SML - 未绑定(bind)的变量或构造函数

我有下一个代码:

datatype expr = K of string| Number2 of expr * (expr list);
datatype number = Number1 of string | Number3 of int;
 fun append (nil, l2) = l2
  | append (x::xs, l2) = x::append(xs, l2);
 fun map [] = []
    | map (h::t) = (What h)::(map t);
fun What (K x) = [Number1(x)]
    |What (Number2 (t,[])) = Number3(0)::What(t)
    |What (Number2 (y,a::b)) =  append(What(a), map(b));

它无法识别函数“What”。(未绑定(bind)的变量或构造函数)。我该如何修复它，让它知道“什么”功能？

谢谢。

最佳答案

SML 中的声明从上到下工作，因此 map 看不到 What 。切换顺序无济于事，因为 What 不会看到 map ，给出同样的错误。相反，您需要使用 and 同时声明相互递归的函数:

fun map [] = []
  | map (h::t) = (What h)::(map t)
and What (K x) = [Number1(x)]
  | What (Number2 (t,[])) = Number3(0)::What(t)
  | What (Number2 (y,a::b)) =  append(What(a), map(b))

关于SML - 未绑定(bind)的变量或构造函数，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/8009873/