Variadics and slice spread

A rest parameter is a slice inside the function and a Go variadic at its call boundary.

Source

import go fmt from "fmt";

function sum(prefix: int, ...values: int[]): int {
  let total = prefix;
  for (const value of values) { total += value; }
  return total;
}

function last<T>(fallback: T, ...values: T[]): T {
  if (len(values) === 0) { return fallback; }
  return values[len(values) - 1];
}

function main(): void {
  const values = [3, 4, 5];
  fmt.Println(sum(10), sum(10, 1, 2), sum(10, values...));
  fmt.Println(last("empty"), last("empty", "hello", "planet"));
}

Run

keika check variadics.km
keika run variadics.km

Expected output:

10 13 22
empty planet

Call rules

• A rest parameter uses ...values: T[] and must be last.
• Callers may pass zero or more individual T values.
• One final T[] may be expanded with values....
• Fixed arguments still appear before the expanded slice.
• Generic inference includes both individual and expanded arguments.

The same form works for functions, methods, interfaces, arrows, function types, and constructors. A plain T[] parameter is not interchangeable with a variadic signature.

The documentation suite also rejects spreading string[] into ...values: int[], verifying that spread does not bypass element typing.

See Functions and generics.