What does 'int ?. tree' mean in an SML error message?

问题

I have the following SML code that I wrote for a class:

fun lookup (cmp: 'a * 'a -> order) (x: 'a, t: 'a tree) : 'a option =
    case t of
      Empty => NONE
    | Node(l,y,r) =>
      case cmp(x,y) of
        EQUAL => SOME y
      | LESS => lookup (cmp) (x,r)
      | GREATER => lookup (cmp) (x,l)

In testing this with:

val SOME 3 = lookup Int.compare (3, Node(Empty,3,Empty));

And getting the following error back:

stdIn:153.1-166.12 Error: operator and operand don't agree [tycon mismatch]
  operator domain: int * int ?.tree
  operand:         int * int tree
  in expression:
    (lookup Int.compare) (3,Node (Empty,3,Empty))

What does the ?. mean?

回答1:

This is usually to do with restricted visibility across modules. What does your 'tree' definition look like? You might need to tell the compiler that your 'tree' type in one module is the same as the one in another module.

来源：https://stackoverflow.com/questions/12830731/what-does-int-tree-mean-in-an-sml-error-message