Higher-order functions

The best type to use for the parameter of a higher-order function is Callable (implied by the & sigil), a role common to all function-like objects. For an example of defining and calling a second-order function, see Functional Composition.

Convenient syntax is provided for anonymous functions, either a bare block, or a parametrized block introduced with ->, which serves as a "lambda":

sub twice(&todo) {
   todo(); todo(); # declaring &todo also defines bare function
}
twice { say "Boing!" }
# output:
# Boing!
# Boing!

sub twice-with-param(&todo) {
    todo(0); todo(1);
}
twice-with-param -> $time {
   say "{$time+1}: Hello!"
}
# output:
# 1: Hello!
# 2: Hello!