Sink parameters

A sink parameter accepts ownership of a managed value:

fun deliver(message: sink String) {
    println(message);
}

The call uses ordinary argument syntax:

deliver("temporary");

Named variables remain usable

Passing a named variable has copy semantics. The caller may continue using it:

var saved = "keep me";
deliver(saved);
println(saved);

This behavior is part of the language contract. It does not change between debug and optimized builds.

Temporaries can transfer directly

A temporary has no later source-level use:

deliver("prefix:" + toString(42));

The compiler may transfer that value into the parameter instead of creating an extra owned copy.

Last-use optimization

The compiler may also optimize a proven final use of a local into a transfer, provided the program behaves exactly as if an ordinary copy occurred.

Zap does not expose a public move expression. Callers do not need to reason about whether optimization consumed a variable.

When to use sink

Use sink T when the function stores an owned value or passes ownership deeper into the program:

class Mailbox {
    priv latest: String;

    pub fun store(message: sink String) {
        self.latest = message;
    }
}

Use an ordinary parameter for a stable value copy. Use ref for in-place mutation and noescape StringView for a borrow that cannot leave the call.