Guide

Lambdas & function values

A lambda is a function declaration without the name — one mental model for every function. Captures are by value, and map/filter/fold come built in.

fn make_adder(n: Int) -> fn(Int) -> Int = fn(x: Int) -> Int = x + n

fn main() -> Void:
    let add10 = make_adder(10)
    print("{add10(32)}")

    let words = ["poly", "tone"]
    let loud = words.map(fn(w: Text) -> Text = w.upper())
    print(loud.join(" + "))

Function types mirror declarations: fn(Int, Text) -> Bool. Lambdas are fully annotated and expression-bodied, so a signature never requires reading a body. A lambda captures the locals it mentions at creation time, as copies — mutating a capture is a compile error, not a footgun.