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//! Minimal S-expression parser shared by the Dune, Common Lisp, and Racket
//! manifest parsers.
//!
//! Supports:
//! - `;` line comments
//! - `#| … |#` block comments (Common Lisp / Racket)
//! - `'expr` → `(quote expr)` reader macro
//! - `` `expr `` and `,expr` — quasiquote / unquote (inner form passed through)
//! - `[…]` bracket lists (Racket — treated as `(…)`)
//! - `#:sym` and `:kw` atoms stored as-is
/// An S-expression value.
#[derive(Debug, Clone, PartialEq)]
pub enum Sexp {
/// A symbol, keyword (`:foo`, `#:bar`), number, or other bare token.
Atom(String),
/// A string literal — content only, **without** surrounding `"` characters.
Str(String),
/// A parenthesised or bracketed list.
List(Vec<Sexp>),
}
impl Sexp {
/// Parse all top-level S-expressions from `input`.
pub fn parse(input: &str) -> Vec<Self> {
let chars: Vec<char> = input.chars().collect();
let mut pos = 0;
let mut out = Vec::new();
while pos < chars.len() {
let before = pos;
skip_ws(&chars, &mut pos);
if pos >= chars.len() {
break;
}
if let Some(tok) = read_sexp(&chars, &mut pos) {
out.push(tok);
} else if pos == before {
// Nothing consumed and no token — skip one char to avoid spinning.
pos += 1;
}
}
out
}
/// Return the atom string if this is `Sexp::Atom`.
pub fn as_atom(&self) -> Option<&str> {
match self {
Self::Atom(s) => Some(s),
_ => None,
}
}
/// Return the string content if this is `Sexp::Str`.
pub fn as_str(&self) -> Option<&str> {
match self {
Self::Str(s) => Some(s),
_ => None,
}
}
/// Return the text content whether this is `Sexp::Atom` or `Sexp::Str`.
pub fn as_text(&self) -> Option<&str> {
match self {
Self::Atom(s) | Self::Str(s) => Some(s),
_ => None,
}
}
/// Return the children if this is `Sexp::List`.
pub fn as_list(&self) -> Option<&[Sexp]> {
match self {
Self::List(v) => Some(v),
_ => None,
}
}
/// If this is a `List` whose first element is `Atom(head)`, return the
/// remaining items (everything after the head).
pub fn tagged_list(&self, head: &str) -> Option<&[Sexp]> {
let items = self.as_list()?;
match items.first() {
Some(Sexp::Atom(h)) if h == head => Some(&items[1..]),
_ => None,
}
}
}
/// Find the value after a keyword atom in a flat token slice (case-sensitive).
///
/// Searches for `Atom(key)` and returns the next item.
pub fn kw_arg<'a>(items: &'a [Sexp], key: &str) -> Option<&'a Sexp> {
let mut iter = items.iter();
while let Some(tok) = iter.next() {
if let Sexp::Atom(k) = tok
&& k == key
{
return iter.next();
}
}
None
}
// ============================================================================
// Internal parser
// ============================================================================
fn skip_ws(chars: &[char], pos: &mut usize) {
while *pos < chars.len() {
match chars[*pos] {
' ' | '\t' | '\n' | '\r' => *pos += 1,
';' => {
while *pos < chars.len() && chars[*pos] != '\n' {
*pos += 1;
}
}
'#' if *pos + 1 < chars.len() && chars[*pos + 1] == '|' => {
*pos += 2;
while *pos + 1 < chars.len() {
if chars[*pos] == '|' && chars[*pos + 1] == '#' {
*pos += 2;
break;
}
*pos += 1;
}
}
_ => break,
}
}
}
fn read_sexp(chars: &[char], pos: &mut usize) -> Option<Sexp> {
skip_ws(chars, pos);
if *pos >= chars.len() {
return None;
}
match chars[*pos] {
'(' | '[' => {
let close = if chars[*pos] == '(' { ')' } else { ']' };
*pos += 1;
let mut children = Vec::new();
loop {
skip_ws(chars, pos);
if *pos >= chars.len() {
break;
}
if chars[*pos] == close {
*pos += 1;
break;
}
let before = *pos;
if let Some(tok) = read_sexp(chars, pos) {
children.push(tok);
} else if *pos == before {
*pos += 1; // skip unexpected char
}
}
Some(Sexp::List(children))
}
')' | ']' => {
*pos += 1;
None
}
'"' => {
*pos += 1;
let mut s = String::new();
while *pos < chars.len() && chars[*pos] != '"' {
if chars[*pos] == '\\' {
*pos += 1;
if *pos < chars.len() {
s.push(unescape_char(chars[*pos]));
*pos += 1;
}
} else {
s.push(chars[*pos]);
*pos += 1;
}
}
if *pos < chars.len() {
*pos += 1; // closing "
}
Some(Sexp::Str(s))
}
'\'' => {
*pos += 1;
let inner = read_sexp(chars, pos)?;
Some(Sexp::List(vec![Sexp::Atom("quote".to_string()), inner]))
}
'`' => {
*pos += 1;
read_sexp(chars, pos)
}
',' => {
*pos += 1;
// ,@ splice — consume '@' too
if *pos < chars.len() && chars[*pos] == '@' {
*pos += 1;
}
read_sexp(chars, pos)
}
_ => {
let mut s = String::new();
while *pos < chars.len()
&& !matches!(
chars[*pos],
' ' | '\t' | '\n' | '\r' | '(' | ')' | '[' | ']' | '"' | ';' | '\'' | '`' | ','
)
{
s.push(chars[*pos]);
*pos += 1;
}
if s.is_empty() {
None
} else {
Some(Sexp::Atom(s))
}
}
}
}
fn unescape_char(c: char) -> char {
match c {
'n' => '\n',
't' => '\t',
'r' => '\r',
_ => c,
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_atoms_and_strings() {
let forms = Sexp::parse(r#"hello "world" :key #:sym"#);
assert_eq!(forms.len(), 4);
assert_eq!(forms[0].as_atom(), Some("hello"));
assert_eq!(forms[1].as_str(), Some("world"));
assert_eq!(forms[2].as_atom(), Some(":key"));
assert_eq!(forms[3].as_atom(), Some("#:sym"));
}
#[test]
fn test_list() {
let forms = Sexp::parse("(define x 42)");
assert_eq!(forms.len(), 1);
let items = forms[0].as_list().unwrap();
assert_eq!(items[0].as_atom(), Some("define"));
assert_eq!(items[1].as_atom(), Some("x"));
assert_eq!(items[2].as_atom(), Some("42"));
}
#[test]
fn test_quote_sugar() {
let forms = Sexp::parse("'(a b c)");
assert_eq!(forms.len(), 1);
let items = forms[0].as_list().unwrap();
assert_eq!(items[0].as_atom(), Some("quote"));
let inner = items[1].as_list().unwrap();
assert_eq!(inner.len(), 3);
}
#[test]
fn test_bracket_list() {
let forms = Sexp::parse("[a b c]");
assert_eq!(forms.len(), 1);
let items = forms[0].as_list().unwrap();
assert_eq!(items.len(), 3);
}
#[test]
fn test_line_comment() {
let forms = Sexp::parse("; comment\n(foo)");
assert_eq!(forms.len(), 1);
assert!(forms[0].as_list().is_some());
}
#[test]
fn test_block_comment() {
let forms = Sexp::parse("#| block comment |# (foo)");
assert_eq!(forms.len(), 1);
}
#[test]
fn test_tagged_list() {
let forms = Sexp::parse("(define x 1)");
assert_eq!(
forms[0].tagged_list("define"),
Some(&forms[0].as_list().unwrap()[1..])
);
}
#[test]
fn test_kw_arg() {
let forms = Sexp::parse("(:version \"1.0\" :name \"pkg\")");
let items = forms[0].as_list().unwrap();
let ver = kw_arg(items, ":version").unwrap();
assert_eq!(ver.as_str(), Some("1.0"));
}
#[test]
fn test_as_text() {
let forms = Sexp::parse(r#"atom "str""#);
assert_eq!(forms[0].as_text(), Some("atom"));
assert_eq!(forms[1].as_text(), Some("str"));
}
}