Type Elaboration Example 2

        fun sumof []     = 0
          | sumof (h::t) = h + sumof t;

Compiler says to itself:

Argument is [] , so must be some kind of list, say 'a list
h::t is also a list, and so h must have type 'a and t must have type 'a list
The return value is 0, that's an int.
We're adding h to the return value of sumof, which is an int
h must also be an int -- now we know what 'a is.
t and h::t must be int list
Everything else checks out OK

Then it prints:

        val sumof = fn : int list -> int

If we had written 0.0 instead of 0, the compiler would have deduced real list -> real.

If we had written true instead of 0, the compiler would have reported a type error.