Mon, 11 Sep 2006

Ocaml : Code for Variant Types and Pattern Matching.

Since my blog post on Ocaml's Variant Types and Pattern Matching I've had two requests for the code, so here it is: 


type expr_t =
    |  Var of string
    |  Plus of expr_t * expr_t
    |  Times of expr_t * expr_t
    |  Divide of expr_t * expr_t


let rec simplify expr =
    match expr with
        |   Times (a, Plus (b, c)) ->
                Plus (simplify (Times (a, b)), simplify (Times (a, c)))
        |   Divide (Divide (a, b), Divide (c, d)) ->
                Divide (simplify (Times (a, d)), simplify (Times (b, c)))
        |   anything -> anything (* Comment : default case *)


let rec str_of_expr expr =
    match expr with
        |   Var v -> v
        |   Plus (a, b) ->
                "(" ^ (str_of_expr a) ^ "+" ^ (str_of_expr b) ^ ")"
        |   Times (a, b) ->
                (str_of_expr a) ^ "*" ^ (str_of_expr b)
        |   Divide (a, b) ->
                (str_of_expr a) ^ " / " ^ (str_of_expr b)


let _ =
    let expr = Times (Var "x", Plus (Var "y", Var "z")) in
    Printf.printf "  orig : %s\n" (str_of_expr expr) ;
    let expr = simplify expr in
    Printf.printf "  new  : %s\n" (str_of_expr expr)

The above code is a single self contained program; to run that program, save it to to a file named say "cas.ml" then run it (assuming you have ocaml installed) using the command "ocaml cas.ml" which should result in the following output: 


  orig : x*(y+z)
  new  : (x*y+x*z)

Obviously, this is just just a demo, but it should be pretty clear that this code could easily be extended to something more useful.

Posted at: 18:41 | Category: CodeHacking/Ocaml | Permalink