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

The twice function for example 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 support generics, also known as parametric polymorphism.

This works by instead of specifying a concrete type, a type variable is used which stands in for whatever specific type is being used when the function is called. These type variable 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.