// Function signatures can be declared as types:

type Filter = fn (string) string

// Functions can accept function types as arguments:

fn filter(s string, f Filter) string {
    return f(s)
}

// Declare a function with a matching signature:

fn uppercase(s string) string {
    return s.to_upper()
}

fn main() {
    // A function can be assigned to a matching type:

    my_filter := Filter(uppercase)

    // You don't strictly need the `Filter` cast - it's only used
    // here to illustrate how these types are compatible.

    // All of the following prints "HELLO WORLD":

    println(filter('Hello world', my_filter))
    println(filter('Hello world', uppercase))
    println(filter('Hello world', fn (s string) string {
        return s.to_upper()
    }))
}