bynk_syntax/

parser.rs

//! Hand-written recursive-descent parser for Bynk v0.
//!
//! Token grammar in spec §4. The expression parser uses one function per
//! precedence level (§4.4). Errors carry spans and short fix-oriented
//! messages; the parser does not currently attempt synchronisation, which
//! means at most one parse error is reported per compilation.

use crate::ast::*;
use crate::error::CompileError;
use crate::lexer::{Token, TokenKind, comment_body, doc_block_content, has_blank_line_between};
use crate::span::Span;
mod declarations;
mod expressions;
mod statements;
mod types;

/// Side-channel store for line-comment trivia (v1.1 LSP spec §3.5).
///
/// Built once up-front by [`split_trivia`] from the raw lexer token stream.
/// Comments are removed from the token stream the parser walks; their text
/// is filed into `leading` (comments on lines preceding a content token)
/// and `trailing` (a single comment on the same line as a content token).
/// The parser consumes entries through [`TriviaTable::take_leading`] and
/// [`TriviaTable::take_trailing`] as it recognises declarations.
#[derive(Debug, Default)]
struct TriviaTable {
    /// `leading[i]` holds the comment-body texts that appear immediately
    /// before content token `i` (zero or more `--` lines, in source order,
    /// not separated from the token by another content token).
    leading: Vec<Vec<String>>,
    /// `trailing[i]` holds an optional comment on the same source line as
    /// content token `i`. Only one trailing comment is recorded per token
    /// because a single `--` consumes the rest of the line.
    trailing: Vec<Option<String>>,
    /// Any pending leading comments at end-of-file (no content token
    /// followed). Used to preserve file-trailing comments.
    epilogue: Vec<String>,
}

impl TriviaTable {
    fn take_leading(&mut self, index: usize) -> Vec<String> {
        match self.leading.get_mut(index) {
            Some(v) => std::mem::take(v),
            None => Vec::new(),
        }
    }

    fn take_trailing(&mut self, index: usize) -> Option<String> {
        self.trailing.get_mut(index).and_then(|s| s.take())
    }

    fn take_epilogue(&mut self) -> Vec<String> {
        std::mem::take(&mut self.epilogue)
    }
}

/// Remove `Comment` trivia tokens from `tokens` and bin them into a
/// [`TriviaTable`] keyed against the surviving content tokens. A comment
/// on the same source line as the preceding content token is recorded as
/// that token's *trailing* trivia; everything else is *leading* for the
/// next content token.
fn split_trivia(tokens: &[Token], source: &str) -> (Vec<Token>, TriviaTable) {
    let mut filtered: Vec<Token> = Vec::with_capacity(tokens.len());
    let mut table = TriviaTable::default();
    let mut pending_leading: Vec<String> = Vec::new();
    let mut last_content_end: Option<usize> = None;
    for tok in tokens {
        if tok.kind == TokenKind::Comment {
            let body = comment_body(source, tok.span).to_string();
            // If nothing has been buffered as leading for the next token and
            // there is no newline between the previous content token and
            // this comment, it trails that token.
            if pending_leading.is_empty()
                && let Some(prev_end) = last_content_end
                && !source[prev_end..tok.span.start].contains('\n')
            {
                let last_idx = filtered.len() - 1;
                // Only attach if no trailing already recorded (shouldn't
                // happen because `--` consumes through end-of-line).
                if table.trailing[last_idx].is_none() {
                    table.trailing[last_idx] = Some(body);
                    continue;
                }
            }
            pending_leading.push(body);
            continue;
        }
        filtered.push(*tok);
        table.leading.push(std::mem::take(&mut pending_leading));
        table.trailing.push(None);
        last_content_end = Some(tok.span.end);
    }
    table.epilogue = pending_leading;
    (filtered, table)
}

/// Parse a token slice into a [`Commons`] AST.
///
/// Accepts either form of v0.3 commons file:
/// - Brace form: `commons name { items... }` (v0–v0.2 compatible).
/// - Fragment form: `commons name uses... items...` to EOF (v0.3).
pub fn parse(tokens: &[Token], source: &str) -> Result<Commons, Vec<CompileError>> {
    match parse_unit(tokens, source)? {
        SourceUnit::Commons(c) => Ok(c),
        SourceUnit::Context(ctx) => Err(vec![
            CompileError::new(
                "bynk.parse.unexpected_context",
                ctx.span,
                "expected a `commons` declaration but found a `context` declaration",
            )
            .with_note(
                "contexts must be compiled as part of a project — pass the source directory, e.g. `bynkc compile --target bundle --output out src`",
            ),
        ]),
        SourceUnit::Suite(t) => Err(vec![
            CompileError::new(
                "bynk.parse.unexpected_suite",
                t.span,
                "expected a `commons` declaration but found a `suite` declaration",
            )
            .with_note(
                "tests must be compiled as part of a project — pass the source directory, e.g. `bynkc compile --target bundle --output out src`",
            ),
        ]),
        SourceUnit::Adapter(a) => Err(vec![
            CompileError::new(
                "bynk.parse.unexpected_adapter",
                a.span,
                "expected a `commons` declaration but found an `adapter` declaration",
            )
            .with_note(
                "adapters must be compiled as part of a project — pass the source directory, e.g. `bynkc compile --target bundle --output out src`",
            ),
        ]),
    }
}

/// Parse a token slice into a [`SourceUnit`] with error recovery, returning a
/// best-effort partial AST plus the full list of parse errors and warnings.
///
/// Used by the LSP: item-level recovery skips past a malformed declaration to
/// the next top-level item, so multiple errors are reported per compilation
/// rather than just the first. Compared to [`parse_unit`], this never bails;
/// if no SourceUnit could be parsed at all (e.g. the file is empty or the
/// header itself fails) the returned `Option` is `None`.
pub fn parse_unit_with_recovery(
    tokens: &[Token],
    source: &str,
) -> (Option<SourceUnit>, Vec<CompileError>) {
    let (filtered, trivia) = split_trivia(tokens, source);
    let mut warnings = Vec::new();
    let mut p = Parser::new(&filtered, source, trivia, &mut warnings);
    p.recover_mode = true;
    let unit_opt = match p.parse_unit() {
        Ok(u) => {
            // v0.113: a file may hold more than one top-level unit (an atomic
            // `commons` + `suite` file, DECISION S). Consume any further units
            // so trailing declarations are not mis-reported as stray tokens; the
            // editor view is keyed on the first (primary) unit. A genuinely
            // malformed trailing declaration is still surfaced via recovery.
            while p.peek().is_some() {
                match p.parse_unit() {
                    Ok(_) => {}
                    Err(e) => {
                        p.recovered_errors.push(e);
                        break;
                    }
                }
            }
            Some(u)
        }
        Err(e) => {
            p.recovered_errors.push(e);
            None
        }
    };
    let mut all_errors = p.recovered_errors;
    all_errors.append(&mut warnings);
    (unit_opt, all_errors)
}

/// Parse a token slice into a [`SourceUnit`] — either a commons or a context.
///
/// Each `.bynk` file is exactly one declaration of one kind.
pub fn parse_unit(tokens: &[Token], source: &str) -> Result<SourceUnit, Vec<CompileError>> {
    let (filtered, trivia) = split_trivia(tokens, source);
    let mut warnings = Vec::new();
    let mut p = Parser::new(&filtered, source, trivia, &mut warnings);
    let result = match p.parse_unit() {
        Ok(u) => {
            if let Some(extra) = p.peek() {
                Err(vec![
                    CompileError::new(
                        "bynk.parse.extra_tokens",
                        extra.span,
                        "unexpected token after top-level declaration",
                    )
                    .with_note(
                        "a `.bynk` file contains exactly one `commons` or `context` declaration",
                    ),
                ])
            } else {
                Ok(u)
            }
        }
        Err(e) => Err(vec![e]),
    };
    // Warnings (e.g. orphan doc blocks) are returned as errors in v0.3 — there
    // is no separate warning channel yet; the test harness matches on category.
    if !warnings.is_empty() {
        match result {
            Ok(_) => return Err(warnings),
            Err(mut errs) => {
                errs.append(&mut warnings);
                return Err(errs);
            }
        }
    }
    result
}

/// Parse a token slice into **all** the top-level [`SourceUnit`]s in one file
/// (v0.113, testing track slice 1b). A `.bynk` file may hold more than one
/// top-level declaration — an *atomic* file with `commons`/`context` **and** a
/// `suite` together (DECISION S) — so the compiler parses a `Vec`, not a single
/// unit. Test-ness is a property of each declaration, not of the file.
///
/// Bails on the first malformed declaration (like [`parse_unit`], not the
/// recovering LSP path). An empty file is an error.
pub fn parse_units(tokens: &[Token], source: &str) -> Result<Vec<SourceUnit>, Vec<CompileError>> {
    let (filtered, trivia) = split_trivia(tokens, source);
    let mut warnings = Vec::new();
    let mut p = Parser::new(&filtered, source, trivia, &mut warnings);
    let mut units = Vec::new();
    let mut errors: Vec<CompileError> = Vec::new();
    while p.peek().is_some() {
        match p.parse_unit() {
            Ok(u) => units.push(u),
            Err(e) => {
                errors.push(e);
                break;
            }
        }
    }
    let eof = p.eof_span();
    // `p` (and thus its `&mut warnings` borrow) is no longer used past here, so
    // the local `warnings` are readable again.
    errors.append(&mut warnings);
    if !errors.is_empty() {
        return Err(errors);
    }
    if units.is_empty() {
        return Err(vec![CompileError::new(
            "bynk.parse.unexpected_eof",
            eof,
            "expected `commons`, `context`, or `suite` to start the file, found end of file",
        )]);
    }
    Ok(units)
}

/// A signed numeric literal in refinement-bound position (v0.21): `InRange`
/// bounds are either both `Int` or both `Float`.
enum SignedNumLit {
    Int(IntBound),
    Float(FloatBound),
}

struct Parser<'a> {
    tokens: &'a [Token],
    source: &'a str,
    pos: usize,
    /// Accumulated non-fatal diagnostics. v0.3 uses this for orphan-doc
    /// warnings, which are emitted as errors with a distinguishable category.
    warnings: &'a mut Vec<CompileError>,
    /// When true, the item-level loops catch errors from individual item
    /// parses, push them into `recovered_errors`, and skip forward to the
    /// next top-level item boundary instead of bailing. Used by the LSP via
    /// [`parse_unit_with_recovery`]; disabled in the normal `parse` path so
    /// existing single-error behaviour is preserved.
    recover_mode: bool,
    /// Errors collected during recovery-mode parsing. Only populated when
    /// `recover_mode` is true.
    recovered_errors: Vec<CompileError>,
    /// Line-comment trivia separated from the token stream. See
    /// [`TriviaTable`].
    trivia: TriviaTable,
}

impl<'a> Parser<'a> {
    fn new(
        tokens: &'a [Token],
        source: &'a str,
        trivia: TriviaTable,
        warnings: &'a mut Vec<CompileError>,
    ) -> Self {
        Self {
            tokens,
            source,
            pos: 0,
            warnings,
            recover_mode: false,
            recovered_errors: Vec::new(),
            trivia,
        }
    }

    /// Comments immediately preceding the current peek position. Consumed
    /// (the table entry is cleared) so the same comments are not attached
    /// to two nodes.
    fn take_leading_trivia(&mut self) -> Vec<String> {
        self.trivia.take_leading(self.pos)
    }

    /// Trailing comment, if any, on the same source line as the most
    /// recently consumed content token. Call AFTER finishing a declaration
    /// or statement, while `self.pos` points one past its last token.
    fn take_trailing_trivia(&mut self) -> Option<String> {
        if self.pos == 0 {
            return None;
        }
        self.trivia.take_trailing(self.pos - 1)
    }

    /// Handle a per-item parse error. In recovery mode, record the error and
    /// advance to the next sync point so the item loop can continue; otherwise
    /// propagate as a hard failure.
    fn handle_item_err(&mut self, e: CompileError) -> Result<(), CompileError> {
        if self.recover_mode {
            self.recovered_errors.push(e);
            self.recover_to_top_item();
            Ok(())
        } else {
            Err(e)
        }
    }

    /// Skip forward to the next top-level item boundary: either a top-level
    /// declaration keyword (`type`, `fn`, `uses`, `consumes`, `exports`,
    /// `capability`, `provides`, `service`, `agent`), a closing brace, or
    /// end-of-input. Used only in recovery mode.
    fn recover_to_top_item(&mut self) {
        while let Some(t) = self.peek() {
            match t.kind {
                TokenKind::Type
                | TokenKind::Fn
                | TokenKind::Uses
                | TokenKind::Consumes
                | TokenKind::Exports
                | TokenKind::Capability
                | TokenKind::Provides
                | TokenKind::Service
                | TokenKind::Agent
                | TokenKind::Suite
                | TokenKind::Case
                | TokenKind::RBrace
                | TokenKind::Commons
                | TokenKind::Context => return,
                _ => {
                    self.bump();
                }
            }
        }
    }

    fn peek(&self) -> Option<Token> {
        self.tokens.get(self.pos).copied()
    }

    fn peek_kind(&self) -> Option<TokenKind> {
        self.peek().map(|t| t.kind)
    }

    /// The token `n` positions ahead of the cursor (`nth(0)` == `peek()`).
    fn nth(&self, n: usize) -> Option<Token> {
        self.tokens.get(self.pos + n).copied()
    }

    fn nth_kind(&self, n: usize) -> Option<TokenKind> {
        self.nth(n).map(|t| t.kind)
    }

    /// The source text of the token `n` positions ahead, or `""` if none.
    fn nth_text(&self, n: usize) -> &'a str {
        self.nth(n).map(|t| self.slice(t.span)).unwrap_or("")
    }

    /// The span of the most recently consumed token (`self.pos - 1`). Falls back
    /// to the current token's span when nothing has been consumed yet.
    fn prev_span(&self) -> Span {
        self.tokens
            .get(self.pos.wrapping_sub(1))
            .or_else(|| self.peek_ref())
            .map(|t| t.span)
            .unwrap_or_default()
    }

    fn peek_ref(&self) -> Option<&Token> {
        self.tokens.get(self.pos)
    }

    fn bump(&mut self) -> Option<Token> {
        let t = self.peek();
        if t.is_some() {
            self.pos += 1;
        }
        t
    }

    fn eat(&mut self, kind: TokenKind) -> Option<Token> {
        if self.peek_kind() == Some(kind) {
            self.bump()
        } else {
            None
        }
    }

    fn slice(&self, span: Span) -> &'a str {
        &self.source[span.range()]
    }

    /// True when the next token sits on a later line than `prev`. Used to
    /// keep a `[` that opens a new line out of the postfix type-application
    /// form: `f` followed by `[1, 2]` on the next line is an identifier and
    /// a list literal, not `f[…]` (v0.20b).
    fn next_token_on_new_line(&self, prev: Span) -> bool {
        match self.peek() {
            Some(t) if prev.end <= t.span.start => {
                self.source[prev.end..t.span.start].contains('\n')
            }
            _ => false,
        }
    }

    /// Span pointing at the end of input — used for "unexpected EOF" reports.
    fn eof_span(&self) -> Span {
        let end = self.source.len();
        Span::new(end.saturating_sub(1), end)
    }

    fn expect(&mut self, kind: TokenKind, ctx: &str) -> Result<Token, CompileError> {
        match self.peek() {
            Some(t) if t.kind == kind => {
                self.bump();
                Ok(t)
            }
            Some(t) => Err(CompileError::new(
                "bynk.parse.expected_token",
                t.span,
                format!(
                    "expected {} {ctx}, found {}",
                    kind.describe(),
                    t.kind.describe()
                ),
            )),
            None => Err(CompileError::new(
                "bynk.parse.unexpected_eof",
                self.eof_span(),
                format!("expected {} {ctx}, found end of file", kind.describe()),
            )),
        }
    }

    fn expect_ident(&mut self, ctx: &str) -> Result<Ident, CompileError> {
        match self.peek() {
            Some(t) if t.kind == TokenKind::Ident => {
                self.bump();
                Ok(Ident {
                    name: self.slice(t.span).to_string(),
                    span: t.span,
                })
            }
            // v0.5 contextual keyword `on` doubles as an identifier in
            // expression / field-access positions so users can name fields and
            // parameters using it. It retains its keyword meaning only at
            // handler-decl-level (`on call(...)`).
            //
            // v0.7 / v0.112: `suite` and `case` are contextual too — they
            // introduce the suite declaration and its cases, but are perfectly
            // valid commons/context/field names otherwise.
            Some(t) if matches!(t.kind, TokenKind::On | TokenKind::Suite | TokenKind::Case) => {
                self.bump();
                Ok(Ident {
                    name: self.slice(t.span).to_string(),
                    span: t.span,
                })
            }
            Some(t) if is_reserved_keyword(t.kind) => Err(CompileError::new(
                "bynk.parse.reserved_keyword",
                t.span,
                format!(
                    "expected identifier {ctx}, but `{}` is a reserved keyword",
                    self.slice(t.span)
                ),
            )
            .with_note("rename the identifier to something that is not a keyword")),
            Some(t) => Err(CompileError::new(
                "bynk.parse.expected_token",
                t.span,
                format!("expected identifier {ctx}, found {}", t.kind.describe()),
            )),
            None => Err(CompileError::new(
                "bynk.parse.unexpected_eof",
                self.eof_span(),
                format!("expected identifier {ctx}, found end of file"),
            )),
        }
    }

    // -- top level --

    /// Consume an optional doc block at the current position, returning the
    /// (content, end-of-doc span) pair. Returns None if the next token is not
    /// a doc block.
    fn take_doc_block(&mut self) -> Option<(String, Span)> {
        if self.peek_kind() == Some(TokenKind::DocBlock) {
            let t = self.bump().unwrap();
            let body = doc_block_content(self.source, t.span);
            return Some((body, t.span));
        }
        None
    }

    /// Collect all line-comment trivia leading the next declaration plus
    /// the optional doc block. Comments may appear both *before* and
    /// *between* the doc and the declaration; the spec canonicalises both
    /// groups above the doc, so we concatenate them.
    fn collect_item_lead(&mut self) -> (Vec<String>, Option<(String, Span)>) {
        let mut leading = self.take_leading_trivia();
        let doc = self.take_doc_block();
        if doc.is_some() {
            leading.extend(self.take_leading_trivia());
        }
        (leading, doc)
    }

    /// Attach a parsed doc block to a following declaration unless a blank
    /// line separates them, in which case the doc is orphaned (warning).
    fn finalize_doc(&mut self, doc: Option<(String, Span)>, next_span: Span) -> Option<String> {
        let (content, doc_span) = doc?;
        // A blank line between the doc and the next decl orphans the doc.
        if has_blank_line_between(self.source, doc_span.end, next_span.start) {
            self.warnings.push(
                CompileError::new(
                    "bynk.parse.orphan_doc_block",
                    doc_span,
                    "documentation block is separated from the following declaration by a blank line; it will not be attached",
                )
                .with_note(
                    "remove the blank line to attach the doc to the next declaration, \
                     or remove the doc block if it is not meant to document anything",
                ),
            );
            return None;
        }
        Some(content)
    }
}

/// Parse the body of a lexed double-quoted string literal (the lexeme,
/// including surrounding quotes), applying the v0 escape rules.
fn parse_string_literal(lexeme: &str, span: Span) -> Result<String, CompileError> {
    let bytes = lexeme.as_bytes();
    debug_assert!(bytes.first() == Some(&b'"') && bytes.last() == Some(&b'"'));
    let inner = &lexeme[1..lexeme.len() - 1];
    let mut out = String::with_capacity(inner.len());
    let mut chars = inner.chars();
    while let Some(c) = chars.next() {
        if c == '\\' {
            match chars.next() {
                Some('n') => out.push('\n'),
                Some('t') => out.push('\t'),
                Some('"') => out.push('"'),
                Some('\\') => out.push('\\'),
                other => {
                    return Err(CompileError::new(
                        "bynk.lex.bad_escape",
                        span,
                        format!(
                            "invalid escape sequence `\\{}` in string literal",
                            other.map(|c| c.to_string()).unwrap_or_default()
                        ),
                    )
                    .with_note("supported escapes: \\n \\t \\\" \\\\"));
                }
            }
        } else {
            out.push(c);
        }
    }
    Ok(out)
}

fn is_reserved_keyword(kind: TokenKind) -> bool {
    use TokenKind::*;
    matches!(
        kind,
        Commons
            | Type
            | Fn
            | Where
            | And
            | True
            | False
            | Int
            | String
            | Bool
            | Let
            | If
            | Else
            | Ok
            | Err
            | Result
            | ValidationError
            | Enum
            | Match
            | Option
            | Record
            | Self_
            | Some
            | None
            | Is
            | Opaque
            | Uses
            | Context
            | Consumes
            | Exports
            | Transparent
            | Agent
            | As
            | Capability
            | Effect
            | Given
            | On
            | Http
            | Provides
            | Service
            | Actor
            | By
            | Expect
            | Suite
            | Case
    )
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::lexer::tokenize;

    fn parse_str(src: &str) -> Result<Commons, Vec<CompileError>> {
        let toks = tokenize(src).map_err(|e| vec![e])?;
        parse(&toks, src)
    }

    fn parse_recover_str(src: &str) -> (Option<SourceUnit>, Vec<CompileError>) {
        let toks = match tokenize(src) {
            Ok(t) => t,
            Err(e) => return (None, vec![e]),
        };
        parse_unit_with_recovery(&toks, src)
    }

    #[test]
    fn recovery_skips_garbage_between_decls() {
        // Two `type` declarations separated by garbage. Recovery should
        // accept both and report one error for the garbage between them.
        let src = "commons x {\n\
                   type A = Int where NonNegative\n\
                   ??? !!!\n\
                   type B = String where NonEmpty\n\
                   }";
        let (unit, errors) = parse_recover_str(src);
        let unit = unit.expect("recovery should produce a partial AST");
        let SourceUnit::Commons(c) = unit else {
            panic!("expected commons")
        };
        // Both type decls should have been collected despite the garbage.
        let names: Vec<_> = c
            .items
            .iter()
            .map(|i| match i {
                CommonsItem::Type(t) => t.name.name.clone(),
                _ => panic!("expected only types"),
            })
            .collect();
        assert!(
            names.contains(&"A".to_string()) && names.contains(&"B".to_string()),
            "expected both A and B; got {names:?}",
        );
        assert!(!errors.is_empty(), "expected at least one parse error");
    }

    #[test]
    fn recovery_handles_bad_first_decl_then_good_second() {
        // First decl is malformed (missing `=`); second is well-formed.
        let src = "commons x {\n\
                   type A Int where NonNegative\n\
                   type B = String where NonEmpty\n\
                   }";
        let (unit, errors) = parse_recover_str(src);
        let unit = unit.expect("recovery should produce a partial AST");
        let SourceUnit::Commons(c) = unit else {
            panic!("expected commons")
        };
        let names: Vec<_> = c
            .items
            .iter()
            .filter_map(|i| match i {
                CommonsItem::Type(t) => Some(t.name.name.clone()),
                _ => None,
            })
            .collect();
        assert!(
            names.contains(&"B".to_string()),
            "B should be parsed after A's failure; got {names:?}"
        );
        assert!(!errors.is_empty(), "expected at least one parse error");
    }

    #[test]
    fn doc_block_attaches_to_type() {
        let c =
            parse_str("commons x {\n---\nA descriptive doc.\n---\ntype T = Int where Positive\n}")
                .unwrap();
        let CommonsItem::Type(t) = &c.items[0] else {
            panic!()
        };
        assert!(t.documentation.is_some());
        assert!(
            t.documentation
                .as_ref()
                .unwrap()
                .contains("A descriptive doc.")
        );
    }

    #[test]
    fn interpolated_string_parses_into_parts() {
        // v0.43: `"Hi, \(name)!"` splits into chunk / hole / chunk.
        let c = parse_str("commons x\n\nfn f(name: String) -> String {\n  \"Hi, \\(name)!\"\n}\n")
            .unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!("expected fn")
        };
        let ExprKind::InterpStr(parts) = &f.body.tail.kind else {
            panic!("expected InterpStr, got {:?}", f.body.tail.kind)
        };
        assert_eq!(parts.len(), 3);
        assert!(matches!(&parts[0], InterpPart::Chunk(s) if s == "Hi, "));
        assert!(
            matches!(&parts[1], InterpPart::Hole(h) if matches!(&h.kind, ExprKind::Ident(id) if id.name == "name"))
        );
        assert!(matches!(&parts[2], InterpPart::Chunk(s) if s == "!"));
    }

    #[test]
    fn interpolated_hole_parses_a_full_expression() {
        // A hole holds an arbitrary expression, not just an identifier.
        let c =
            parse_str("commons x\n\nfn f(a: Int, b: Int) -> String {\n  \"sum = \\(a + b)\"\n}\n")
                .unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!("expected fn")
        };
        let ExprKind::InterpStr(parts) = &f.body.tail.kind else {
            panic!("expected InterpStr")
        };
        assert!(matches!(&parts[1], InterpPart::Hole(h) if matches!(&h.kind, ExprKind::BinOp(..))));
    }

    #[test]
    fn empty_interpolation_hole_is_rejected() {
        let errs = parse_str("commons x\n\nfn f() -> String {\n  \"\\()\"\n}\n").unwrap_err();
        assert!(
            errs.iter()
                .any(|e| e.category == "bynk.parse.empty_interpolation"),
            "expected empty_interpolation; got {errs:?}"
        );
    }

    #[test]
    fn fragment_form_parses() {
        let c = parse_str("commons x.y\n\ntype T = Int where NonNegative\n").unwrap();
        assert_eq!(c.form, CommonsForm::Fragment);
        assert_eq!(c.items.len(), 1);
    }

    #[test]
    fn uses_parses() {
        let c = parse_str("commons x\n\nuses other.lib\n").unwrap();
        assert_eq!(c.uses.len(), 1);
        assert_eq!(c.uses[0].target.joined(), "other.lib");
    }

    fn parse_unit_str(src: &str) -> Result<SourceUnit, Vec<CompileError>> {
        let toks = tokenize(src).map_err(|e| vec![e])?;
        parse_unit(&toks, src)
    }

    #[test]
    fn minimal_context_parses() {
        let u = parse_unit_str("context commerce.orders {}").unwrap();
        let SourceUnit::Context(c) = u else {
            panic!("expected context");
        };
        assert_eq!(c.name.joined(), "commerce.orders");
        assert!(c.items.is_empty());
    }

    #[test]
    fn context_consumes_and_exports_parse() {
        let src = "context commerce.orders {\n  uses commerce.money\n  consumes commerce.payment\n  exports opaque { OrderId }\n  exports transparent { OrderError }\n  type OrderId = String where Matches(\"ORD-[0-9]+\")\n  type OrderError = enum { CartEmpty, BadInput }\n}";
        let u = parse_unit_str(src).unwrap();
        let SourceUnit::Context(c) = u else { panic!() };
        assert_eq!(c.uses.len(), 1);
        assert_eq!(c.consumes.len(), 1);
        assert_eq!(c.exports.len(), 2);
        assert_eq!(c.exports[0].kind, ExportKind::Type(Visibility::Opaque));
        assert_eq!(c.exports[1].kind, ExportKind::Type(Visibility::Transparent));
    }

    #[test]
    fn context_fragment_form_parses() {
        let src = "context x.y\n\nuses other.lib\nconsumes other.ctx\nexports opaque { T }\n\ntype T = Int where NonNegative\n";
        let u = parse_unit_str(src).unwrap();
        let SourceUnit::Context(c) = u else { panic!() };
        assert_eq!(c.form, CommonsForm::Fragment);
        assert_eq!(c.uses.len(), 1);
        assert_eq!(c.consumes.len(), 1);
        assert_eq!(c.exports.len(), 1);
    }

    #[test]
    fn opaque_type_parses() {
        let c = parse_str("commons x { type T = opaque Int where NonNegative }").unwrap();
        let CommonsItem::Type(t) = &c.items[0] else {
            panic!()
        };
        assert!(matches!(t.body, TypeBody::Opaque { .. }));
    }

    #[test]
    fn empty_commons() {
        let c = parse_str("commons fitness.units {}").unwrap();
        assert_eq!(c.name.joined(), "fitness.units");
        assert!(c.items.is_empty());
    }

    #[test]
    fn one_type_decl() {
        let c = parse_str("commons x { type Metres = Int where NonNegative }").unwrap();
        assert_eq!(c.items.len(), 1);
        let CommonsItem::Type(t) = &c.items[0] else {
            panic!()
        };
        assert_eq!(t.name.name, "Metres");
        match &t.body {
            TypeBody::Refined {
                base, refinement, ..
            } => {
                assert_eq!(*base, BaseType::Int);
                assert!(refinement.is_some());
            }
            _ => panic!("expected refined body"),
        }
    }

    #[test]
    fn function_decl() {
        let c = parse_str("commons x { fn add(a: Int, b: Int) -> Int { a + b } }").unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!()
        };
        assert_eq!(f.name.ident().name, "add");
        assert_eq!(f.params.len(), 2);
    }

    #[test]
    fn chained_comparison_is_error() {
        let errs = parse_str("commons x { fn f(a: Int, b: Int, c: Int) -> Bool { a < b < c } }")
            .unwrap_err();
        assert_eq!(errs[0].category, "bynk.parse.non_associative");
    }

    #[test]
    fn chained_equality_is_error() {
        let errs = parse_str("commons x { fn f(a: Int, b: Int, c: Int) -> Bool { a == b == c } }")
            .unwrap_err();
        assert_eq!(errs[0].category, "bynk.parse.non_associative");
    }

    #[test]
    fn let_statement_parses() {
        let c = parse_str("commons x { fn f(n: Int) -> Int { let y = n + 1\n y } }").unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!()
        };
        assert_eq!(f.body.statements.len(), 1);
        match &f.body.statements[0] {
            Statement::Let(l) => {
                assert_eq!(l.name.name, "y");
                assert!(l.type_annot.is_none());
            }
            _ => panic!("expected a pure `let` statement"),
        }
    }

    #[test]
    fn let_with_annotation() {
        let c = parse_str("commons x { fn f(n: Int) -> Int { let y: Int = n\n y } }").unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!()
        };
        match &f.body.statements[0] {
            Statement::Let(l) => assert!(l.type_annot.is_some()),
            _ => panic!("expected a pure `let` statement"),
        }
    }

    #[test]
    fn if_else_parses_as_expression() {
        let c = parse_str("commons x { fn f(b: Bool) -> Int { if b { 1 } else { 0 } } }").unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!()
        };
        assert!(matches!(f.body.tail.kind, ExprKind::If { .. }));
    }

    #[test]
    fn else_if_chain_parses() {
        let c = parse_str(
            "commons x { fn f(n: Int) -> Int { if n < 0 { -1 } else if n == 0 { 0 } else { 1 } } }",
        )
        .unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!()
        };
        let ExprKind::If { else_block, .. } = &f.body.tail.kind else {
            panic!()
        };
        // The else-branch is a block whose tail is another `If`.
        assert!(else_block.statements.is_empty());
        assert!(matches!(else_block.tail.kind, ExprKind::If { .. }));
    }

    #[test]
    fn ok_and_err_parse_as_expressions() {
        let c = parse_str("commons x { fn f(n: Int) -> Result[Int, String] { Ok(n) } }").unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!()
        };
        assert!(matches!(f.body.tail.kind, ExprKind::Ok(_)));

        let c =
            parse_str("commons x { fn f(n: Int) -> Result[Int, String] { Err(\"x\") } }").unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!()
        };
        assert!(matches!(f.body.tail.kind, ExprKind::Err(_)));
    }

    #[test]
    fn question_postfix_parses() {
        let c = parse_str(
            "commons x { type T = Int where Positive\n fn f(n: Int) -> Result[T, ValidationError] { let x = T.of(n)?\n Ok(x) } }",
        )
        .unwrap();
        let CommonsItem::Fn(f) = &c.items[1] else {
            panic!()
        };
        let Statement::Let(l) = &f.body.statements[0] else {
            panic!("expected a pure `let` statement");
        };
        assert!(matches!(l.value.kind, ExprKind::Question(_)));
    }

    #[test]
    fn constructor_call_parses() {
        let c = parse_str(
            "commons x { type T = Int where Positive\n fn f(n: Int) -> Result[T, ValidationError] { T.of(n) } }",
        )
        .unwrap();
        let CommonsItem::Fn(f) = &c.items[1] else {
            panic!()
        };
        // v0.2: T.of(n) parses as a MethodCall with receiver Ident("T"); the
        // checker reinterprets it as a static call by noticing T is a type.
        let ExprKind::MethodCall {
            receiver, method, ..
        } = &f.body.tail.kind
        else {
            panic!("expected MethodCall, got {:?}", f.body.tail.kind)
        };
        let ExprKind::Ident(id) = &receiver.kind else {
            panic!("expected receiver Ident");
        };
        assert_eq!(id.name, "T");
        assert_eq!(method.name, "of");
    }

    #[test]
    fn result_type_ref_parses() {
        let c = parse_str("commons x { fn f(n: Int) -> Result[Int, String] { Ok(n) } }").unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!()
        };
        assert!(matches!(f.return_type, TypeRef::Result(_, _, _)));
    }

    #[test]
    fn result_missing_arg_count_errors() {
        let errs = parse_str("commons x { fn f(n: Int) -> Result[Int] { Ok(n) } }").unwrap_err();
        assert_eq!(errs[0].category, "bynk.parse.generic_arg_count");
    }

    #[test]
    fn field_access_parses_in_v0_2() {
        // v0.2: field access is supported (the type checker validates the
        // field exists on the receiver's type). Parser-level acceptance:
        let c =
            parse_str("commons x { type R = { foo: Int }\n fn f(r: R) -> Int { r.foo } }").unwrap();
        let CommonsItem::Fn(f) = &c.items[1] else {
            panic!()
        };
        assert!(matches!(f.body.tail.kind, ExprKind::FieldAccess { .. }));
    }

    // -- v1.1 trivia attachment --

    #[test]
    fn leading_line_comment_attaches_to_next_decl() {
        let src = "commons x {\n-- explain the type\ntype T = Int where NonNegative\n}";
        let c = parse_str(src).unwrap();
        let CommonsItem::Type(t) = &c.items[0] else {
            panic!()
        };
        assert_eq!(t.trivia.leading, vec![" explain the type".to_string()]);
        assert!(t.trivia.trailing.is_none());
    }

    #[test]
    fn trailing_line_comment_attaches_to_prev_decl() {
        let src = "commons x {\ntype T = Int where NonNegative  -- trailing note\n}";
        let c = parse_str(src).unwrap();
        let CommonsItem::Type(t) = &c.items[0] else {
            panic!()
        };
        assert!(t.trivia.leading.is_empty());
        assert_eq!(t.trivia.trailing.as_deref(), Some(" trailing note"));
    }

    #[test]
    fn grouped_leading_comments_attach_together() {
        let src = "commons x {\n-- one\n-- two\n-- three\ntype T = Int where Positive\n}";
        let c = parse_str(src).unwrap();
        let CommonsItem::Type(t) = &c.items[0] else {
            panic!()
        };
        assert_eq!(
            t.trivia.leading,
            vec![" one".to_string(), " two".to_string(), " three".to_string()],
        );
    }

    #[test]
    fn comment_with_doc_block_keeps_both() {
        // Both `-- intro` and the doc block should attach to the type decl.
        let src = "commons x {\n-- intro\n---\ndocs\n---\ntype T = Int where Positive\n}";
        let c = parse_str(src).unwrap();
        let CommonsItem::Type(t) = &c.items[0] else {
            panic!()
        };
        assert_eq!(t.trivia.leading, vec![" intro".to_string()]);
        assert_eq!(t.documentation.as_deref(), Some("docs"));
    }

    #[test]
    fn comment_before_let_statement_attaches() {
        let src = "commons x {\nfn f(n: Int) -> Int {\n-- pick a value\nlet y = n + 1\ny\n}\n}";
        let c = parse_str(src).unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!()
        };
        let Statement::Let(l) = &f.body.statements[0] else {
            panic!()
        };
        assert_eq!(l.trivia.leading, vec![" pick a value".to_string()]);
    }

    #[test]
    fn comment_before_tail_attaches_to_block_tail() {
        let src = "commons x {\nfn f(n: Int) -> Int {\nlet y = n + 1\n-- result\ny\n}\n}";
        let c = parse_str(src).unwrap();
        let CommonsItem::Fn(f) = &c.items[0] else {
            panic!()
        };
        assert_eq!(f.body.tail_leading_comments, vec![" result".to_string()],);
    }

    #[test]
    fn trailing_file_comment_becomes_unit_trailing() {
        // A comment after the last item but before EOF (fragment form)
        // becomes the commons body's trailing comments so the formatter
        // can preserve it.
        let src = "commons x\n\ntype T = Int where Positive\n-- afterword\n";
        let c = parse_str(src).unwrap();
        assert_eq!(c.trailing_comments, vec![" afterword".to_string()]);
    }
}