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Callable structs

Let a small object express an operation.

callables.voidDownload source ↓
value struct multiplier:
    amount: i32
func (m multiplier) call(value: i32) -> i32:
    return m.amount * value
struct counter:
    value: i32
func (c counter) call(value: i32) -> i32:
    c.value += value
    return c.value
func make() -> multiplier:
    return multiplier(amount = 3)
func apply<t>(action: t, value: i32) -> i32:
    return action(value)
func entry(value: i32) -> i32:
    var m = multiplier(amount = 2)
    var c = counter()
    var alias = c
    if apply(m, 3) != 6:
        return -100
    var first = m(value)
    var second = alias(2)
    return first + second + make()(1) + c(1)
Input 5Expected result 18

Define call on a receiver

A compatible call method lets an instance be invoked with ordinary function-call syntax. The multiplier value stores an amount and multiplies its argument.

Keep value and reference behavior

The managed counter mutates its shared state when called through an alias. The multiplier remains a value. Using call does not create a new ownership or memory model.

Pass an operation to a generic

apply accepts a concrete callable and invokes it. A returned temporary can also be called, as in make()(1). Input 5 returns 18. Callable structs obey the normal visibility, receiver-effect and readonly rules.