标准ML二叉树数据类型

Question

我知道这个问题之前已经被问过了，但以前的问题都没有答案为我工作，所以我会尝试一种不同的方法。

我已经做到了这一点：

> datatype which = STRING of string | INT of int; 
datatype which = INT of int | STRING of string 
> datatype whichTree = Empty | Leaf of which | Node of whichTree*whichTree; 
datatype whichTree = Empty | Leaf of which | Node of whichTree * whichTree 

，但是当我尝试建立一个树

> val mytree = Node(Leaf(which 2), Leaf(which 6)); 

我得到的错误。

Error-Value or constructor (which) has not been declared Found near 
Node(Leaf(which(2)), Leaf(which(6))) 
Error-Value or constructor (which) has not been declared Found near 
Node(Leaf(which(2)), Leaf(which(6))) 
Static errors (pass2) 

Answer 1

which是数据类型的名称;它不是一个构造函数。相反，你必须创建一棵树如下：

> val mytree = Node(Leaf(INT 2), Leaf(STRING "6"));