Pacomb is a parsing library that compiles grammars to combinators prior to parsing together with a PPX extension to write parsers inside OCaml files.

The advantages of Pacomb are

• Grammars as first class values defined in your OCaml files. This is an example from the distribution:

(* The three levels of priorities ) type p = Atom | Prod | Sum let%parser rec ( This includes each priority level in the next one ) expr p = Atom < Prod < Sum ( all other rule are selected by their priority level ) ; (p=Atom) (x::FLOAT) => x ; (p=Atom) '(' (e::expr Sum) ')' => e ; (p=Prod) (x::expr Prod) '' (y::expr Atom) => x*.y ; (p=Prod) (x::expr Prod) '/' (y::expr Atom) => x/.y ; (p=Sum ) (x::expr Sum ) '+' (y::expr Prod) => x+.y ; (p=Sum ) (x::expr Sum ) '-' (y::expr Prod) => x-.y

• Good performances:

  • on non ambiguous grammars, 2 to 3 time slower compared to ocamlyacc
  • on ambiguous grammars O(N^3 ln(N)) can be achieved.

• Parsing from left to right (despite the use of combinators) allowing not to keep the whole input in memory and allowing to parse streams.

• Dependant sequence allowing for self extensible grammars (like new infix with a given priority in a given example).

• Managing of blanks that for instance allows for nested language using different kind of comments or blanks.

• Support for cache and merge for ambiguous grammars (to get O(N^3 ln(N)))

• Enough support for utf8 to write parser for a language using utf8.

• Comes with documentation and various examples illustrating most possibilities.

All this makes Pacomb a promising solution to write languages in OCaml.