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0.2.0	Aug 16, 2024

#2980 in Rust patterns

Used in peggen

MIT/Apache

26KB
522 lines

Peggen

A parser generator for parsing expression grammar (PEG) that use inline macros to specify PEG operations.

How is it different from (...)?

/	Conceptual	User Experience	Performance	Error Handling
PEST	PEST only annotates text. Peggen generates AST directly from your text.	In most cases, you still want rust `enum`s for your AST, which is directly provided by Peggen, but you have to manually create `enums` from PEST rules.	PEST use an optimizer to memorize your grammar rules, and use memorization for better performance; Peggen doesn't use memorization, arguably this gives better performance over memorization for most grammars.	/
Chumsky	Chumsky provides parser combinators. Peggen is a parser generator.	Both Chumsky and Peggen provides ast directly. However, Peggen supports arena allocation.	Chumsky deallocates successful sub-rules when a rule fails; Peggen uses a internal representation to eliminate deallocation.	/
LALRPOP	Peggen is PEG-based; LALRPOP uses LR(1) grammar.	Peggen is more intuitive to use than LALRPOP; LR(1) grammar is hard to extend and debug.	LALRPOP has better performance over Peggen.	LR(1) grammar can report errors far away from normally percepted cause; Peggen allows you to capture errors from customary cause.

Performance

I roughly tested the peggen on a sample json file against chumsky.

CPU Model: Intel(R) Core(TM) i7-14700HX

Suprisingly, Peggen is faster than Chumsky.

Here are some numbers:

Peggen : 867913 ns/iter
Chumsky: 1555256 ns/iter

Example: Json Parser

You can write a json parser in the following several lines:

#[derive(Debug, ParseImpl, Space, Num, EnumAstImpl)]
pub enum Json {
    #[rule(r"null")]
    Null,
    #[rule(r"{0:`false|true`}")]
    Bool(bool),
    #[rule(r"{0:`-?(0|[1-9][0-9]*)\.([0-9]+)`}")]
    Flt(f32),
    #[rule("{0:`0|-?[1-9][0-9]*`}")]
    Num(i32),
    #[rule(r#""{0:`[^"]*`}""#)]
    Str(String),
    #[rule(r#"\{ [*0: "{0:`[^"]*`}" : {1} , ][?0: "{0:`[^"]*`}" : {1} ] \}"#)]
    Obj(RVec<(String, Json)>),
    #[rule(r"\[ [*0: {0} , ][?0: {0} ] \]")]
    Arr(RVec<Json>)
}

Roadmap

Optimizations:
- Rule dispatch: filter rules by the first symbol, instead of trying each of them.
- Thinner tag: currently each tag in internal representation is 3-pointers wide, I want to make them thinner.
Error Handling:
- Custom error handlers when error handlers fail.

Dependencies

~4–12MB
~136K SLoC