SML中的类型模式匹配

Question

我正在为SML中的Programming Languages类做简单的练习，练习是计算树是否平衡。下面的代码是什么，我认为将是一个有效的解决方案：

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; 

然而，当我尝试解释我碰到下面的错误运行此：

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NODE应该是IntTree类型，为什么不工作？

Answer 1

我没有一个SML编译器现在，所以我不能对此进行测试，但尝试改变balanced(n1)到(balanced (NODE n1))（同样与balanced(n2))。