Question

我有一个创建map_reverse函数的赋值。

let rec map_rev func = function
        [] -> [] 
    |   h::t -> map_rev func t @ func h;;

let f x = x * 7;;

open Printf
let a = map_rev f [1;2;10;20;400]
let () = List.iter (printf "%d ") a

现在，编译器不允许这一行：

let a = map_rev f [1;2;10;20;400]

，因为：

'This expression has type 'a * 'b but an expression was expected of type 'c -> 'd list'.

有人能指出我的错误吗？为什么期望＆＃c; c - ＆gt;列表？

Answer 1

地图反转功能的类型应为('a -> 'b) -> 'a list -> 'b list。您的map_rev函数的类型为('a -> 'b list) -> 'a list -> 'b list。所以关键是弄清楚编译器认为func返回列表的原因。

Answer 2

let rec map_rev func = function
        [] -> []
    |   h::t -> map_rev func t @ [func h ];;

Ocaml反向并映射

2 个答案: