Guide

Generic functions

Square-bracket type parameters, inferred at every call — there is exactly one way to call any function.

import lists

fn label[T](x: T, tag: Text) -> Text:
    return "{tag}: {x}"

fn main() -> Void:
    let names = ["do", "re", "mi"]
    print("{lists.index_of(names, "re")}")
    print("{lists.reversed(names)}")
    print(label(42, "int"))
    print(label([1.5], "list"))

Since Sprint 32. No turbofish and no call-site type arguments: the declaration rule (every type parameter must appear in a parameter type) makes inference total, and conflicting arguments teach — 'cannot infer index_of: argument 2 is Text, but the arguments so far need Int'. Bodies are checked once with opaque type parameters; monomorphization then generates readable instances like index_of[Int], transitively and across modules. The VM never sees a type parameter.