标准 ML 二叉树数据类型

Question

我知道这个问题以前曾被问过，但之前问题中的答案都不适合我，所以我会尝试不同的方法。

我已经这样做了：

> 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

但是当我尝试构建一棵树时，

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

我遇到了错误。

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，您必须按如下方式创建一棵树：

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