forall

Writes a polymorphic (higher-rank) function type: forall T. T -> T.

A parameter of that type accepts a generic function that the callee can use at several types. Constraints go on the binder: forall T: Num. T -> T.

fn id<T>(T x) -> T {
    return x;
}

fn both(forall T. T -> T f) -> (int, string) {
    return (f(1), f("one"));
}

fn main() {
    let pair = both(id);
}

See also#

• fn: Declares a function, a method, or (written in an expression) an anonymous function.
• let: Declares a mutable local variable.
• const: Declares a binding that cannot be reassigned: a local, a module constant, or a class field that is set only by new.
• static: Declares a module-level or class-level slot shared by the whole program, or a method without self.
• readonly: A read-only view of a value: nothing reachable through it can be changed.
• if: Runs a block when a bool condition is true.
• else: Introduces the branch of an if that runs when its condition is false (or its pattern does not match).
• while: Repeats a block while a bool condition is true.