Overview of Variadic Packs

A positional variadic parameter begins with %... and must be the last positional parameter:

::count_tail FUNCTION(%first I32, %...rest I32): I32
{
  RETURN PACK_SIZE(rest) AS I32;
}

::second_tail FUNCTION(%first I32, %...rest I32): I32
{
  RETURN PACK_ARG(rest, 1);
}

ASSERT(count_tail(% [1, 2, 3]) == 2);
ASSERT(second_tail(% [1, 2, 7]) == 7);

Pack operations

• PACK_SIZE(pack) is the compile-time number of arguments.
• PACK_ARG(pack, index) selects one argument.
• PACK_ARG_TYPE(pack, index) names that argument's type.

A heterogeneous AUTO pack preserves each argument type:

::first FUNCTION(%...values AUTO): PACK_ARG_TYPE(values, 0)
{
  RETURN PACK_ARG(values, 0);
}

AUTO(tag) on the pack instead requires every element to bind the same deduced type:

::same_type_count FUNCTION(%...values AUTO(t)): I32
{
  RETURN PACK_SIZE(values) AS I32;
}

Static expansion can iterate a pack:

::sum_pack FUNCTION(%...values I32): I32
{
  VAR result I32 := 0;
  STATIC_VAR index U64 := 0;
  STATIC_WHILE(index < PACK_SIZE(values))
  {
    result += PACK_ARG(values, index);
    STATIC_EVAL index++;
  }
  RETURN result;
}

An out-of-range PACK_ARG or PACK_ARG_TYPE is a compilation error.

Keyword argument packs

A final @KWARGS ... parameter collects unmatched named arguments. The parameter's value is a composite, with named fields inspected by COMPOSITE_* operations and passed to another callable with APPLY. %... and PACK_* continue to operate on positional packs.

Reference

See the Variadic Packs Reference for the complete language rules, constraints, and technical edge cases.