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Better delimiter errors #916

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e6894cd
Added parser
zesterer Nov 19, 2025
d84d7e7
Added DelimitedBy::with_mismatched_end, improved handling of mismatch…
zesterer Nov 19, 2025
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25 changes: 21 additions & 4 deletions examples/json.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -35,6 +35,7 @@ fn parser<'a>() -> impl Parser<'a, &'a str, Json, extra::Err<Rich<'a, char>>> {
.then(exp.or_not())
.to_slice()
.map(|s: &str| s.parse().unwrap())
.labelled("number")
.boxed();

let escape = just('\\')
Expand Down Expand Up @@ -68,6 +69,8 @@ fn parser<'a>() -> impl Parser<'a, &'a str, Json, extra::Err<Rich<'a, char>>> {
.to_slice()
.map(ToString::to_string)
.delimited_by(just('"'), just('"'))
.labelled("string")
.as_context()
.boxed();

let array = value
Expand All @@ -86,9 +89,16 @@ fn parser<'a>() -> impl Parser<'a, &'a str, Json, extra::Err<Rich<'a, char>>> {
.recover_with(via_parser(end()))
.recover_with(skip_then_retry_until(any().ignored(), end())),
)
.labelled("array")
.as_context()
.boxed();

let member = string.clone().then_ignore(just(':').padded()).then(value);
let member = string
.clone()
.then_ignore(just(':').padded())
.then(value)
.labelled("object member")
.as_context();
let object = member
.clone()
.separated_by(just(',').padded().recover_with(skip_then_retry_until(
Expand All @@ -104,12 +114,14 @@ fn parser<'a>() -> impl Parser<'a, &'a str, Json, extra::Err<Rich<'a, char>>> {
.recover_with(via_parser(end()))
.recover_with(skip_then_retry_until(any().ignored(), end())),
)
.labelled("object")
.as_context()
.boxed();

choice((
just("null").to(Json::Null),
just("true").to(Json::Bool(true)),
just("false").to(Json::Bool(false)),
just("null").to(Json::Null).labelled("null"),
just("true").to(Json::Bool(true)).labelled("boolean"),
just("false").to(Json::Bool(false)).labelled("boolean"),
number.map(Json::Num),
string.map(Json::Str),
array.map(Json::Array),
Expand Down Expand Up @@ -150,6 +162,11 @@ fn main() {
.with_message(e.reason().to_string())
.with_color(Color::Red),
)
.with_labels(e.contexts().next().map(|ctx| {
Label::new(((), ctx.span().into_range()))
.with_message(format!("while parsing this {}", ctx.pattern()))
.with_color(Color::Yellow)
}))
.finish()
.print(Source::from(&src))
.unwrap()
Expand Down
43 changes: 30 additions & 13 deletions examples/mini_ml.rs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -78,7 +78,8 @@ fn lexer<'src>(
.repeated()
.collect()
.delimited_by(just('('), just(')'))
.labelled("token tree")
.with_mismatched_end(one_of("]})"))
.labelled("expression")
.as_context()
.map(Token::Parens),
))
Expand Down Expand Up @@ -137,7 +138,9 @@ fn parser<'tokens, 'src: 'tokens>() -> impl Parser<
lhs,
rhs: Box::new(rhs),
then: Box::new(then),
}),
})
.labelled("expression")
.as_context(),
));

choice((
Expand Down Expand Up @@ -183,8 +186,6 @@ fn parser<'tokens, 'src: 'tokens>() -> impl Parser<
})
.boxed(),
])
.labelled("expression")
.as_context()
})
}

Expand Down Expand Up @@ -257,8 +258,8 @@ impl Solver<'_> {
format!("Type mismatch between {a_info} and {b_info}"),
("mismatch occurred here".to_string(), span),
vec![
(format!("{a_info}"), self.vars[a.0].1),
(format!("{b_info}"), self.vars[b.0].1),
(format!("{a_info}"), self.vars[a.0].1, Color::Yellow),
(format!("{b_info}"), self.vars[b.0].1, Color::Yellow),
],
self.src,
),
Expand Down Expand Up @@ -408,7 +409,7 @@ impl<'src> Vm<'src> {
fn failure(
msg: String,
label: (String, SimpleSpan),
extra_labels: impl IntoIterator<Item = (String, SimpleSpan)>,
extra_labels: impl IntoIterator<Item = (String, SimpleSpan, Color)>,
src: &str,
) -> ! {
let fname = "example";
Expand All @@ -420,10 +421,10 @@ fn failure(
.with_message(label.0)
.with_color(Color::Red),
)
.with_labels(extra_labels.into_iter().map(|label2| {
Label::new((fname, label2.1.into_range()))
.with_message(label2.0)
.with_color(Color::Yellow)
.with_labels(extra_labels.into_iter().map(|(pat, span, col)| {
Label::new((fname, span.into_range()))
.with_message(pat)
.with_color(col)
}))
.finish()
.print(sources([(fname, src)]))
Expand All @@ -440,8 +441,24 @@ fn parse_failure(err: &Rich<impl fmt::Display>, src: &str) -> ! {
.unwrap_or_else(|| "end of input".to_string()),
*err.span(),
),
err.contexts()
.map(|(l, s)| (format!("while parsing this {l}"), *s)),
err.reason()
.unclosed_delimiter()
.map(|ud| {
(
"this delimiter was never closed".to_string(),
*ud,
Color::Blue,
)
})
.into_iter()
.chain(err.contexts().map(|ctx| {
(
format!("while parsing this {}", ctx.pattern()),
*ctx.span(),
Color::Yellow,
)
}))
.take(1),
src,
)
}
Expand Down
65 changes: 33 additions & 32 deletions examples/nano_rust.rs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -300,7 +300,10 @@ where
// Blocks are expressions but delimited with braces
let block = expr
.clone()
// Empty blocks are permitted
.or(empty().map_with(|(), e| (Expr::Value(Value::Null), e.span())))
.delimited_by(just(Token::Ctrl('{')), just(Token::Ctrl('}')))
.with_mismatched_end(one_of([Token::Ctrl(']'), Token::Ctrl(')')]))
// Attempt to recover anything that looks like a block but contains errors
.recover_with(via_parser(nested_delimiters(
Token::Ctrl('{'),
Expand All @@ -310,7 +313,9 @@ where
(Token::Ctrl('['), Token::Ctrl(']')),
],
|span| (Expr::Error, span),
)));
)))
.labelled("block")
.as_context();

let if_ = recursive(|if_| {
just(Token::If)
Expand Down Expand Up @@ -343,30 +348,9 @@ where
(Expr::Then(Box::new(a), Box::new(b)), e.span())
});

let block_recovery = nested_delimiters(
Token::Ctrl('{'),
Token::Ctrl('}'),
[
(Token::Ctrl('('), Token::Ctrl(')')),
(Token::Ctrl('['), Token::Ctrl(']')),
],
|span| (Expr::Error, span),
);

block_chain
.labelled("block")
// Expressions, chained by semicolons, are statements
.or(inline_expr.clone())
.recover_with(skip_then_retry_until(
block_recovery.ignored().or(any().ignored()),
one_of([
Token::Ctrl(';'),
Token::Ctrl('}'),
Token::Ctrl(')'),
Token::Ctrl(']'),
])
.ignored(),
))
.foldl_with(
just(Token::Ctrl(';')).ignore_then(expr.or_not()).repeated(),
|a, b, e| {
Expand Down Expand Up @@ -395,27 +379,39 @@ fn funcs_parser<'tokens, 'src: 'tokens, I>() -> impl Parser<
where
I: ValueInput<'tokens, Token = Token<'src>, Span = Span>,
{
let ident = select! { Token::Ident(ident) => ident };
let ident = select! { Token::Ident(ident) => ident }.labelled("identifier");

// Argument lists are just identifiers separated by commas, surrounded by parentheses
let args = ident
.separated_by(just(Token::Ctrl(',')))
.allow_trailing()
.collect()
.delimited_by(just(Token::Ctrl('(')), just(Token::Ctrl(')')))
.labelled("function args");
.with_mismatched_end(one_of([Token::Ctrl(']'), Token::Ctrl('}')]))
.labelled("argument list")
.as_context()
.recover_with(via_parser(nested_delimiters(
Token::Ctrl('('),
Token::Ctrl(')'),
[
(Token::Ctrl('{'), Token::Ctrl('}')),
(Token::Ctrl('['), Token::Ctrl(']')),
],
|_| Vec::new(),
)));

let func = just(Token::Fn)
.ignore_then(
ident
.map_with(|name, e| (name, e.span()))
.labelled("function name"),
)
.ignore_then(ident.map_with(|name, e| (name, e.span())))
.then(args)
.map_with(|start, e| (start, e.span()))
.then(
expr_parser()
// Empty function bodies are permitted
.or(empty().map_with(|(), e| (Expr::Value(Value::Null), e.span())))
.delimited_by(just(Token::Ctrl('{')), just(Token::Ctrl('}')))
.with_mismatched_end(one_of([Token::Ctrl(']'), Token::Ctrl(')')]))
.labelled("function body")
.as_context()
// Attempt to recover anything that looks like a function body but contains errors
.recover_with(via_parser(nested_delimiters(
Token::Ctrl('{'),
Expand Down Expand Up @@ -610,9 +606,14 @@ fn main() {
.with_message(e.reason().to_string())
.with_color(Color::Red),
)
.with_labels(e.contexts().map(|(label, span)| {
Label::new((filename.clone(), span.into_range()))
.with_message(format!("while parsing this {label}"))
.with_labels(e.reason().unclosed_delimiter().map(|ud| {
Label::new((filename.clone(), ud.into_range()))
.with_message("this delimiter was never closed")
.with_color(Color::Blue)
}))
.with_labels(e.contexts().take(1).map(|ctx| {
Label::new((filename.clone(), ctx.span().into_range()))
.with_message(format!("while parsing this {}", ctx.pattern()))
.with_color(Color::Yellow)
}))
.finish()
Expand Down
2 changes: 1 addition & 1 deletion examples/sample.json
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,7 @@
"origin": 981339097,
},
"though": ttrue asasjk,
"invalid": "\uDFFF",
"invalid": "hello \uDFFF world",
"activity": "value",
"office": -342325541.1937506,
"noise": false,
Expand Down
60 changes: 53 additions & 7 deletions src/combinator.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1445,40 +1445,86 @@ where
}

/// See [`Parser::delimited_by`].
pub struct DelimitedBy<A, B, C, OB, OC> {
pub struct DelimitedBy<A, B, C, OB, OC, D = crate::primitive::Never, OD = Infallible> {
pub(crate) parser: A,
pub(crate) start: B,
pub(crate) end: C,
pub(crate) invalid_end: D,
#[allow(dead_code)]
pub(crate) phantom: EmptyPhantom<(OB, OC)>,
pub(crate) phantom: EmptyPhantom<(OB, OC, OD)>,
}

impl<A: Copy, B: Copy, C: Copy, OB, OC> Copy for DelimitedBy<A, B, C, OB, OC> {}
impl<A: Clone, B: Clone, C: Clone, OB, OC> Clone for DelimitedBy<A, B, C, OB, OC> {
impl<A: Copy, B: Copy, C: Copy, OB, OC, D: Copy, OD> Copy for DelimitedBy<A, B, C, OB, OC, D, OD> {}
impl<A: Clone, B: Clone, C: Clone, OB, OC, D: Clone, OD> Clone
for DelimitedBy<A, B, C, OB, OC, D, OD>
{
fn clone(&self) -> Self {
Self {
parser: self.parser.clone(),
start: self.start.clone(),
end: self.end.clone(),
invalid_end: self.invalid_end.clone(),
phantom: EmptyPhantom::new(),
}
}
}

impl<A, B, C, OB, OC, D, OD> DelimitedBy<A, B, C, OB, OC, D, OD> {
/// Indicate that when the given mismatched pattern is found, errors should be marked as 'unclosed delimiters'.
///
/// This can result in improved error messages in certain cases.
pub fn with_mismatched_end<D2, OD2>(self, end: D2) -> DelimitedBy<A, B, C, OB, OC, D2, OD2> {
DelimitedBy {
parser: self.parser,
start: self.start,
end: self.end,
invalid_end: end,
phantom: EmptyPhantom::new(),
}
}
}

impl<'src, I, E, A, B, C, OA, OB, OC> Parser<'src, I, OA, E> for DelimitedBy<A, B, C, OB, OC>
impl<'src, I, E, A, B, C, OA, OB, OC, D, OD> Parser<'src, I, OA, E>
for DelimitedBy<A, B, C, OB, OC, D, OD>
where
I: Input<'src>,
E: ParserExtra<'src, I>,
A: Parser<'src, I, OA, E>,
B: Parser<'src, I, OB, E>,
C: Parser<'src, I, OC, E>,
D: Parser<'src, I, OD, E>,
I::Span: Clone,
{
#[inline(always)]
fn go<M: Mode>(&self, inp: &mut InputRef<'src, '_, I, E>) -> PResult<M, OA> {
let before_start = inp.cursor();
self.start.go::<Check>(inp)?;
let start_span = inp.span_since(&before_start);
let a = self.parser.go::<M>(inp)?;
self.end.go::<Check>(inp)?;
Ok(a)
// let before_errors = inp.errors.secondary.len();
let before_end = inp.save();
let old_alt = inp.take_alt();
let scope_span = inp.span_since(&before_start);
let end_res = self.end.go::<Check>(inp);
let res = if end_res.is_err() {
let mut new_alt = inp.take_alt().expect("error, but no alt");
inp.rewind(before_end);
if self.invalid_end.go::<Check>(inp).is_ok() {
new_alt
.err
.in_delimited(start_span.clone(), scope_span.clone());
}
inp.errors.alt = old_alt;
inp.add_alt_err(&new_alt.pos, new_alt.err);
Err(())
} else {
inp.errors.alt = old_alt;
Ok(a)
};
// for err in inp.errors.secondary_errors_since(before_errors) {
// err.err.in_delimited(start_span.clone(), scope_span.clone());
// }
res
}

go_extra!(OA);
Expand Down
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