Functions

Functions are first-class values. They can be bound by let, stored in fields, passed as arguments, and returned from other functions.

Function literals

fn(params) -> Return body. The body is a single expression, or a { … } block expression.

double = fn(x: i32) -> i32 x * 2
sum_sq = fn(x: i32, y: i32) -> i32 { let s = x + y; s * s }

Higher-order 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)

Function types

The type fn(T1, T2, …) -> R describes a callable. Use it for fields that hold callbacks.

type Step {
  apply: fn(i32) -> i32
}