assert_eq!(d.left.as_deref(), left);
                assert_eq!(d.right, "y");
                assert_eq!(d.body.len(), 1);
            }
            other => panic!("expected an explicit verb definition, got {other:?}"),
        }
    }

    #[rstest]
    #[case("f =. 13 : 'y + 1'", "tacit definitions")]
    fn definition_forms_libjay_has_not_are_named(#[case] src: &str, #[case] msg: &str) {
        let e = err(src);
        assert_eq!(e.kind, ErrorKind::NotYet);
        assert!(e.msg.contains(msg), "{}", e.msg);
    }

    /// `1 :` and `2 :` say the part of speech; a `{{ }}` leaves it to the
    /// operand names its body uses.
    #[rstest]
    #[case("f =. 1 : 'y + 1'", Some(false))]
    #[case("f =. 2 : 'u v y'", Some(true))]
    #[case("f =. {{ y + 1 }}", None)]
    #[case("f =. {{ u y }}", Some(false))]
    #[case("f =. {{ m + y }}", Some(false))]
    #[case("f =. {{ u v y }}", Some(true))]
    #[case("f =. {{ v y }}", Some(true))]
    #[case("f =. {{ n + y }}", Some(true))]
    #[case("f =. {{ u n y }}", Some(true))]
    #[case("f =. {{)a\nu y\n}}", Some(false))]
    #[case("f =. {{)c\nu v y\n}}", Some(true))]
    #[case("f =. {{)v\ny\n}}", None)]
    fn an_explicit_definitions_part_of_speech(#[case] src: &str, #[case] want: Option<bool>) {
        match (one(src), want) {
            (Expr::ModDef { name, conjunction, .. }, Some(conj)) => {
                assert_eq!(name, "f");
                assert_eq!(conjunction, conj, "{src:?}");
            }
            (Expr::VerbDef { name, .. }, None) => assert_eq!(name, "f"),
            (other, _) => panic!("expected {want:?} for {src:?}, got {other:?}"),
        }
    }

    #[test]
    fn a_control_word_outside_a_definition_is_a_parse_error() {
        let e = err("if. 1 do. 2 end.");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert!(e.msg.contains("only meaningful inside an explicit definition"), "{}", e.msg);
    }

    #[test]
    fn multiple_assignment_is_not_supported_yet() {
        let e = err("'a b' =. 1 2");
        assert_eq!(e.kind, ErrorKind::NotYet);
        assert!(e.msg.contains("multiple assignment"), "{}", e.msg);
    }

    // -------------------------------------------------------- interpolation

    #[test]
    fn a_hole_is_a_noun() {
        let e = one("{a} + 1");
        let (_, x, y) = dyad_of(&e);
        match x {
            Expr::Param(i, s) => {
                assert_eq!(i, 0);
                assert_eq!(s, Span::new(0, 3));
            }
            other => panic!("expected a parameter, got {other:?}"),
        }
        assert_eq!(ints(&y), vec![1]);
        assert_eq!(e.span(), Span::new(0, 7));
    }

    #[test]
    fn holes_are_numbered_and_shared_by_name() {
        let sp = SourceParts::from_source("{a} + {b} + {a}").expect("source parts");
        assert_eq!(sp.param_names, vec!["a".to_string(), "b".to_string()]);
        let e = parse(&sp).expect("parse").pop().expect("one sentence");
        let (_, x, y) = dyad_of(&e);
        assert!(matches!(x, Expr::Param(0, _)));
        let (_, x2, y2) = dyad_of(&y);
        assert!(matches!(x2, Expr::Param(1, _)));
        assert!(matches!(y2, Expr::Param(0, _)));
    }

    #[rstest]
    #[case("3j4", 3.0, 4.0)]
    #[case("_1j_2", -1.0, -2.0)]
    #[case("1e1j2", 10.0, 2.0)]
    #[case("2ad90", 0.0, 2.0)]
    #[case("1ad180", -1.0, 0.0)]
    fn complex_literals(#[case] src: &str, #[case] re: f64, #[case] im: f64) {
        let a = konst(&one(src));
        assert_eq!(a.dtype(), DType::Complex);
        let z = a.as_complex_slice().expect("complex data")[0];
        assert!((z[0] - re).abs() < 1e-12 && (z[1] - im).abs() < 1e-12, "{z:?}");
    }

    #[test]
    fn a_hole_takes_a_verb_like_any_noun() {
        let (v, y) = monad_of(&one("+/ {data}"));
        assert!(matches!(v, Verb::Reduce(_)));
        assert!(matches!(y, Expr::Param(0, _)));
    }

    #[test]
    fn braces_inside_a_string_are_not_holes() {
        let sp = SourceParts::from_source("'{a}'").expect("source parts");
        assert!(sp.param_names.is_empty());
        let a = konst(&parse(&sp).expect("parse")[0]);
        assert_eq!(a.data, Data::Char(vec!['{', 'a', '}'].into()));
    }

    #[test]
    fn parts_of_one_sentence_lex_across_a_hole() {
        // The t-string path: literal parts with a hole between them.
        let sp = SourceParts::from_parts(&["1 + ", " * 2"], &["v"]);
        assert_eq!(sp.display, "1 + {v} * 2");
        let e = parse(&sp).expect("parse").pop().expect("one sentence");
        let (_, x, y) = dyad_of(&e);
        assert_eq!(ints(&x), vec![1]);
        let (_, x2, y2) = dyad_of(&y);
        assert!(matches!(x2, Expr::Param(0, _)));
        assert_eq!(ints(&y2), vec![2]);
    }

    #[test]
    fn spans_of_later_sentences_index_the_whole_source() {
        let src = "5\n1 + 2";
        let s = stmts(src);
        assert_eq!(s[1].span(), Span::new(2, 7));
        assert_eq!(&src[2..7], "1 + 2");
    }

    // --------------------------------------------------------------- errors

    #[test]
    fn unknown_word_reports_its_span() {
        let e = err("1 [. 2");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert_eq!(e.msg, "unknown word: [.");
        assert_eq!(e.span, Some(Span::new(2, 4)));
    }

    #[test]
    fn an_inflected_unknown_word_is_reported_whole() {
        let e = err("1 ]: 2");
        assert_eq!(e.msg, "unknown word: ]:");
        assert_eq!(e.span, Some(Span::new(2, 4)));
    }

    /// The exact suffixes read; the forms that spell no number do not.
    #[rstest]
    #[case("1.5x", 0, 4)]
    #[case("1e10x", 0, 5)]
    fn a_fractional_extended_literal_is_ill_formed(
        #[case] src: &str,
        #[case] start: usize,
        #[case] end: usize,
    ) {
        let e = err(src);
        assert_eq!(e.kind, ErrorKind::Parse);
        assert!(e.msg.contains("invalid number"), "{}", e.msg);
        assert_eq!(e.span, Some(Span::new(start, end)));
    }

    #[test]
    fn a_malformed_number_is_a_parse_error() {
        let e = err("1.2.3");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert!(e.msg.contains("invalid number"), "{}", e.msg);
    }

    #[test]
    fn an_unbalanced_sentence_is_a_syntax_error() {
        // The parenthesis itself is what is wrong, so that is what the
        // span covers.
        let e = err("(1 + 2");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert!(e.msg.contains("no closing"), "{}", e.msg);
        assert_eq!(e.span, Some(Span::new(0, 1)));
    }

    #[test]
    fn a_stray_right_parenthesis_is_a_syntax_error() {
        let e = err("1 + 2)");
        assert_eq!(e.kind, ErrorKind::Parse);
        assert!(e.msg.contains("no opening"), "{}", e.msg);
        assert_eq!(e.span, Some(Span::new(5, 6)));
    }

    #[test]
    fn the_error_of_a_later_sentence_points_at_that_sentence() {
        let e = err("1 + 2\n3 [. 4");
        assert_eq!(e.span, Some(Span::new(8, 10)));
    }
}