Fork me on GitHub
Source file: parametric-polymorphism.fut

# Parametric polymorphism

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.

``````def pair 'a 'b (x: a) (y: b) : (a,b) =
(x,y)``````

We do not pass the type parameters `a` and `b` explicitly when we call `pair`. Rather, they are automatically inferred from the value parameters.

``def a_pair = pair 1 true``

Although using explicit type parameters is recommended, a polymorphic type can also be inferred automatically:

``````def 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:

``````def 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 [1] [2,3]` is illegal, but `pair [1] [2,3]` is fine.