rusty_lr

A Bison-like Parser generator & Compiler frontend for Rust generating optimised IELR(1), LALR(1) parser tables, with deterministic LR and non-deterministic LR (GLR) parsing.

RustyLR is a parser generator that converts context-free grammars into IELR(1)/LALR(1) tables and supporting deterministic LR and non-deterministic GLR parsing strategies. It supports custom reduce actions in Rust, with beautiful diagnostics. Highly inspired by tools like bison, it uses a similar syntax while integrating seamlessly with Rust's ecosystem. It constructs optimized state machines, ensuring efficient and reliable parsing.

Features

• Custom Reduce Actions: Define custom actions in Rust, allowing you to build custom data structures easily.
• Automatic Optimization: Reduces parser table size and improves performance by grouping terminals with identical behavior across parser states.
• Multiple Parsing Strategies: Supports minimal-LR(1), LALR(1) parser tables, and GLR parsing strategy.
• Detailed Diagnostics: Detects grammar conflicts, verbose conflict resolution stages, and optimization stages.
• Static & Runtime Conflict Resolution: Provides mechanisms to resolve conflicts at compile time or runtime.
• Location Tracking: Tracks the location of every token in the parse tree, useful for error reporting and debugging.
• State Machine Debugging: The rustylr executable provides a --state option that allows you to debug and visualize the generated state machine. This is useful for understanding how the parser will behave and for identifying potential issues in the grammar.

Quick Start: Using the rustylr Executable

The recommended way to use RustyLR is with the standalone rustylr executable. It's faster, provides richer grammar diagnostics, and includes commands for debugging state machines directly.

Here is a step-by-step guide to get you started.

1. Add rusty_lr to your dependencies

First, add the rusty_lr runtime library to your project's Cargo.toml. The generated parser code will depend on it.

[dependencies]
rusty_lr = "..." # Use the same version as the executable

2. Install the rustylr executable

You can install the executable from crates.io using cargo:

cargo install rustylr

3. Create a grammar file

Create a file named src/grammar.rs. This file will contain your token definitions and grammar rules. Any Rust code above the %% separator will be copied directly to the generated output file.

// src/grammar.rs
// This code is copied to the generated file.
pub enum MyToken {
    Num(i32),
    Plus,
}

%% // Grammar rules start here.

%tokentype MyToken;
%start E;
%left plus; // Specify left-associativity for the 'plus' token.

// Define tokens and how they map to MyToken variants.
%token num MyToken::Num(_);
%token plus MyToken::Plus;

// Define grammar rules and their return types.
// E(i32) means the non-terminal E returns an i32.
// In the action blocks `{ ... }`, you can refer to the values of symbols
// on the right-hand side by their names (e.g., `e1`, `e2`, `num`).
E(i32): e1=E plus e2=E { e1 + e2 }
      | num { let MyToken::Num(num) = num else { unreachable!(); };
        num
      }
      ;

4. Generate the parser code

Run the rustylr executable to process your grammar file. This command will generate src/parser.rs from src/grammar.rs.

rustylr src/grammar.rs src/parser.rs

5. Use the generated parser in your code

Finally, include the newly generated src/parser.rs as a module in your main.rs or lib.rs and use it to parse a token stream.

// In src/main.rs

// Include the generated parser module.
mod parser;
// Bring the token enum into scope.
use parser::MyToken;

fn main() {
    // Example token stream for "1 + 2"
    let tokens = vec![MyToken::Num(1), MyToken::Plus, MyToken::Num(2)]; 

    let parser = parser::EParser::new();        // Assumes 'E' is your start symbol
    let mut context = parser::EContext::new();
    let mut userdata = (); // No userdata in this example.

    for token in tokens {
        match context.feed(&parser, token, &mut userdata) {
            Ok(_) => {}
            Err(e) => {
                eprintln!("Parse error: {}", e);
                return;
            }
        }
    }

    // Get the final parsed result.
    match context.accept(&parser) {
        Ok(result) => {
            let final_result: i32 = result;
            println!("Parsed result: {}", final_result); // Should print "3"
        },
        Err(e) => {
            eprintln!("Failed to produce a final result: {}", e);
        }
    }
}

Important: Ensure the version of the rustylr executable you run matches the version of the rusty_lr crate in your Cargo.toml. Mismatched versions can lead to build errors.

Generated Code Structure

The generated code will include several structs and enums:

• <Start>Parser: A struct that holds the parser table. (LR docs) (GLR docs)

• <Start>Context: A struct that maintains the current parsing state and symbol values. (LR docs) (GLR docs)

• <Start>State: A type representing a parser state and its associated table.

• <Start>Rule: A type representing a production rule. (docs)

• <Start>NonTerminals: An enum representing all non-terminal symbols in the grammar. (docs)

Working with Context

You can also get contextual information from the <Start>Context struct:

let mut context = <Start>Context::new();

// ... parsing ...

context.expected_token();    // Get expected (terminal, non-terminal) symbols for current state
context.can_feed(&term);     // Check if a terminal symbol can be fed
context.trace();             // Get all `%trace` non-terminals currently being parsed
println!("{}", context.backtrace()); // Print backtrace of the parser state
println!("{}", context);     // Print tree structure of the parser state (`tree` feature)

The Feed Method

The generated code includes a feed method that processes tokens:

context.feed(&parser, term, &mut userdata); // Feed a terminal symbol and update the state machine
context.feed_location(&parser, term, &mut userdata, term_location); // Feed a terminal symbol with location tracking

This method returns Ok(()) if the token was successfully parsed, or an Err if there was an error.

Note: The actual method signatures differ slightly when building a GLR parser.

GLR Parsing

RustyLR offers built-in support for Generalized LR (GLR) parsing, enabling it to handle ambiguous or nondeterministic grammars that traditional LR(1) or LALR(1) parsers cannot process. See GLR.md for details.

Error Handling and Conflict Resolution

RustyLR provides multiple mechanisms for handling semantic errors and resolving conflicts during parsing:

• Panic Mode Error Recovery: Use the error token for panic-mode error recovery
• Operator Precedence: Set precedence with %left, %right, %precedence for terminals
• Reduce Rule Priority: Set priority with %dprec for production rules
• Runtime Errors: Return Err from reduce actions to handle semantic errors

See SYNTAX.md - Resolving Conflicts for detailed information.

Location Tracking

Track the location of tokens and non-terminals for better error reporting and debugging:

Expr: exp1=Expr '+' exp2=Expr {
    println!("Location of exp1: {:?}", @exp1);
    println!("Location of exp2: {:?}", @exp2);
    println!("Location of this expression: {:?}", @$); // @$ is the location of the non-terminal itself
    exp1 + exp2
}
| Expr error Expr {
    println!("Error at: {:?}", @error); // @error is the location of the error token
    0 // Return a default value
}

See SYNTAX.md - Location Tracking for detailed information.

State Machine Debugging

The rustylr executable includes a powerful --state option for debugging the generated parser's state machine. This feature allows you to inspect the details of each state, including its production rules, expected tokens, and transitions to other states. It is an invaluable tool for diagnosing grammar ambiguities, understanding shift/reduce conflicts, and verifying that the parser behaves as expected.

To use it, run rustylr with the --state flag, followed by your grammar file:

rustylr --state src/grammar.rs

This will output a detailed, color-coded representation of the state machine directly in your terminal, making it easy to trace the parser's logic.

This visualization helps you understand the parsing process step-by-step and is particularly useful for debugging complex grammars.

Examples

• Calculator (enum version): A numeric expression parser using custom token enums
• Calculator (u8 version): A numeric expression parser using byte tokens
• JSON Validator: A JSON syntax validator
• Lua 5.4 syntax parser: A complete Lua language parser
• C language parser: A C language parser
• Bootstrap parser: RustyLR's own syntax parser is written in RustyLR itself

Cargo Features

• build: Enables build script tools for generating parsers at compile time.
• tree: Enables automatic syntax tree construction for debugging purposes. Makes Context implement Display for pretty-printing.

Grammar Syntax

RustyLR's grammar syntax is inspired by traditional Yacc/Bison formats. See SYNTAX.md for detailed grammar definition syntax.

Contributing

Contributions are welcome! Please feel free to open an issue or submit a pull request.

Project Structure

This project is organized as a Cargo workspace with the following crates:

• rusty_lr/: The main end-user library that provides the public API. This is what users add to their Cargo.toml.
• rusty_lr_core/: Core parsing engine containing the fundamental data structures, algorithms, and runtime components for both deterministic (src/parser/deterministic) and non-deterministic (src/parser/nondeterministic) parsing.
• rusty_lr_parser/: The main code generation engine that parses RustyLR's grammar syntax, builds parser tables, and generates the actual parser code. This is the core of the parser generation process.
• rusty_lr_derive/: Procedural macro interface that wraps rusty_lr_parser to provide the lr1! macro for inline grammar definitions.
• rusty_lr_buildscript/: Build script interface that wraps rusty_lr_parser for generating parser code at compile time when using the build feature.
• rusty_lr_executable/: Standalone rustylr executable for command-line parser generation.
• scripts/: Development and testing scripts

The crates have the following dependency relationships:

• rusty_lr depends on rusty_lr_core, rusty_lr_derive, and rusty_lr_buildscript (optional)
• rusty_lr_derive and rusty_lr_buildscript depend on rusty_lr_parser
• rusty_lr_parser depends on rusty_lr_core
• rusty_lr_executable depends on rusty_lr_buildscript

graph TD;
    subgraph User Facing
        rusty_lr;
        rusty_lr_executable;
    end

    subgraph Internal
        rusty_lr_derive;
        rusty_lr_buildscript;
        rusty_lr_parser;
        rusty_lr_core;
    end

    rusty_lr --> rusty_lr_core;
    rusty_lr --> rusty_lr_derive;
    rusty_lr --> rusty_lr_buildscript;

    rusty_lr_derive --> rusty_lr_parser;
    rusty_lr_buildscript --> rusty_lr_parser;
    
    rusty_lr_executable --> rusty_lr_buildscript;

    rusty_lr_parser --> rusty_lr_core;

About the Versioning

RustyLR consists of two big parts:

• executable (rustylr), the code generator
• runtime (rusty_lr), the main library

Since the cargo automatically uses the latest patch in major.minor.patch version of a crate, we increase the patch number only if the generated code is compatible with the runtime. That is, for any user who is not using buildscript or proc-macro, and using the executable-generated code itself, any code change that could make compile errors with the previous generated code will result in a minor version bump.

License

This project is dual-licensed under either of the following licenses, at your option:

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