This expression annotates the type $⟨$ty$\mathrm{⟩}$ to the expression $⟨$exp$\mathrm{⟩}$. The type of $⟨$exp$\mathrm{⟩}$ must be either same as or more general than $⟨$ty$\mathrm{⟩}$. The following shows an example.

# [] : int list;
val it = [] : int list
# 1 : intInf;
val it = 1 : intInf
# fn x => x : int;
val it = fn : int -> int
# fn x => x : ’a -> ’a;
val it = fn : [’a. (’a -> ’a) -> ’a -> ’a]

As seen in the last example, $⟨$ty$\mathrm{⟩}$ may contain type variables. Type variables in a type annotation are never instantiated. Unless there exists an explicit declaration of such type variables, the scope of type variables is the whole of the inner-most val declaration in which the type variables occur. See Section 23.1 for the type variable declarations and their scopes.