Higher-order Functions

Functions that take and return other functions.

Functions can take and return other functions. This is how the collection builtins like map, filter, and fold are parameterised.

adder = fn(x: i32) -> fn(i32) -> i32 fn(y: i32) -> i32 x + y
add3  = adder(3)
seven = add3(4)

doubled = map([1, 2, 3], fn(x: i64) -> i64 x * 2)