CODEWITHSUNDEEP
ocaml
Javran
Javran

Reputation: 3434

what's wrong with my "curry2" implementation?

I'm new to OCaml, and trying to write some basic functions.

I found the find_new function(this function is basically finding a list of elements that only appear in the first list) doesn't work if I use curry2 to define it.

let curry2 f (x,y) = f x y

let flip f x y = f y x

let compose f g x = g (f x)
let rec elem v xs = match xs with
         [] -> false
       | (h::t) -> if v = h then true else elem v t

let rec filter fb l = match l with
    [] -> []
  | (h::t) -> if fb h
                then h::(filter fb t)
                else (filter fb t)

let x = [5;6;7;3] ;;
let y = [5;6;7;5] ;;
let z = [7;5;6;5] ;;
let a = [3;5;8;9] ;;

let find_new_ xs ys = filter (compose ((flip elem) ys) not) xs ;;
let find_new (xs,ys) = find_new_ xs ys;; (* works *)
(* let find_new = curry2 find_new_;; *)  (* doesn't work *)

find_new (x,[3]);;
find_new (x,[3;5]);;
find_new (x,[3;6]);;
find_new ([x;y;z],[y]);;
find_new ([x;y;z],[y;x]);;

if I use the second definition of find_new (one that commented out), the error info was:

Error: This expression has type int list
       but an expression was expected of type int

So I'm wondering what's wrong with my code?

Upvotes: 1

Views: 136

Answers (1)

Jeffrey Scofield
Jeffrey Scofield

Reputation: 66818

This looks like the value restriction. Try defining with eta expansion

let find_new pair = curry2 find_new_ pair

See the OCaml FAQ A function obtained through partial application is not polymorphic enough.

Upvotes: 3

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