这个 ML 代码的输出是什么？

Question

fun square(x:int) = x*x;

map (~, map (square, map (~, [1, 2, 3] )));

是否只是三个数字（1、4、9）的平方？ 我尝试在新泽西州的 SML 上运行它，但一直收到此错误...

- fun square(x:int) = x*x;
val square = fn : int -> int
- map(~, map(square, map(~,[1,2,3])));
stdIn:3.11-3.35 Error: operator and operand don't agree [tycon mismatch]
  operator domain: 'Z -> 'Y
  operand:         ('X -> 'X) * int list
  in expression:
    map (~,1 :: 2 :: <exp> :: <exp>)
stdIn:3.4-3.36 Error: operator and operand don't agree [tycon mismatch]
  operator domain: 'Z -> 'Y
  operand:         (int -> int) * _
  in expression:
    map (square,map (~,<exp> :: <exp>))
stdIn:3.1-3.36 Error: operator and operand don't agree [tycon mismatch]
  operator domain: 'Z -> 'Y
  operand:         ('X -> 'X) * _
  in expression:
    map (~,map (square,map <exp>))
-

Answer 1

正如您发现的那样，此代码将打印的唯一内容是错误消息。原因是map 是一个柯里化函数，它的参数不采用元组形式。

要解决此问题，您需要将map 作为柯里化函数调用（即map f xs 而不是map (f, xs)）。