A polymorphic function is one in which one of the arguments can be of any type. We write a polymorphic function by first listing the type parameters in which the function is polymorphic.
let pair 'a 'b (x: a) (y: b) : (a,b) = (x,y)
We do not pass the type parameters
b explicitly when we call
pair. Rather, they are automatically inferred from the value parameters.
let a_pair = pair 1 true
Although using explicit type parameters is recommended, a polymorphic type can also be inferred automatically:
let pair_implicit x y = (x,y)
We can use type parameters to make a function less polymorphic than it would normally be. For example, we can write a variant of
pair where we require that both arguments have the same type:
let pair_same 'a (x: a) (y: a) : (a,a) = (x,y)
When a polymorphic function is called with an array argument, all arrays passed for the same polymorphic type must be of the same size. For example,
pair_same  [2,3] is illegal, but
pair  [2,3] is fine.