PaComb implements a representation of grammars with semantic actions (values
returned as a result of parsing). Parsing is performed by compiling grammars
defined with the Grammar module (or indirectly though a PPX extension) to
the combinators of the Combinator module. The library offers scanner less
parsing, but the Lex module provide a notion of terminals and blanks
that give a simple way to write grammars in two phases, as usual.
The main advantage of PaComb and similar solutions, contrary to ocamlyacc, is that grammars (compiled or not) are first class values. This allows using the full power of OCaml for manipulating grammars. For example, this is very useful when working with syntax extension mechanisms.
Importantly, the performances of PaComb are very good: it is only two to five times slower than grammars generated by ocamlyacc, which is a compiler.
Defining languages using the Grammar module directly is cumbersome. For that
reason, PaComb provides a BNF-like PPX syntax extension (enabled using the
-ppx pacomb.ppx compilation flag).
A complete documentation is available via ocamldoc (make doc)
Pacomb also support: self extensible grammars, ambiguous grammars (with merge), late rejection of rule via raising exception from action code, priority and others.
A complete documentation is available
As teaser, the usual calculator example:
(* 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
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