For an older version of this repository based on sedlex, refer to the sedlex branch.

Writing an OCaml parser with nice error messages should be easy - and now it is! Nice Parser comes in two parts:

1. lib/: The nice_parser library (API) consolidates boilerplate code and wraps your auto-generated parser in a nice interface with beautiful error messages. In types: Nice_parser.Make : functor(P : RAW_PARSER) -> NICE_PARSER.
2. example/: The example parser lets you get started on your own parser in seconds. It is based on nice_parser and standard tools:
   • Menhir, a LR(1) parser generator.
   • ocamllex, OCaml's built-in lexer generator.
   • Jane Street's dune and base, the de-facto standard built tool and standard library for OCaml.

Using the library and the skeleton, you can get started on your own parser in seconds:

git clone https://github.com/smolkaj/nice-parser.git  # clone this repository
cd nice-parser && rm -r lib && mv example src         # use example as starting point
opam install . --deps-only --with-test                # install dependencies
dune build                                            # try to build...
dune exec src/bin/main.exe                            # ...and run your parser!

You should see the following output (the error message relies on OCaml >= 4.08's new source highlighting mechanism):

Trying to parse "(a b (c d) e)".
-> (List ((Atom a) (Atom b) (List ((Atom c) (Atom d))) (Atom e)))

Trying to parse "(long_atom_with_0123)".
-> (List ((Atom long_atom_with_0123)))

Trying to parse "
    ( so far so good
          but (this is)) illegal (isnt it?)
    (* parsing will fail ^^^^^^^ here *)
  ".
Fatal error: exception Line 3, characters 25-32:
3 |           but (this is)) illegal (isnt it?)
                             ^^^^^^^
Error: [parser] unexpected token

The API is documented here. The example skeleton should be self-explanatory.

Ideally, use OCaml 4.08 or higher (for beautiful error messages). All required dependencies can be installed using the opam package manager. The project can be built using dune.

opam install . --deps-only --with-test
dune build
dune runtest

Suggestions and changes are welcome. Please submit pull requests, or open issues.