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//! A friendly parser combinator crate that makes writing LL(1) parsers with error recovery easy. //! //! ## Example //! //! Here follows a [Brainfuck](https://en.wikipedia.org/wiki/Brainfuck) parser. See `examples/` for the full interpreter. //! //! ``` //! use chumsky::prelude::*; //! //! #[derive(Clone)] //! enum Instr { //! Invalid, //! Left, Right, //! Incr, Decr, //! Read, Write, //! Loop(Vec<Self>) //! } //! //! fn parser() -> impl Parser<char, Vec<Instr>, Error = Simple<char>> { //! use Instr::*; //! recursive(|bf| bf.delimited_by('[', ']').map(|xs| xs.map_or(Invalid, Loop)) //! .or(just('<').to(Left)) //! .or(just('>').to(Right)) //! .or(just('+').to(Incr)) //! .or(just('-').to(Decr)) //! .or(just(',').to(Read)) //! .or(just('.').to(Write)) //! .repeated()) //! } //! ``` //! //! ## Features //! //! - Generic combinator parsing //! - Error recovery //! - Recursive parsers //! - Text-specific parsers & utilities //! - Custom error types //! //! ## What is LL(k) parsing? //! //! We can think of parsing like navigating a maze that has no cycles. As we walk towards what we hope is the exit we //! will, from time to time, come across splits in the path. Our job is to decide which path to take in order to reach //! the exit. How do we do this? //! //! Thankfully, we have signposts at every turning in the form of upcoming tokens to parse. Unfortunately, these //! signposts don't tell us exactly where to go: they just give us an idea of what obstacles might be lying ahead. We //! also have a way to see into the future using 'token lookahead'. Lookahead means that at every split in the path, we //! get a chance to peer into the distance to see what lies ahead on each possible path before deciding to commit to //! it. The distance that we're allowed to look is the 'k' in LL(k) parsing. Parsers capable of taking a walk into the //! future, realising they've taken a wrong turn, and retracing their steps are known as 'backtracking' parsers for //! this reason. //! //! ## LL(1) parsing: limitations and advantages //! //! Chumsky is designed for LL(1) parsing. This means that at each split in the path, we only get to see the very next //! token in advance before making a choice about which path to proceed down. In effect, we need to be able to figure //! out how to proceed using only the very next token in the input sequence. No backtracking allowed! //! //! This may initially seem like a serious limitation and it is, at least in theory. In practice, we can eliminate many //! of these limitations by deploying *parsing passes*. A common example of multi-pass parsing is the way in which //! compilers for C-like languages generally perform an initial lexing (also called 'tokenization') pass to split //! the source code into logical groups of characters known as 'tokens' and later feed these tokens into a secondary //! pass to generate the program's abstract syntax tree. This isn't just a workaround for the limitations of an LL(1) //! parser either: splitting parsing into multiple passes makes writing and validating each pass simpler and allows for //! tools like syntax highlighters that make use of earlier passes only to be built on top of the same codebase. //! //! All of this considered, LL(1) parsers are powerful tools that continue to be a relevant and effective route for //! parsing even (and, perhaps, *in particular*) the latest generations of programming languages. //! //! Why use LL(1) parsing when more powerful backtracking alternatives exist? The answer, in my view, comes down to //! three main points. //! //! - **Simplicity**: LL(1) grammars are usually easier to write (and, arguably, read) than more complex grammars. //! //! - **Performance**: LL(1) parsing does not require backtracking (because backtracking is impossible). This means //! that parsing can only ever proceed in the forward direction, meaning that parsing has *linear* (i.e: **O(n)**) //! time complexity. Compare this to other parsing techniques, for which exponential time is virtually the norm. //! //! - **Error recovery**: Because parsers of LL(1) grammars do not perform backtracking, there are fewer potential //! parse paths to consider: this makes it more likely that errors can be recovered from without misinterpreting //! syntax and makes it more likely that errors relate to the syntax that the user is attempting to write. #![deny(missing_docs)] // TODO: Enable when stable //#![feature(once_cell)] /// Combinators that allow combining and extending existing parsers. pub mod combinator; /// Error types, traits and utilities. pub mod error; /// Traits that allow chaining parser outputs together. pub mod chain; /// Parser primitives that accept specific token patterns. pub mod primitive; /// Recursive parsers (parser that include themselves within their patterns). pub mod recursive; /// Token streams and behaviours. pub mod stream; /// Text-specific parsers and utilities. pub mod text; pub use crate::error::{Error, Span}; use crate::{ chain::Chain, combinator::*, primitive::*, stream::*, }; use std::{ iter::Peekable, marker::PhantomData, rc::Rc, // TODO: Enable when stable //lazy::OnceCell, }; /// Commonly used functions, traits and types. pub mod prelude { pub use super::{ error::{Error as _, Simple}, text::{TextParser as _, whitespace}, primitive::{any, end, filter, filter_map, just, one_of, none_of, seq}, recursive::recursive, text, Parser, BoxedParser, }; } fn or_zip_with<T, F: FnOnce(T, T) -> T>(a: Option<T>, b: Option<T>, f: F) -> Option<T> { match (a, b) { (Some(a), Some(b)) => Some(f(a, b)), (a, b) => a.or(b), } } fn zip_or<T, F: FnOnce(T, T) -> T>(a: Option<T>, b: T, f: F) -> T { match a { Some(a) => f(a, b), None => b, } } type ParserFn<'a, I, O, E> = dyn Fn(&mut dyn Stream<I, <E as Error<I>>::Span>, &mut Vec<E>) -> (usize, Result<(O, Option<E>), E>) + 'a; /// A trait implemented by parsers. /// /// Parsers take a stream of tokens of type `I` and attempt to parse them into a value of type `O`. In doing so, they /// may encounter errors. These need not be fatal to the parsing process: syntactic errors can be recovered from and a /// valid output may still be generated alongside any syntax errors that were encountered along the way. Usually, this /// output comes in the form of an [Abstract Syntax Tree](https://en.wikipedia.org/wiki/Abstract_syntax_tree) (AST). /// /// Parsers currently only support LL(1) grammars. More concretely, this means that the rules that compose this parser /// are only permitted to 'look' a single token into the future to determine the path through the grammar rules to be /// taken by the parser. Unlike other techniques, such as recursive decent, arbitrary backtracking is not permitted. /// The reasons for this are numerous, but perhaps the most obvious is that it makes error detection and recovery /// significantly simpler and easier. In the future, this crate may be extended to support more complex grammars. /// /// LL(1) parsers by themselves are not particularly powerful. Indeed, even very old languages such as C cannot parsed /// by an LL(1) parser in a single pass. However, this limitation quickly vanishes (and, indeed, makes the design of /// both the language and the parser easier) when one introduces multiple passes. For example, C compilers generally /// have a lexical pass prior to the main parser that groups the input characters into tokens. pub trait Parser<I, O> { /// The type of errors emitted by this parser. type Error: Error<I>; // TODO when default associated types are stable: = Simple<I>; /// Parse a stream with all the bells & whistles. You can use this to implement your own parser combinators. Note /// that both the signature and semantic requirements of this function are very likely to change in later versions. /// Where possible, prefer more ergonomic combinators provided elsewhere in the crate rather than implementing your /// own. fn parse_inner<S: Stream<I, <Self::Error as Error<I>>::Span>>(&self, stream: &mut S, errors: &mut Vec<Self::Error>) -> (usize, Result<(O, Option<Self::Error>), Self::Error>) where Self: Sized; /// Parse an iterator of tokens, yielding an output if possible, and any errors encountered along the way. /// /// If you don't care about producing an output if errors are encountered, use `Parser::parse` instead. fn parse_recovery<S: IntoStream<I, <Self::Error as Error<I>>::Span>>(&self, stream: S) -> (Option<O>, Vec<Self::Error>) where Self: Sized { let mut errors = Vec::new(); match self.parse_inner(&mut stream.into_stream(), &mut errors).1 { Ok((o, _)) => (Some(o), errors), Err(e) => { errors.push(e); (None, errors) }, } } /// Parse an iterator of tokens, yielding an output *or* any errors that were encountered along the way. /// /// If you wish to attempt to produce an output even if errors are encountered, use `Parser::parse_recovery`. fn parse<S: IntoStream<I, <Self::Error as Error<I>>::Span>>(&self, stream: S) -> Result<O, Vec<Self::Error>> where Self: Sized { let (output, errors) = self.parse_recovery(stream); if errors.len() > 0 { Err(errors) } else { Ok(output.expect("Parsing failed, but no errors were emitted. This is troubling, to say the least.")) } } /// Map the output of this parser to aanother value. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// #[derive(Debug, PartialEq)] /// enum Token { Word(String), Num(u64) } /// /// let word = filter::<_, _, Simple<char>>(|c: &char| c.is_alphabetic()) /// .repeated_at_least(1) /// .collect::<String>() /// .map(Token::Word); /// /// let num = filter::<_, _, Simple<char>>(|c: &char| c.is_ascii_digit()) /// .repeated_at_least(1) /// .collect::<String>() /// .map(|s| Token::Num(s.parse().unwrap())); /// /// let token = word.or(num); /// /// assert_eq!(token.parse("test"), Ok(Token::Word("test".to_string()))); /// assert_eq!(token.parse("42"), Ok(Token::Num(42))); /// ``` fn map<U, F: Fn(O) -> U>(self, f: F) -> Map<Self, F, O> where Self: Sized { Map(self, f, PhantomData) } /// Map the output of this parser to another value, making use of the pattern's span. fn map_with_span<U, F: Fn(O, Option<<Self::Error as Error<I>>::Span>) -> U>(self, f: F) -> MapWithSpan<Self, F, O> where Self: Sized { MapWithSpan(self, f, PhantomData) } /// Map the primary error of this parser to another value. /// /// This does not map error emitted by sub-patterns within the parser. fn map_err<F: Fn(Self::Error) -> Self::Error>(self, f: F) -> MapErr<Self, F> where Self: Sized { MapErr(self, f) } /// Label the pattern parsed by this parser for more useful error messages. /// /// This is useful when you want to give users a more useful description of an expected pattern than simply a list /// of possible initial tokens. For example, it's common to use the term "expression" at a catch-all for a number /// of different constructs in many languages. /// /// This does not label recovered errors generated by sub-patterns within the parser, only error *directly* emitted /// by the parser. /// /// This does not label errors where the labelled pattern consumed input (i.e: in unambiguous cases). /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let frac = text::digits() /// .chain(just('.')) /// .chain::<char, _, _>(text::digits()) /// .collect::<String>() /// .padded_by(end()) /// .labelled("number"); /// /// assert_eq!(frac.parse("42.3"), Ok("42.3".to_string())); /// assert_eq!(frac.parse("hello"), Err(vec![Simple::expected_label_found(Some(0..1), "number", Some('h'))])); /// assert_eq!(frac.parse("42!"), Err(vec![Simple::expected_token_found(Some(2..3), vec!['.'], Some('!'))])); /// ``` fn labelled<L: Into<<Self::Error as Error<I>>::Pattern> + Clone>(self, label: L) -> Label<Self, L> where Self: Sized { Label(self, label) } /// Transform all outputs of this parser to a pretermined value. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// #[derive(Clone, Debug, PartialEq)] /// enum Op { Add, Sub, Mul, Div } /// /// let op = just::<_, Simple<char>>('+').to(Op::Add) /// .or(just('-').to(Op::Sub)) /// .or(just('*').to(Op::Mul)) /// .or(just('/').to(Op::Div)); /// /// assert_eq!(op.parse("+"), Ok(Op::Add)); /// assert_eq!(op.parse("/"), Ok(Op::Div)); /// ``` fn to<U: Clone>(self, x: U) -> To<Self, O, U> where Self: Sized { To(self, x, PhantomData) } /// Left-fold the output of the parser into a single value, where the output is of type `(_, Vec<_>)`. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let int = text::int::<Simple<char>>() /// .collect::<String>() /// .map(|s| s.parse().unwrap()); /// /// let sum = int /// .then(just('+').padding_for(int).repeated()) /// .foldl(|a, b| a + b); /// /// assert_eq!(sum.parse("1+12+3+9"), Ok(25)); /// assert_eq!(sum.parse("6"), Ok(6)); /// ``` fn foldl<A, B, F: Fn(A, B) -> A>(self, f: F) -> Foldl<Self, F, A, B> where Self: Parser<I, (A, Vec<B>)> + Sized { Foldl(self, f, PhantomData) } /// Right-fold the output of the parser into a single value, where the output is of type `(Vec<_>, _)`. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let int = text::int::<Simple<char>>() /// .collect::<String>() /// .map(|s| s.parse().unwrap()); /// /// let signed = just('+').to(1) /// .or(just('-').to(-1)) /// .repeated() /// .then(int) /// .foldr(|a, b| a * b); /// /// assert_eq!(signed.parse("3"), Ok(3)); /// assert_eq!(signed.parse("-17"), Ok(-17)); /// assert_eq!(signed.parse("--+-+-5"), Ok(5)); /// ``` fn foldr<'a, A, B, F: Fn(A, B) -> B + 'a>(self, f: F) -> Foldr<Self, F, A, B> where Self: Parser<I, (Vec<A>, B)> + Sized { Foldr(self, f, PhantomData) } /// Ignore the output of this parser, yielding `()` as an output instead. /// /// This can be used to reduce the cost of passing by avoiding unnecessary allocations (most collections containing /// [ZSTs](https://doc.rust-lang.org/nomicon/exotic-sizes.html#zero-sized-types-zsts) do /// [not allocate](https://doc.rust-lang.org/std/vec/struct.Vec.html#guarantees)). For example, it's common to want /// to ignore whitespace in many grammars. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// // A parser that parses any number of whitespace characters without allocating /// let whitespace = filter::<_, _, Simple<char>>(|c: &char| c.is_whitespace()) /// .ignored() /// .repeated(); /// /// assert_eq!(whitespace.parse(" "), Ok(vec![(); 4])); /// assert_eq!(whitespace.parse(" hello"), Ok(vec![(); 2])); /// ``` fn ignored(self) -> Ignored<Self, O> where Self: Sized { To(self, (), PhantomData) } /// Collect the output of this parser into a collection. /// /// This is commonly useful for collecting [`Vec<char>`] outputs into [`String`]s. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let word = filter::<_, _, Simple<char>>(|c: &char| c.is_alphabetic()) /// .repeated() /// .collect::<String>(); /// /// assert_eq!(word.parse("hello"), Ok("hello".to_string())); /// ``` fn collect<C: core::iter::FromIterator<O::Item>>(self) -> Map<Self, fn(O) -> C, O> where Self: Sized, O: IntoIterator { self.map(|items| C::from_iter(items.into_iter())) } /// Parse one thing and then another thing, yielding a tuple of the two outputs. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let word = filter::<_, _, Simple<char>>(|c: &char| c.is_alphabetic()) /// .repeated_at_least(1) /// .collect::<String>(); /// let two_words = word.padded_by(just(' ')).then(word); /// /// assert_eq!(two_words.parse("dog cat"), Ok(("dog".to_string(), "cat".to_string()))); /// assert!(two_words.parse("hedgehog").is_err()); /// ``` fn then<U, P: Parser<I, U>>(self, other: P) -> Then<Self, P> where Self: Sized { Then(self, other) } /// Parse one thing and then another thing, attempting to chain the two outputs into a [`Vec`]. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let int = just('-').or_not() /// .chain(filter::<_, _, Simple<char>>(|c: &char| c.is_ascii_digit() && *c != '0') /// .chain(filter::<_, _, Simple<char>>(|c: &char| c.is_ascii_digit()).repeated())) /// .or(just('0').map(|c| vec![c])) /// .padded_by(end()) /// .collect::<String>() /// .map(|s| s.parse().unwrap()); /// /// assert_eq!(int.parse("0"), Ok(0)); /// assert_eq!(int.parse("415"), Ok(415)); /// assert_eq!(int.parse("-50"), Ok(-50)); /// assert!(int.parse("-0").is_err()); /// assert!(int.parse("05").is_err()); /// ``` fn chain<T, U, P: Parser<I, U, Error = Self::Error>>(self, other: P) -> Map<Then<Self, P>, fn((O, U)) -> Vec<T>, (O, U)> where Self: Sized, U: Chain<T>, O: Chain<T>, { self.then(other).map(|(a, b)| { let mut v = Vec::with_capacity(a.len() + b.len()); a.append(&mut v); b.append(&mut v); v }) } /// Flatten a nested collection. fn flatten<T, Inner>(self) -> Map<Self, fn(O) -> Vec<T>, O> where Self: Sized, O: IntoIterator<Item = Inner>, Inner: IntoIterator<Item = T>, { self.map(|xs| xs.into_iter().map(|xs| xs.into_iter()).flatten().collect()) } /// Parse one thing and then another thing, yielding only the output of the latter. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let zeroes = filter::<_, _, Simple<char>>(|c: &char| *c == '0').ignored().repeated(); /// let digits = filter(|c: &char| c.is_ascii_digit()).repeated(); /// let integer = zeroes /// .padding_for(digits) /// .collect::<String>() /// .map(|s| s.parse().unwrap()); /// /// assert_eq!(integer.parse("00064"), Ok(64)); /// assert_eq!(integer.parse("32"), Ok(32)); /// ``` fn padding_for<U, P: Parser<I, U>>(self, other: P) -> PaddingFor<Self, P, O, U> where Self: Sized { Map(Then(self, other), |(_, u)| u, PhantomData) } /// Parse one thing and then another thing, yielding only the output of the former. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let word = filter::<_, _, Simple<char>>(|c: &char| c.is_alphabetic()) /// .repeated_at_least(1) /// .collect::<String>(); /// /// let punctuated = word /// .padded_by(just('!').or(just('?')).or_not()); /// /// let sentence = punctuated /// .padded() // Allow for whitespace gaps /// .repeated(); /// /// assert_eq!( /// sentence.parse("hello! how are you?"), /// Ok(vec![ /// "hello".to_string(), /// "how".to_string(), /// "are".to_string(), /// "you".to_string(), /// ]), /// ); /// ``` fn padded_by<U, P: Parser<I, U>>(self, other: P) -> PaddedBy<Self, P, O, U> where Self: Sized { Map(Then(self, other), |(o, _)| o, PhantomData) } // fn then_catch(self, end: I) -> ThenCatch<Self, I> where Self: Sized { ThenCatch(self, end) } /// Parse the pattern surrounded by the given delimiters, performing error recovery where possible. /// /// If parsing of the inner pattern is successful, the output is `Some(_)`. If an error occurs, the output is /// `None`. /// /// The delimiters are assumed to allow nesting, so error recovery will attempt to balance the delimiters where /// possible. A syntax error within the delimiters should not prevent correct parsing of tokens beyond the /// delimiters. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// // A LISP-style S-expression /// #[derive(Debug, PartialEq)] /// enum SExpr { /// Error, /// Ident(String), /// Num(u64), /// List(Vec<SExpr>), /// } /// /// let ident = filter::<_, _, Simple<char>>(|c: &char| c.is_alphabetic()) /// .repeated_at_least(1) /// .collect::<String>(); /// /// let num = text::int() /// .collect::<String>() /// .map(|s| s.parse().unwrap()); /// /// let s_expr = recursive(|s_expr| s_expr /// .padded() /// .repeated() /// .delimited_by('(', ')') /// .map(|list| list.map_or(SExpr::Error, SExpr::List)) /// .or(ident.map(SExpr::Ident)) /// .or(num.map(SExpr::Num))); /// /// // A valid input /// assert_eq!( /// s_expr.parse_recovery("(add (mul 42 3) 15)"), /// ( /// Some(SExpr::List(vec![ /// SExpr::Ident("add".to_string()), /// SExpr::List(vec![ /// SExpr::Ident("mul".to_string()), /// SExpr::Num(42), /// SExpr::Num(3), /// ]), /// SExpr::Num(15), /// ])), /// Vec::new(), // No errors! /// ), /// ); /// /// // An input with a syntax error at position 11! Thankfully, we're able to recover /// // and still produce a useful output for later compilation stages (i.e: type-checking). /// assert_eq!( /// s_expr.parse_recovery("(add (mul ! 3) 15)"), /// ( /// Some(SExpr::List(vec![ /// SExpr::Ident("add".to_string()), /// SExpr::Error, /// SExpr::Num(15), /// ])), /// vec![Simple::expected_token_found(Some(10..11), vec!['(', '0', ')'], Some('!'))], // A syntax error! /// ), /// ); /// ``` fn delimited_by(self, start: I, end: I) -> DelimitedBy<Self, I> where Self: Sized { DelimitedBy(self, start, end) } /// Parse one thing or, on failure, another thing. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let op = just::<_, Simple<char>>('+') /// .or(just('-')) /// .or(just('*')) /// .or(just('/')); /// /// assert_eq!(op.parse("+"), Ok('+')); /// assert_eq!(op.parse("/"), Ok('/')); /// assert!(op.parse("!").is_err()); /// ``` fn or<P: Parser<I, O>>(self, other: P) -> Or<Self, P> where Self: Sized { Or(self, other) } /// Attempt to parse something, but only if it exists. /// /// If parsing of the pattern is successful, the output is `Some(_)`. Otherwise, the output is `None`. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let word = filter::<_, _, Simple<char>>(|c: &char| c.is_alphabetic()) /// .repeated_at_least(1) /// .collect::<String>(); /// /// let word_or_question = word /// .then(just('?').or_not()); /// /// assert_eq!(word_or_question.parse("hello?"), Ok(("hello".to_string(), Some('?')))); /// assert_eq!(word_or_question.parse("wednesday"), Ok(("wednesday".to_string(), None))); /// ``` fn or_not(self) -> OrNot<Self> where Self: Sized { OrNot(self) } /// Parse an expression any number of times (including zero times). /// /// Input is eagerly parsed. Be aware that the parser will accept no occurences of the pattern too. Consider using /// [`Parser::repeated_at_least`] instead if it better suits your use-case. /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let num = filter::<_, _, Simple<char>>(|c: &char| c.is_ascii_digit()) /// .repeated_at_least(1) /// .collect::<String>() /// .map(|s| s.parse().unwrap()); /// /// let sum = num.then(just('+').padding_for(num).repeated()) /// .foldl(|a, b| a + b); /// /// assert_eq!(sum.parse("2+13+4+0+5"), Ok(24)); /// ``` fn repeated(self) -> Repeated<Self> where Self: Sized { Repeated(self, 0) } /// Parse an expression at least a given number of times. /// /// Input is eagerly parsed. If `n` is 0, this function is equivalent to [`Parser::repeated`] /// /// # Examples /// /// ``` /// use chumsky::prelude::*; /// /// let long_word = filter::<_, _, Simple<char>>(|c: &char| c.is_alphabetic()) /// .repeated_at_least(5) /// .collect::<String>(); /// /// assert_eq!(long_word.parse("hello"), Ok("hello".to_string())); /// assert!(long_word.parse("hi").is_err()); /// ``` fn repeated_at_least(self, n: usize) -> Repeated<Self> where Self: Sized { Repeated(self, n) } /// Box the parser, yielding a parser that performs parsing through dynamic dispatch. /// /// Boxing a parser might be useful for: /// /// - Passing a parser over an FFI boundary /// /// - Getting around compiler implementation problems with long types such as /// [this](https://github.com/rust-lang/rust/issues/54540). /// /// - Places where you need to name the type of a parser /// /// Boxing a parser is loosely equivalent to boxing other combinators, such as [`Iterator`]. fn boxed<'a>(self) -> BoxedParser<'a, I, O, Self::Error> where Self: Sized + 'a { BoxedParser(Rc::new(move |mut stream: &mut dyn Stream<I, <Self::Error as Error<I>>::Span>, errors| self.parse_inner(&mut stream, errors))) } } /// See [`Parser::boxed`]. /// /// This type is a [`repr(transparent)`](https://doc.rust-lang.org/nomicon/other-reprs.html#reprtransparent) wrapper /// around its inner value. /// /// Due to current implementation details, the inner value is not, in fact, a [`Box`], but is an [`Rc`] to facilitate /// efficient cloning. This is likely to change in the future. Unlike [`Box`], [`Rc`] has no size guarantees: although /// it is *currently* the same size as a raw pointer. // TODO: Don't use an Rc #[repr(transparent)] pub struct BoxedParser<'a, I, O, E: Error<I>>(Rc<ParserFn<'a, I, O, E>>); impl<'a, I, O, E: Error<I>> Clone for BoxedParser<'a, I, O, E> { fn clone(&self) -> Self { Self(self.0.clone()) } } impl<'a, I, O, E: Error<I>> Parser<I, O> for BoxedParser<'a, I, O, E> { type Error = E; fn parse_inner<S: Stream<I, <Self::Error as Error<I>>::Span>>(&self, stream: &mut S, errors: &mut Vec<Self::Error>) -> (usize, Result<(O, Option<E>), E>) { (self.0)(stream, errors) } }