Generic functions

Up until now each function has accepted precisely one type for each of its arguments.

The twice function in the previous lesson on higher order functions only worked with functions that would take and return ints. This is overly restrictive, it should be possible to use this function with any type, so long as the function and the initial value are compatible.

To enable this, Gleam supports generics, also known as parametric polymorphism.

This works by using a type variable instead of specifying a concrete type. It stands in for whatever specific type is being used when the function is called. These type variables are written with a lowercase name.

Type variables are not like an any type, they get replaced with a specific type each time the function is called. Try uncommenting twice(10, exclaim) to see the compiler error from trying to use a type variable as an int and a string at the same time.