Continuation-Passing Style Parser Generator

Overview

This program is a tool similar to ocamlyacc and menhir that generates LALR(1) parsers. However, unlike those, it uses the continuation-passing style, which allows for better type inference and more efficient compiler optimizations.

Features

• Specializes in generating LALR(1) parsers, with support for LR(0), SLR, and LR(1) parsers.
• Largely compatible with ocamlyacc, supporting:
  • %left, %right, and %nonassoc precedence and associativity declarations
  • $1, $2, etc., for referencing semantic values
  • Compatibility with the Parsing module, including functions like Parsing.symbol_start
• Supports a subset of menhir-specific features:
  • Named semantic values using the id=symbol syntax
  • Keywords such as $startpos and $loc
  • Parametric rules using the rule(param1, param2) syntax
  • Full support for menhir's standard library
  • Shorthand notations *, +, and ?
  • %inline rules
  • Anonymous rules
• Experimental/non-standard:
  • %when conditions

Planned but not yet supported:

• Error recovery
• Parametric semantic actions

Installation

To install cpspg using opam, you can pin the package directly from the GitHub repository:

opam pin add cpspg git+https://github.com/fram-lang/cpspg.git

This will fetch the latest version of cpspg from the repository and install it.

Alternatively, you can build cpspg manually from the source code:

git clone git+https://github.com/fram-lang/cpspg.git
cd cpspg
dune build --profile=release
dune install

Usage

To generate a parser from a .mly grammar definition, use the following command:

cpspg [-o OUTPUT] INPUT

To integrate this tool into a dune build process, add the following rule to your dune file:

(rule
 (deps Parser.mly)
 (targets Parser.ml Parser.mli)
 (action
  (chdir %{workspace_root} (run cpspg %{deps} %{targets}))))

Building

This project is managed using dune. To build it, execute:

dune build

Note that lib/Parser.ml is bootstrapped from lib/Parser.mly. When code generation changes, it can be promoted to a new version:

dune build @bootstrap
dune promote

Why Use Continuation-Passing Style?

LR parsers are essentially finite automata with a stack containing various semantic values. Representing such a stack in a functional language is challenging. Traditional tools like ocamlyacc and menhir employ workarounds — ocamlyacc uses Obj.repr as an escape hatch, while menhir utilizes GADTs. These approaches make it difficult to typecheck parsers without explicit type declarations. CPS enables the generation of simpler, more type-safe code that is also more readable (sort of).