#lemon #parser #generator #lalr1


Famous Lemon Parser Generator implemented in rust as library with API

5 releases

0.1.4 Feb 3, 2021
0.1.3 Aug 20, 2020
0.1.2 Aug 19, 2020
0.1.1 Aug 17, 2020
0.1.0 Aug 16, 2020

#28 in Parser tooling

21 downloads per month
Used in 2 crates (via lemon-tree-derive)

MIT license

3.5K SLoC


Famous Lemon Parser Generator implemented in rust as library with API.


extern crate lemon_mint;

use std::fs::File;
use std::sync::Arc;
use lemon_mint::LemonMintBuilder;

fn main()
{	let builder = LemonMintBuilder::new().load_y
	(	&Arc::new("source.y".to_string()), // fake source name, that will appear in error messages
		"	%trace {>> }
			%extra_argument {()}
			%left PLUS MINUS.
			%left TIMES DIVIDE.
			%token_type {f64}
			%type Expr {super::Expr}
			%type Exprs {Vec<super::Expr>}
			%type Program {Vec<super::Expr>}

			Program ::= Exprs(exprs). exprs
			Program ::= Exprs(exprs) NEW_LINE. exprs

			Exprs ::= Expr(item).                       vec![item]
			Exprs ::= Exprs(items) NEW_LINE Expr(item). let mut items = items; items.push(item); items

			Expr ::= NUM(value). super::Expr {value}
			Expr ::= PAR_OPEN Expr(a) PAR_CLOSE. a
			Expr ::= PLUS Expr(a). a
			Expr ::= MINUS Expr(a). let mut a = a; a.value = -a.value; a
			Expr ::= Expr(a) PLUS Expr(b). super::Expr{value: a.value + b.value}
			Expr ::= Expr(a) MINUS Expr(b). super::Expr{value: a.value - b.value}
			Expr ::= Expr(a) TIMES Expr(b). super::Expr{value: a.value * b.value}
			Expr ::= Expr(a) DIVIDE Expr(b). super::Expr{value: a.value / b.value}

			%code {
				use code::{Parser, Token};

                #[derive(Debug, PartialEq)]
				pub struct Expr {value: f64}

				fn main()
				{	let mut parser = Parser::new(()); // () is our extra argument, that will be accessible in actions, and also through parser.extra

					parser.add_token(Token::NUM, 15.0).unwrap();
					parser.add_token(Token::DIVIDE, 0.0).unwrap();
					parser.add_token(Token::NUM, 5.0).unwrap();
					parser.add_token(Token::NEW_LINE, 0.0).unwrap();

					let result = parser.end().unwrap(); // if Program
                    assert_eq!(result, vec![Expr {value: 3.0}]);
                    println!(\"Result: {:?}\", result);

	let lemon = builder.try_into_lemon().unwrap();
	let mut out_rust = File::create("/tmp/main.rs").unwrap();
	let mut out_y = File::create("/tmp/main.y").unwrap();
	lemon.gen_rust(&mut out_rust).unwrap();
	lemon.gen_log(&mut out_y, false, false).unwrap();

The first step is to create LemonMintBuilder object, and use it's methods to describe the desired parser. It's possible to load "y"-grammar from a file with load_y_file(), or from a string with load_y(), as shown in the example above, or you can set each parser rule with individual methods, like set_start_symbol(), set_token_type(), add_type(), add_rule(), and others.

Then the builder object can be converted to LemonMint object that represents the parser transition tables. This step is done with the try_into_lemon() method, as shown above.

And the last step is to generate a rust file with parser tables and it's driver code. This is done with gen_rust() method. Optionally you can generate log file with gen_log(), as classic Lemon program does.


Lemon-mint uses y-grammar similar to what classic Lemon parser uses. There are a few distinctions:

  • If a symbol name contains at least one lowercase letter, it's considered nonterminal. In classic Lemon only the first letter matters.
  • To enable trace, use %trace directive. In Lemon need to call ParseTrace() function.
  • No need for %name directive, because in rust each file is separate module.
  • No need for destructors.

The generated parser will be separated to 2 submodules: code and rules. There are only 2 interesting things in code module: Parser and Token. See in above example how they're used. The rules module wraps actions, and you don't need to use it directly. If your actions use types, constants or functions global space, they can be accessed like super::Expr, or crate::Expr, or so.

The following directives are supported:

  • %token_type
  • %type
  • %default_type
  • %start_symbol
  • %trace
  • %extra_argument - only it's type. The name is always extra. Default type is ().
  • %left
  • %right
  • %nonassoc
  • %fallback
  • %code or %include - are the same

The syntax is free and permissive:

%start_symbol {Unit}
/* or */
%start_symbol Unit.
/* or */
%start_symbol Unit

Braces allow to specify multiline value, till matching closing brace. Supported line and multiline C-style comments.

No runtime deps


  • with-debug