Add a parser-combinator crate

Parser-combinators are one of the simpler tools for building ad-hoc
parsers. They're a good fit because they are...

* Small: each parser / parser-combinator is around 10 LOC.
* Functional: helix_core strives to be a functional set of utilities
  usable throughout the rest of the editor.
* Flexible: use them to build any sort of ad-hoc parser. In the child
  commit, we'll parse LSP Snippet syntax using these new parser
  combinators.

Why not use an existing parser-combinator crate? Existing popular
parser-combinator crates have histories of making breaking changes
(for example nom and combine).

> Implementation note: I tried to not introduce a new trait since the
> types can be expressed in terms of `impl Fn`s. The trait is necessary
> to build `seq` implementations without a proc macro though, and also
> allows us to use `&'static str`s very conveniently: see the trait
> implementation for `&'static str`.

2 files changed, 574 insertions, 0 deletions

diff --git a/helix-parsec/Cargo.toml b/helix-parsec/Cargo.toml
new file mode 100644
index 00000000..562df8dd
--- /dev/null
+++ b/helix-parsec/Cargo.toml
@@ -0,0 +1,14 @@
+[package]
+name = "helix-parsec"
+version = "0.6.0"
+authors = ["Blaž Hrastnik <blaz@mxxn.io>"]
+edition = "2021"
+license = "MPL-2.0"
+description = "Parser combinators for Helix"
+categories = ["editor"]
+repository = "https://github.com/helix-editor/helix"
+homepage = "https://helix-editor.com"
+include = ["src/**/*", "README.md"]
+
+[dependencies]
+regex = "1"
diff --git a/helix-parsec/src/lib.rs b/helix-parsec/src/lib.rs
new file mode 100644
index 00000000..c86a1a05
--- /dev/null
+++ b/helix-parsec/src/lib.rs
@@ -0,0 +1,560 @@
+//! Parser-combinator functions
+//!
+//! This module provides parsers and parser combinators which can be used
+//! together to build parsers by functional composition.
+
+use regex::Regex;
+
+// This module implements parser combinators following https://bodil.lol/parser-combinators/.
+// `sym` (trait implementation for `&'static str`), `map`, `pred` (filter), `one_or_more`,
+// `zero_or_more`, as well as the `Parser` trait originate mostly from that post.
+// The remaining parsers and parser combinators are either based on
+// https://github.com/archseer/snippets.nvim/blob/a583da6ef130d2a4888510afd8c4e5ffd62d0dce/lua/snippet/parser.lua#L5-L138
+// or are novel.
+
+// When a parser matches the input successfully, it returns `Ok((next_input, some_value))`
+// where the type of the returned value depends on the parser. If the parser fails to match,
+// it returns `Err(input)`.
+type ParseResult<'a, Output> = Result<(&'a str, Output), &'a str>;
+
+/// A parser or parser-combinator.
+///
+/// Parser-combinators compose multiple parsers together to parse input.
+/// For example, two basic parsers (`&'static str`s) may be combined with
+/// a parser-combinator like [or] to produce a new parser.
+///
+/// ```
+/// use helix_parsec::{or, Parser};
+/// let foo = "foo"; // matches "foo" literally
+/// let bar = "bar"; // matches "bar" literally
+/// let foo_or_bar = or(foo, bar); // matches either "foo" or "bar"
+/// assert_eq!(Ok(("", "foo")), foo_or_bar.parse("foo"));
+/// assert_eq!(Ok(("", "bar")), foo_or_bar.parse("bar"));
+/// assert_eq!(Err("baz"), foo_or_bar.parse("baz"));
+/// ```
+pub trait Parser<'a> {
+    type Output;
+
+    fn parse(&self, input: &'a str) -> ParseResult<'a, Self::Output>;
+}
+
+// Most parser-combinators are written as higher-order functions which take some
+// parser(s) as input and return a new parser: a function that takes input and returns
+// a parse result. The underlying implementation of [Parser::parse] for these functions
+// is simply application.
+#[doc(hidden)]
+impl<'a, F, T> Parser<'a> for F
+where
+    F: Fn(&'a str) -> ParseResult<T>,
+{
+    type Output = T;
+
+    fn parse(&self, input: &'a str) -> ParseResult<'a, Self::Output> {
+        self(input)
+    }
+}
+
+/// A parser which matches the string literal exactly.
+///
+/// This parser succeeds if the next characters in the input are equal to the given
+/// string literal.
+///
+/// Note that [str::parse] interferes with calling [Parser::parse] on string literals
+/// directly; this trait implementation works when used within any parser combinator
+/// but does not work on its own. To call [Parser::parse] on a parser for a string
+/// literal, use the [token] parser.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{or, Parser};
+/// let parser = or("foo", "bar");
+/// assert_eq!(Ok(("", "foo")), parser.parse("foo"));
+/// assert_eq!(Ok(("", "bar")), parser.parse("bar"));
+/// assert_eq!(Err("baz"), parser.parse("baz"));
+/// ```
+impl<'a> Parser<'a> for &'static str {
+    type Output = &'a str;
+
+    fn parse(&self, input: &'a str) -> ParseResult<'a, Self::Output> {
+        match input.get(0..self.len()) {
+            Some(actual) if actual == *self => Ok((&input[self.len()..], &input[0..self.len()])),
+            _ => Err(input),
+        }
+    }
+}
+
+// Parsers
+
+/// A parser which matches the given string literally.
+///
+/// This function is a convenience for interpreting string literals as parsers
+/// and is only necessary to avoid conflict with [str::parse]. See the documentation
+/// for the `&'static str` implementation of [Parser].
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{token, Parser};
+/// let parser = token("foo");
+/// assert_eq!(Ok(("", "foo")), parser.parse("foo"));
+/// assert_eq!(Err("bar"), parser.parse("bar"));
+/// ```
+pub fn token<'a>(literal: &'static str) -> impl Parser<'a, Output = &'a str> {
+    literal
+}
+
+/// A parser which matches the pattern described by the given regular expression.
+///
+/// The pattern must match from the beginning of the input as if the regular expression
+/// included the `^` anchor. Using a `^` anchor in the regular expression is
+/// recommended in order to reduce any work done by the regex on non-matching input.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{pattern, Parser};
+/// use regex::Regex;
+/// let regex = Regex::new(r"Hello, \w+!").unwrap();
+/// let parser = pattern(&regex);
+/// assert_eq!(Ok(("", "Hello, world!")), parser.parse("Hello, world!"));
+/// assert_eq!(Err("Hey, you!"), parser.parse("Hey, you!"));
+/// assert_eq!(Err("Oh Hello, world!"), parser.parse("Oh Hello, world!"));
+/// ```
+pub fn pattern<'a>(regex: &'a Regex) -> impl Parser<'a, Output = &'a str> {
+    move |input: &'a str| match regex.find(input) {
+        Some(match_) if match_.start() == 0 => {
+            Ok((&input[match_.end()..], &input[0..match_.end()]))
+        }
+        _ => Err(input),
+    }
+}
+
+/// A parser which matches all values until the specified pattern is found.
+///
+/// If the pattern is not found, this parser does not match. The input up to the
+/// character which returns `true` is returned but not that character itself.
+///
+/// If the pattern function returns true on the first input character, this
+/// parser fails.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{take_until, Parser};
+/// let parser = take_until(|c| c == '.');
+/// assert_eq!(Ok((".bar", "foo")), parser.parse("foo.bar"));
+/// assert_eq!(Err(".foo"), parser.parse(".foo"));
+/// assert_eq!(Err("foo"), parser.parse("foo"));
+/// ```
+pub fn take_until<'a, F>(pattern: F) -> impl Parser<'a, Output = &'a str>
+where
+    F: Fn(char) -> bool,
+{
+    move |input: &'a str| match input.find(&pattern) {
+        Some(index) if index != 0 => Ok((&input[index..], &input[0..index])),
+        _ => Err(input),
+    }
+}
+
+// Variadic parser combinators
+
+/// A parser combinator which matches a sequence of parsers in an all-or-nothing fashion.
+///
+/// The returned value is a tuple containing the outputs of all parsers in order. Each
+/// parser in the sequence may be typed differently.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{seq, Parser};
+/// let parser = seq!("<", "a", ">");
+/// assert_eq!(Ok(("", ("<", "a", ">"))), parser.parse("<a>"));
+/// assert_eq!(Err("<b>"), parser.parse("<b>"));
+/// ```
+#[macro_export]
+macro_rules! seq {
+    ($($parsers: expr),+ $(,)?) => {
+        ($($parsers),+)
+    }
+}
+
+// Seq is implemented using trait-implementations of Parser for various size tuples.
+// This allows sequences to be typed heterogeneously.
+macro_rules! seq_impl {
+    ($($parser:ident),+) => {
+        #[allow(non_snake_case)]
+        impl<'a, $($parser),+> Parser<'a> for ($($parser),+)
+        where
+            $($parser: Parser<'a>),+
+        {
+            type Output = ($($parser::Output),+);
+
+            fn parse(&self, input: &'a str) -> ParseResult<'a, Self::Output> {
+                let ($($parser),+) = self;
+                seq_body_impl!(input, input, $($parser),+ ; )
+            }
+        }
+    }
+}
+
+macro_rules! seq_body_impl {
+    ($input:expr, $next_input:expr, $head:ident, $($tail:ident),+ ; $(,)? $($acc:ident),*) => {
+        match $head.parse($next_input) {
+            Ok((next_input, $head)) => seq_body_impl!($input, next_input, $($tail),+ ; $($acc),*, $head),
+            Err(_) => Err($input),
+        }
+    };
+    ($input:expr, $next_input:expr, $last:ident ; $(,)? $($acc:ident),*) => {
+        match $last.parse($next_input) {
+            Ok((next_input, last)) => Ok((next_input, ($($acc),+, last))),
+            Err(_) => Err($input),
+        }
+    }
+}
+
+seq_impl!(A, B);
+seq_impl!(A, B, C);
+seq_impl!(A, B, C, D);
+seq_impl!(A, B, C, D, E);
+seq_impl!(A, B, C, D, E, F);
+seq_impl!(A, B, C, D, E, F, G);
+seq_impl!(A, B, C, D, E, F, G, H);
+seq_impl!(A, B, C, D, E, F, G, H, I);
+seq_impl!(A, B, C, D, E, F, G, H, I, J);
+
+/// A parser combinator which chooses the first of the input parsers which matches
+/// successfully.
+///
+/// All input parsers must have the same output type. This is a variadic form for [or].
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{choice, or, Parser};
+/// let parser = choice!("foo", "bar", "baz");
+/// assert_eq!(Ok(("", "foo")), parser.parse("foo"));
+/// assert_eq!(Ok(("", "bar")), parser.parse("bar"));
+/// assert_eq!(Err("quiz"), parser.parse("quiz"));
+/// ```
+#[macro_export]
+macro_rules! choice {
+    ($parser: expr $(,)?) => {
+        $parser
+    };
+    ($parser: expr, $($rest: expr),+ $(,)?) => {
+        or($parser, choice!($($rest),+))
+    }
+}
+
+// Ordinary parser combinators
+
+/// A parser combinator which takes a parser as input and maps the output using the
+/// given transformation function.
+///
+/// This corresponds to [Result::map]. The value is only mapped if the input parser
+/// matches against input.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{map, Parser};
+/// let parser = map("123", |s| s.parse::<i32>().unwrap());
+/// assert_eq!(Ok(("", 123)), parser.parse("123"));
+/// assert_eq!(Err("abc"), parser.parse("abc"));
+/// ```
+pub fn map<'a, P, F, T>(parser: P, map_fn: F) -> impl Parser<'a, Output = T>
+where
+    P: Parser<'a>,
+    F: Fn(P::Output) -> T,
+{
+    move |input| {
+        parser
+            .parse(input)
+            .map(|(next_input, result)| (next_input, map_fn(result)))
+    }
+}
+
+/// A parser combinator which succeeds if the given parser matches the input and
+/// the given `filter_map_fn` returns `Some`.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{filter_map, take_until, Parser};
+/// let parser = filter_map(take_until(|c| c == '.'), |s| s.parse::<i32>().ok());
+/// assert_eq!(Ok((".456", 123)), parser.parse("123.456"));
+/// assert_eq!(Err("abc.def"), parser.parse("abc.def"));
+/// ```
+pub fn filter_map<'a, P, F, T>(parser: P, filter_map_fn: F) -> impl Parser<'a, Output = T>
+where
+    P: Parser<'a>,
+    F: Fn(P::Output) -> Option<T>,
+{
+    move |input| match parser.parse(input) {
+        Ok((next_input, value)) => match filter_map_fn(value) {
+            Some(value) => Ok((next_input, value)),
+            None => Err(input),
+        },
+        Err(_) => Err(input),
+    }
+}
+
+/// A parser combinator which succeeds if the first given parser matches the input and
+/// the second given parse also matches.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{reparse_as, take_until, one_or_more, Parser};
+/// let parser = reparse_as(take_until(|c| c == '/'), one_or_more("a"));
+/// assert_eq!(Ok(("/bb", vec!["a", "a"])), parser.parse("aa/bb"));
+/// ```
+pub fn reparse_as<'a, P1, P2, T>(parser1: P1, parser2: P2) -> impl Parser<'a, Output = T>
+where
+    P1: Parser<'a, Output = &'a str>,
+    P2: Parser<'a, Output = T>,
+{
+    filter_map(parser1, move |str| {
+        parser2.parse(str).map(|(_, value)| value).ok()
+    })
+}
+
+/// A parser combinator which only matches the input when the predicate function
+/// returns true.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{filter, take_until, Parser};
+/// let parser = filter(take_until(|c| c == '.'), |s| s == &"123");
+/// assert_eq!(Ok((".456", "123")), parser.parse("123.456"));
+/// assert_eq!(Err("456.123"), parser.parse("456.123"));
+/// ```
+pub fn filter<'a, P, F, T>(parser: P, pred_fn: F) -> impl Parser<'a, Output = T>
+where
+    P: Parser<'a, Output = T>,
+    F: Fn(&P::Output) -> bool,
+{
+    move |input| {
+        if let Ok((next_input, value)) = parser.parse(input) {
+            if pred_fn(&value) {
+                return Ok((next_input, value));
+            }
+        }
+        Err(input)
+    }
+}
+
+/// A parser combinator which matches either of the input parsers.
+///
+/// Both parsers must have the same output type. For a variadic form which
+/// can take any number of parsers, use `choice!`.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{or, Parser};
+/// let parser = or("foo", "bar");
+/// assert_eq!(Ok(("", "foo")), parser.parse("foo"));
+/// assert_eq!(Ok(("", "bar")), parser.parse("bar"));
+/// assert_eq!(Err("baz"), parser.parse("baz"));
+/// ```
+pub fn or<'a, P1, P2, T>(parser1: P1, parser2: P2) -> impl Parser<'a, Output = T>
+where
+    P1: Parser<'a, Output = T>,
+    P2: Parser<'a, Output = T>,
+{
+    move |input| match parser1.parse(input) {
+        ok @ Ok(_) => ok,
+        Err(_) => parser2.parse(input),
+    }
+}
+
+/// A parser combinator which attempts to match the given parser, returning a
+/// `None` output value if the parser does not match.
+///
+/// The parser produced with this combinator always succeeds. If the given parser
+/// succeeds, `Some(value)` is returned where `value` is the output of the given
+/// parser. Otherwise, `None`.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{optional, Parser};
+/// let parser = optional("foo");
+/// assert_eq!(Ok(("bar", Some("foo"))), parser.parse("foobar"));
+/// assert_eq!(Ok(("bar", None)), parser.parse("bar"));
+/// ```
+pub fn optional<'a, P, T>(parser: P) -> impl Parser<'a, Output = Option<T>>
+where
+    P: Parser<'a, Output = T>,
+{
+    move |input| match parser.parse(input) {
+        Ok((next_input, value)) => Ok((next_input, Some(value))),
+        Err(_) => Ok((input, None)),
+    }
+}
+
+/// A parser combinator which runs the given parsers in sequence and returns the
+/// value of `left` if both are matched.
+///
+/// This is useful for two-element sequences in which you only want the output
+/// value of the `left` parser.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{left, Parser};
+/// let parser = left("foo", "bar");
+/// assert_eq!(Ok(("", "foo")), parser.parse("foobar"));
+/// ```
+pub fn left<'a, L, R, T>(left: L, right: R) -> impl Parser<'a, Output = T>
+where
+    L: Parser<'a, Output = T>,
+    R: Parser<'a>,
+{
+    map(seq!(left, right), |(left_value, _)| left_value)
+}
+
+/// A parser combinator which runs the given parsers in sequence and returns the
+/// value of `right` if both are matched.
+///
+/// This is useful for two-element sequences in which you only want the output
+/// value of the `right` parser.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{right, Parser};
+/// let parser = right("foo", "bar");
+/// assert_eq!(Ok(("", "bar")), parser.parse("foobar"));
+/// ```
+pub fn right<'a, L, R, T>(left: L, right: R) -> impl Parser<'a, Output = T>
+where
+    L: Parser<'a>,
+    R: Parser<'a, Output = T>,
+{
+    map(seq!(left, right), |(_, right_value)| right_value)
+}
+
+/// A parser combinator which matches the given parser against the input zero or
+/// more times.
+///
+/// This parser always succeeds and returns the empty Vec when it matched zero
+/// times.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{zero_or_more, Parser};
+/// let parser = zero_or_more("a");
+/// assert_eq!(Ok(("", vec![])), parser.parse(""));
+/// assert_eq!(Ok(("", vec!["a"])), parser.parse("a"));
+/// assert_eq!(Ok(("", vec!["a", "a"])), parser.parse("aa"));
+/// assert_eq!(Ok(("bb", vec![])), parser.parse("bb"));
+/// ```
+pub fn zero_or_more<'a, P, T>(parser: P) -> impl Parser<'a, Output = Vec<T>>
+where
+    P: Parser<'a, Output = T>,
+{
+    move |mut input| {
+        let mut values = Vec::new();
+
+        while let Ok((next_input, value)) = parser.parse(input) {
+            input = next_input;
+            values.push(value);
+        }
+
+        Ok((input, values))
+    }
+}
+
+/// A parser combinator which matches the given parser against the input one or
+/// more times.
+///
+/// This parser combinator acts the same as [zero_or_more] but must match at
+/// least once.
+///
+/// # Examples
+///
+/// ```
+/// use helix_parsec::{one_or_more, Parser};
+/// let parser = one_or_more("a");
+/// assert_eq!(Err(""), parser.parse(""));
+/// assert_eq!(Ok(("", vec!["a"])), parser.parse("a"));
+/// assert_eq!(Ok(("", vec!["a", "a"])), parser.parse("aa"));
+/// assert_eq!(Err("bb"), parser.parse("bb"));
+/// ```
+pub fn one_or_more<'a, P, T>(parser: P) -> impl Parser<'a, Output = Vec<T>>
+where
+    P: Parser<'a, Output = T>,
+{
+    move |mut input| {
+        let mut values = Vec::new();
+
+        match parser.parse(input) {
+            Ok((next_input, value)) => {
+                input = next_input;
+                values.push(value);
+            }
+            Err(err) => return Err(err),
+        }
+
+        while let Ok((next_input, value)) = parser.parse(input) {
+            input = next_input;
+            values.push(value);
+        }
+
+        Ok((input, values))
+    }
+}
+
+/// A parser combinator which matches one or more instances of the given parser
+/// interspersed with the separator parser.
+///
+/// Output values of the separator parser are discarded.
+///
+/// This is typically used to parse function arguments or list items.
+///
+/// # Examples
+///
+/// ```rust
+/// use helix_parsec::{sep, Parser};
+/// let parser = sep("a", ",");
+/// assert_eq!(Ok(("", vec!["a", "a", "a"])), parser.parse("a,a,a"));
+/// ```
+pub fn sep<'a, P, S, T>(parser: P, separator: S) -> impl Parser<'a, Output = Vec<T>>
+where
+    P: Parser<'a, Output = T>,
+    S: Parser<'a>,
+{
+    move |mut input| {
+        let mut values = Vec::new();
+
+        match parser.parse(input) {
+            Ok((next_input, value)) => {
+                input = next_input;
+                values.push(value);
+            }
+            Err(err) => return Err(err),
+        }
+
+        loop {
+            match separator.parse(input) {
+                Ok((next_input, _)) => input = next_input,
+                Err(_) => break,
+            }
+
+            match parser.parse(input) {
+                Ok((next_input, value)) => {
+                    input = next_input;
+                    values.push(value);
+                }
+                Err(_) => break,
+            }
+        }
+
+        Ok((input, values))
+    }
+}