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

Ok, so the first think you should do is write a lexical analyser. That's the function that takes the ‘raw’ input, like ["3"; "-"; "("; "4"; "+"; "2"; ")"], and splits it into a list of tokens (that is, representations of terminal symbols).

You can define a token to be

type token =
    | TokInt of int         (* an integer *)
    | TokBinOp of binop     (* a binary operator *)
    | TokOParen             (* an opening parenthesis *) 
    | TokCParen             (* a closing parenthesis *)     
and binop = Plus | Minus 

The type of the lexer function would be string list -> token list and the ouput of

lexer ["3"; "-"; "("; "4"; "+"; "2"; ")"]

would be something like

[   TokInt 3; TokBinOp Minus; TokOParen; TokInt 4;
    TBinOp Plus; TokInt 2; TokCParen   ]

This will make the job of writing the parser easier, because you won't have to worry about recognising what is a integer, what is an operator, etc.

This is a first, not too difficult step because the tokens are already separated. All the lexer has to do is identify them.

When this is done, you can write a more realistic lexical analyser, of type string -> token list, that takes a actual raw input, such as "3-(4+2)" and turns it into a token list.