Easiest way to get started is through gll-macros:

[dependencies]
gll = "0.0.2"
gll-macros = "0.0.2"

As an example, this is what you might write for a JSON-like syntax, that uses plain identifiers instead of string literals for field names, and allows values to be parenthesized Rust expressions:

mod json_like {
    gll_macros::proc_macro_parser! {
        Value =
            | Null:"null"
            | False:"false"
            | True:"true"
            | Literal:LITERAL
            | Array:{ "[" elems:Value* % "," "]" }
            | Object:{ "{" fields:Field* % "," "}" }
            | InterpolateRust:{ "(" TOKEN_TREE  ")" }
            ;
        Field = name:IDENT ":" value:Value;
    }
}

You can also use a build script to generate the parser (TODO: document).

To parse a string with that grammar:

let tokens = string.parse::<gll::grammer::proc_macro::TokenStream>().unwrap();
json_like::Value::parse(tokens).unwrap().with(|value| {
    // ...
});

All grammars contain a set of named rules, with the syntax Name = rule;. (the order between the rules doesn't matter)

Rules are made out of:

• grouping, using {...}
• string literals, matching input characters / tokens exactly
• character ranges: 'a'..='d' is equivalent to "a"|"b"|"c"|"d"
  • only in scannerless mode
• builtin rules: IDENT, PUNCT, LITERAL, TOKEN_TREE
  • only in proc macro mode
• named rules, referred to by their name
• concatenation: A B - "A followed by B"
• alternation: A | B - "either A or B"
• optionals: A? - "either A or nothing"
• lists: A* - "zero or more As", A - "one or more As"
  • optional separator: A* % "," - "comma-separated As"
  • variant: A* %% "," - "comma-separated As", with an optional trailing comma

Parts of a rule can be labeled with field names, to allow later access to them:

LetDecl = "let" pat:Pat { "=" init:Expr }? ";"; produces:

// Note: generic parameters omitted for brevity.
struct LetDecl {
    pat: Handle<Pat>,
    init: Option<Handle<Expr>>,
}

One Rust-specific convention is that alternation fields are enum variants.

Expr = Lit:LITERAL | Add:{ a:Expr " " b:Expr }; produces:

enum Expr {
    Lit(Handle<LITERAL>),
    Add {
        a: Handle<Expr>,
        b: Handle<Expr>,
    },
}

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.