Parsing grammars using OCaml

Question

I have a task to write a (toy) parser for a (toy) grammar using OCaml and not sure how to start (and proceed with) this problem.

Here\'s a sample Awk grammar:

<

Answer 1

Here is a rough sketch - straightforwardly descend into the grammar and try each branch in order. Possible optimization : tail recursion for single non-terminal in a branch.

exception Backtrack

let parse l =
  let rules = snd awksub_grammar in
  let rec descend gram l =
    let rec loop = function 
      | [] -> raise Backtrack
      | x::xs -> try attempt x l with Backtrack -> loop xs
    in
    loop (rules gram)
  and attempt branch (path,tokens) =
    match branch, tokens with
    | T x :: branch' , h::tokens' when h = x -> 
        attempt branch' ((T x :: path),tokens')
    | N n :: branch' , _ -> 
        let (path',tokens) = descend n ((N n :: path),tokens) in 
        attempt branch' (path', tokens)
    | [], _ -> path,tokens
    | _, _ -> raise Backtrack
  in
  let (path,tail) = descend (fst awksub_grammar) ([],l) in
  tail, List.rev path