Add TODO.xit

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`.

Cargo.lock

@@ -471,8 +471,10 @@ dependencies = [
  "futures-executor",
  "futures-util",
  "helix-core",
+ "helix-parsec",
  "log",
  "lsp-types",
+ "once_cell",
  "serde",
  "serde_json",
  "thiserror",
@@ -481,6 +483,13 @@ dependencies = [
  "which",
 ]
 
+[[package]]
+name = "helix-parsec"
+version = "0.6.0"
+dependencies = [
+ "regex",
+]
+
 [[package]]
 name = "helix-term"
 version = "0.6.0"

Cargo.toml

@@ -7,6 +7,7 @@ members = [
   "helix-lsp",
   "helix-dap",
   "helix-loader",
+  "helix-parsec",
   "xtask",
 ]
 

TODO.xit

@@ -0,0 +1,9 @@
+[x] Parse snippet syntax
+[x] Generate transaction from snippet
+[ ] Store snippet on the Document
+[ ] Map snippet ranges through Document changes
+[ ] Delete placeholder values on-type in `helix_term::commands::insert::insert`
+[ ] Jump between tabstops with Tab and BackTab
+[ ] Support variables
+[ ] Support variable transforms
+[ ] Support choices as completion items

helix-lsp/Cargo.toml

@@ -13,6 +13,7 @@ homepage = "https://helix-editor.com"
 
 [dependencies]
 helix-core = { version = "0.6", path = "../helix-core" }
+helix-parsec = { version = "0.6", path = "../helix-parsec" }
 
 anyhow = "1.0"
 futures-executor = "0.3"
@@ -25,3 +26,4 @@ thiserror = "1.0"
 tokio = { version = "1.21", features = ["rt", "rt-multi-thread", "io-util", "io-std", "time", "process", "macros", "fs", "parking_lot", "sync"] }
 tokio-stream = "0.1.11"
 which = "4.2"
+once_cell = "1.15"

helix-lsp/src/client.rs

@@ -303,7 +303,7 @@ pub(crate) async fn initialize(&self) -> Result<lsp::InitializeResult> {
                 text_document: Some(lsp::TextDocumentClientCapabilities {
                     completion: Some(lsp::CompletionClientCapabilities {
                         completion_item: Some(lsp::CompletionItemCapability {
-                            snippet_support: Some(false),
+                            snippet_support: Some(true),
                             resolve_support: Some(lsp::CompletionItemCapabilityResolveSupport {
                                 properties: vec![
                                     String::from("documentation"),

helix-lsp/src/lib.rs

@@ -1,5 +1,6 @@
 mod client;
 pub mod jsonrpc;
+pub mod snippet;
 mod transport;
 
 pub use client::Client;

helix-lsp/src/snippet.rs

@@ -0,0 +1,440 @@
+use anyhow::{anyhow, Result};
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum CaseChange {
+    Upcase,
+    Downcase,
+    Capitalize,
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum FormatItem<'a> {
+    Text(&'a str),
+    Capture(usize),
+    CaseChange(usize, CaseChange),
+    Conditional(usize, Option<&'a str>, Option<&'a str>),
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub struct Regex<'a> {
+    value: &'a str,
+    replacement: Vec<FormatItem<'a>>,
+    options: Option<&'a str>,
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum SnippetElement<'a> {
+    Tabstop {
+        tabstop: usize,
+    },
+    Placeholder {
+        tabstop: usize,
+        value: Box<SnippetElement<'a>>,
+    },
+    Choice {
+        tabstop: usize,
+        choices: Vec<&'a str>,
+    },
+    Variable {
+        name: &'a str,
+        default: Option<&'a str>,
+        regex: Option<Regex<'a>>,
+    },
+    Text(&'a str),
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub struct Snippet<'a> {
+    elements: Vec<SnippetElement<'a>>,
+}
+
+pub fn parse<'a>(s: &'a str) -> Result<Snippet<'a>> {
+    parser::parse(s).map_err(|rest| anyhow!("Failed to parse snippet. Remaining input: {}", rest))
+}
+
+pub fn into_transaction<'a>(
+    snippet: Snippet<'a>,
+    text: &helix_core::Rope,
+    trigger_offset: usize,
+) -> helix_core::Transaction {
+    use helix_core::{smallvec, Range, Selection, Transaction};
+    use SnippetElement::*;
+
+    let mut insert = String::new();
+    let mut offset = trigger_offset;
+    let mut tabstops: Vec<Range> = Vec::new();
+
+    for element in snippet.elements {
+        match element {
+            Text(text) => {
+                offset += text.chars().count();
+                insert.push_str(text)
+            }
+            Variable {
+                name: _name,
+                regex: None,
+                r#default,
+            } => {
+                // TODO: variables. For now, fall back to the default, which defaults to "".
+                let text = r#default.unwrap_or_default();
+                offset += text.chars().count();
+                insert.push_str(text);
+            }
+            Tabstop { .. } => {
+                // TODO: tabstop indexing: 0 is final cursor position. 1,2,.. are positions.
+                // TODO: merge tabstops with the same index
+                tabstops.push(Range::point(offset));
+            }
+            Placeholder {
+                tabstop: _tabstop,
+                value,
+            } => match value.as_ref() {
+                // https://doc.rust-lang.org/beta/unstable-book/language-features/box-patterns.html
+                // would make this a bit nicer
+                Text(text) => {
+                    let len_chars = text.chars().count();
+                    tabstops.push(Range::new(offset, offset + len_chars));
+                    offset += len_chars;
+                    insert.push_str(text);
+                }
+                other => {
+                    log::error!(
+                            "Discarding snippet: generating a transaction for placeholder contents {:?} is unimplemented.",
+                            other
+                        );
+                    return Transaction::new(text);
+                }
+            },
+            other => {
+                log::error!(
+                    "Discarding snippet: generating a transaction for {:?} is unimplemented.",
+                    other
+                );
+                return Transaction::new(text);
+            }
+        }
+    }
+
+    let transaction = Transaction::change(
+        text,
+        std::iter::once((trigger_offset, trigger_offset, Some(insert.into()))),
+    );
+
+    if let Some(first) = tabstops.first() {
+        transaction.with_selection(Selection::new(smallvec![*first], 0))
+    } else {
+        transaction
+    }
+}
+
+mod parser {
+    use helix_core::regex;
+    use once_cell::sync::Lazy;
+
+    use helix_parsec::*;
+
+    use super::{CaseChange, FormatItem, Regex, Snippet, SnippetElement};
+
+    /*
+    https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#snippet_syntax
+
+        any         ::= tabstop | placeholder | choice | variable | text
+        tabstop     ::= '$' int | '${' int '}'
+        placeholder ::= '${' int ':' any '}'
+        choice      ::= '${' int '|' text (',' text)* '|}'
+        variable    ::= '$' var | '${' var }'
+                        | '${' var ':' any '}'
+                        | '${' var '/' regex '/' (format | text)+ '/' options '}'
+        format      ::= '$' int | '${' int '}'
+                        | '${' int ':' '/upcase' | '/downcase' | '/capitalize' '}'
+                        | '${' int ':+' if '}'
+                        | '${' int ':?' if ':' else '}'
+                        | '${' int ':-' else '}' | '${' int ':' else '}'
+        regex       ::= Regular Expression value (ctor-string)
+        options     ::= Regular Expression option (ctor-options)
+        var         ::= [_a-zA-Z] [_a-zA-Z0-9]*
+        int         ::= [0-9]+
+        text        ::= .*
+        if          ::= text
+        else        ::= text
+    */
+
+    static DIGIT: Lazy<regex::Regex> = Lazy::new(|| regex::Regex::new(r"^[0-9]+").unwrap());
+    static VARIABLE: Lazy<regex::Regex> =
+        Lazy::new(|| regex::Regex::new(r"^[_a-zA-Z][_a-zA-Z0-9]*").unwrap());
+    static TEXT: Lazy<regex::Regex> = Lazy::new(|| regex::Regex::new(r"^[^\$]+").unwrap());
+
+    fn var<'a>() -> impl Parser<'a, Output = &'a str> {
+        pattern(&VARIABLE)
+    }
+
+    fn digit<'a>() -> impl Parser<'a, Output = usize> {
+        filter_map(pattern(&DIGIT), |s| s.parse().ok())
+    }
+
+    fn case_change<'a>() -> impl Parser<'a, Output = CaseChange> {
+        use CaseChange::*;
+
+        choice!(
+            map("upcase", |_| Upcase),
+            map("downcase", |_| Downcase),
+            map("capitalize", |_| Capitalize),
+        )
+    }
+
+    fn format<'a>() -> impl Parser<'a, Output = FormatItem<'a>> {
+        use FormatItem::*;
+
+        choice!(
+            // '$' int
+            map(right("$", digit()), Capture),
+            // '${' int '}'
+            map(seq!("${", digit(), "}"), |seq| Capture(seq.1)),
+            // '${' int ':' '/upcase' | '/downcase' | '/capitalize' '}'
+            map(seq!("${", digit(), ":/", case_change(), "}"), |seq| {
+                CaseChange(seq.1, seq.3)
+            }),
+            // '${' int ':+' if '}'
+            map(
+                seq!("${", digit(), ":+", take_until(|c| c == '}'), "}"),
+                |seq| { Conditional(seq.1, Some(seq.3), None) }
+            ),
+            // '${' int ':?' if ':' else '}'
+            map(
+                seq!(
+                    "${",
+                    digit(),
+                    ":?",
+                    take_until(|c| c == ':'),
+                    ":",
+                    take_until(|c| c == '}'),
+                    "}"
+                ),
+                |seq| { Conditional(seq.1, Some(seq.3), Some(seq.5)) }
+            ),
+            // '${' int ':-' else '}' | '${' int ':' else '}'
+            map(
+                seq!(
+                    "${",
+                    digit(),
+                    ":",
+                    optional("-"),
+                    take_until(|c| c == '}'),
+                    "}"
+                ),
+                |seq| { Conditional(seq.1, None, Some(seq.4)) }
+            ),
+            // Any text
+            map(pattern(&TEXT), Text),
+        )
+    }
+
+    fn regex<'a>() -> impl Parser<'a, Output = Regex<'a>> {
+        let replacement = reparse_as(take_until(|c| c == '/'), one_or_more(format()));
+
+        map(
+            seq!(
+                "/",
+                take_until(|c| c == '/'),
+                "/",
+                replacement,
+                "/",
+                optional(take_until(|c| c == '}')),
+            ),
+            |(_, value, _, replacement, _, options)| Regex {
+                value,
+                replacement,
+                options,
+            },
+        )
+    }
+
+    fn tabstop<'a>() -> impl Parser<'a, Output = SnippetElement<'a>> {
+        map(
+            or(
+                right("$", digit()),
+                map(seq!("${", digit(), "}"), |values| values.1),
+            ),
+            |digit| SnippetElement::Tabstop { tabstop: digit },
+        )
+    }
+
+    fn placeholder<'a>() -> impl Parser<'a, Output = SnippetElement<'a>> {
+        // TODO: why doesn't parse_as work?
+        // let value = reparse_as(take_until(|c| c == '}'), anything());
+        let value = filter_map(take_until(|c| c == '}'), |s| {
+            anything().parse(s).map(|parse_result| parse_result.1).ok()
+        });
+
+        map(seq!("${", digit(), ":", value, "}"), |seq| {
+            SnippetElement::Placeholder {
+                tabstop: seq.1,
+                value: Box::new(seq.3),
+            }
+        })
+    }
+
+    fn choice<'a>() -> impl Parser<'a, Output = SnippetElement<'a>> {
+        map(
+            seq!(
+                "${",
+                digit(),
+                "|",
+                sep(take_until(|c| c == ',' || c == '|'), ","),
+                "|}",
+            ),
+            |seq| SnippetElement::Choice {
+                tabstop: seq.1,
+                choices: seq.3,
+            },
+        )
+    }
+
+    fn variable<'a>() -> impl Parser<'a, Output = SnippetElement<'a>> {
+        choice!(
+            // $var
+            map(right("$", var()), |name| SnippetElement::Variable {
+                name,
+                default: None,
+                regex: None,
+            }),
+            // ${var:default}
+            map(
+                seq!("${", var(), ":", take_until(|c| c == '}'), "}",),
+                |values| SnippetElement::Variable {
+                    name: values.1,
+                    default: Some(values.3),
+                    regex: None,
+                }
+            ),
+            // ${var/value/format/options}
+            map(seq!("${", var(), regex(), "}"), |values| {
+                SnippetElement::Variable {
+                    name: values.1,
+                    default: None,
+                    regex: Some(values.2),
+                }
+            }),
+        )
+    }
+
+    fn text<'a>() -> impl Parser<'a, Output = SnippetElement<'a>> {
+        map(pattern(&TEXT), SnippetElement::Text)
+    }
+
+    fn anything<'a>() -> impl Parser<'a, Output = SnippetElement<'a>> {
+        choice!(tabstop(), placeholder(), choice(), variable(), text())
+    }
+
+    fn snippet<'a>() -> impl Parser<'a, Output = Snippet<'a>> {
+        map(one_or_more(anything()), |parts| Snippet { elements: parts })
+    }
+
+    pub fn parse(s: &str) -> Result<Snippet, &str> {
+        snippet().parse(s).map(|(_input, elements)| elements)
+    }
+
+    #[cfg(test)]
+    mod test {
+        use super::SnippetElement::*;
+        use super::*;
+
+        #[test]
+        fn empty_string_is_error() {
+            assert_eq!(Err(""), parse(""));
+        }
+
+        #[test]
+        fn parse_placeholders_in_function_call() {
+            assert_eq!(
+                Ok(Snippet {
+                    elements: vec![
+                        Text("match("),
+                        Placeholder {
+                            tabstop: 1,
+                            value: Box::new(Text("Arg1")),
+                        },
+                        Text(")")
+                    ]
+                }),
+                parse("match(${1:Arg1})")
+            )
+        }
+
+        #[test]
+        fn parse_placeholders_in_statement() {
+            assert_eq!(
+                Ok(Snippet {
+                    elements: vec![
+                        Text("local "),
+                        Placeholder {
+                            tabstop: 1,
+                            value: Box::new(Text("var")),
+                        },
+                        Text(" = "),
+                        Placeholder {
+                            tabstop: 1,
+                            value: Box::new(Text("value")),
+                        },
+                    ]
+                }),
+                parse("local ${1:var} = ${1:value}")
+            )
+        }
+
+        #[test]
+        fn parse_all() {
+            assert_eq!(
+                Ok(Snippet {
+                    elements: vec![
+                        Text("hello "),
+                        Tabstop { tabstop: 1 },
+                        Tabstop { tabstop: 2 },
+                        Text(" "),
+                        Choice {
+                            tabstop: 1,
+                            choices: vec!["one", "two", "three"]
+                        },
+                        Text(" "),
+                        Variable {
+                            name: "name",
+                            default: Some("foo"),
+                            regex: None
+                        },
+                        Text(" "),
+                        Variable {
+                            name: "var",
+                            default: None,
+                            regex: None
+                        },
+                        Text(" "),
+                        Variable {
+                            name: "TM",
+                            default: None,
+                            regex: None
+                        },
+                    ]
+                }),
+                parse("hello $1${2} ${1|one,two,three|} ${name:foo} $var $TM")
+            );
+        }
+
+        #[test]
+        fn regex_capture_replace() {
+            assert_eq!(
+                Ok(Snippet {
+                    elements: vec![Variable {
+                        name: "TM_FILENAME",
+                        default: None,
+                        regex: Some(Regex {
+                            value: "(.*).+$",
+                            replacement: vec![FormatItem::Capture(1)],
+                            options: None,
+                        }),
+                    }]
+                }),
+                parse("${TM_FILENAME/(.*).+$/$1/}")
+            );
+        }
+    }
+}

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"

helix-parsec/src/lib.rs

@@ -0,0 +1,551 @@
+//! 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_fun` 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),
+    }
+}
+
+/// TODO: name
+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))
+    }
+}

helix-term/src/ui/completion.rs

@@ -109,32 +109,55 @@ fn item_to_transaction(
                 start_offset: usize,
                 trigger_offset: usize,
             ) -> Transaction {
-                let transaction = if let Some(edit) = &item.text_edit {
-                    let edit = match edit {
-                        lsp::CompletionTextEdit::Edit(edit) => edit.clone(),
-                        lsp::CompletionTextEdit::InsertAndReplace(item) => {
-                            unimplemented!("completion: insert_and_replace {:?}", item)
+                use helix_lsp::snippet;
+
+                match item {
+                    CompletionItem {
+                        text_edit: Some(edit),
+                        ..
+                    } => {
+                        let edit = match edit {
+                            lsp::CompletionTextEdit::Edit(edit) => edit.clone(),
+                            lsp::CompletionTextEdit::InsertAndReplace(item) => {
+                                unimplemented!("completion: insert_and_replace {:?}", item)
+                            }
+                        };
+
+                        util::generate_transaction_from_edits(
+                            doc.text(),
+                            vec![edit],
+                            offset_encoding, // TODO: should probably transcode in Client
+                        )
+                    }
+                    CompletionItem {
+                        insert_text: Some(insert_text),
+                        insert_text_format: Some(lsp::InsertTextFormat::SNIPPET),
+                        ..
+                    } => match snippet::parse(insert_text) {
+                        Ok(snippet) => {
+                            snippet::into_transaction(snippet, doc.text(), trigger_offset)
                         }
-                    };
-
-                    util::generate_transaction_from_edits(
-                        doc.text(),
-                        vec![edit],
-                        offset_encoding, // TODO: should probably transcode in Client
-                    )
-                } else {
-                    let text = item.insert_text.as_ref().unwrap_or(&item.label);
-                    // Some LSPs just give you an insertText with no offset ¯\_(ツ)_/¯
-                    // in these cases we need to check for a common prefix and remove it
-                    let prefix = Cow::from(doc.text().slice(start_offset..trigger_offset));
-                    let text = text.trim_start_matches::<&str>(&prefix);
-                    Transaction::change(
-                        doc.text(),
-                        vec![(trigger_offset, trigger_offset, Some(text.into()))].into_iter(),
-                    )
-                };
-
-                transaction
+                        Err(err) => {
+                            log::error!(
+                                "Failed to parse snippet: {:?}, remaining output: {}",
+                                insert_text,
+                                err
+                            );
+                            Transaction::new(doc.text())
+                        }
+                    },
+                    _ => {
+                        let text = item.insert_text.as_ref().unwrap_or(&item.label);
+                        // Some LSPs just give you an insertText with no offset ¯\_(ツ)_/¯
+                        // in these cases we need to check for a common prefix and remove it
+                        let prefix = Cow::from(doc.text().slice(start_offset..trigger_offset));
+                        let text = text.trim_start_matches::<&str>(&prefix);
+                        Transaction::change(
+                            doc.text(),
+                            vec![(trigger_offset, trigger_offset, Some(text.into()))].into_iter(),
+                        )
+                    }
+                }
             }
 
             fn completion_changes(transaction: &Transaction, trigger_offset: usize) -> Vec<Change> {