标准ML：操作符和操作数不同意（循环） [英] Standard ML: Operator and Operand Don't Agree (Circularity)

问题描述

我试图在SML中编写一个函数来翻转列表中的交替元素。这是我的功能：

I'm trying to write a function in SML to flip alternate elements of a list. Here's my function:

 fun flipAlternate(nil) = nil
     | flipAlternate([x]) = x
     | flipAlternate(x::y::xs) = y::x::flipAlternate(xs);

当我在交互式解释器中使用我的文件（Ullman.sml）时，错误：

When I go to use my file (Ullman.sml) in the interactive interpreter, I get this compilation error:

- use "Ullman.sml";
[opening Ullman.sml]
Ullman.sml:5.31-5.54 Error: operator and operand don't agree [circularity]
  operator domain: 'Z list * 'Z list list
  operand:         'Z list * 'Z list
  in expression:
    x :: flipAlternate xs

所以SML说它需要一个整数列表列表，但我只给它一对整数列表？

So SML is saying it requires a list of lists of integers but I'm only giving it a pair of lists of integers?

我在这里有点失落，所以任何帮助都会非常感谢。

I'm a bit lost here so any help would be much appreciated.

谢谢，
bclayman

推荐答案

第二种情况是错误的;你想

Your second case is wrong; you want

fun flipAlternate(nil) = nil
 | flipAlternate([x]) = [x]
 | flipAlternate(x::y::xs) = y::x::flipAlternate(xs);

SML正在研究第二种情况并得出结论

SML is looking at the second case and concluding

flipAlternate :: 'z list list -> 'z list

这与第三种情况下的递归不兼容。

which isn't compatible with the recursion in your third case.

编辑：它知道结果是第一个案例中的 list ，并且结束于参数还有一个列表比第二种情况的结果。

It knows the result is a list from the first case, and concludes the argument has one more list than the result from the second case.

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