我的“curry2”实现有什么问题?

时间:2013-10-10 07:31:36

标签: ocaml

我是OCaml的新手,并尝试编写一些基本功能。

我发现如果我使用find_new来定义curry2函数(此函数基本上找到仅出现在第一个列表中的元素列表)不起作用。

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]);;

如果我使用find_new的第二个定义(注释掉的那个),则错误信息为:

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

所以我想知道我的代码有什么问题?

1 个答案:

答案 0 :(得分:3)

这看起来像价值限制。尝试使用eta扩展进行定义

let find_new pair = curry2 find_new_ pair

请参阅OCaml常见问题A function obtained through partial application is not polymorphic enough