在SML中键入模式匹配

Question

我正在为SML中的Programming Languages类做一个简单的练习，练习是计算树是否平衡。以下代码是我认为有效的解决方案：

datatype IntTree = LEAF of int | NODE of (IntTree * IntTree);

fun balanced (LEAF l) = true 
  | balanced (NODE (LEAF l, NODE n)) = false
  | balanced (NODE (NODE n1, NODE n2)) = balanced(n1) andalso balanced(n2)
  | balanced (NODE (NODE n, LEAF l)) = false
  | balanced (NODE (LEAF l1, LEAF l2)) = true;

但是，当我尝试在解释器中运行它时，我收到以下错误：

stdIn:98.42-98.54 Error: operator and operand don't agree [tycon mismatch]
  operator domain: IntTree
  operand:         IntTree * IntTree
  in expression:
    balanced n1
stdIn:98.63-98.75 Error: operator and operand don't agree [tycon mismatch]
  operator domain: IntTree
  operand:         IntTree * IntTree
  in expression:
    balanced n2

但是NODE应该是IntTree类型，为什么这不起作用？

Answer 1

我目前没有SML编译器，所以我无法对此进行测试，但请尝试将balanced(n1)更改为(balanced (NODE n1))（与balanced(n2))相同。