Variadic parameters for Traits

Mojo
, ,
1

Problem

The following code is currently not possible:

alias ComponentType = Copyable & Movable

trait ArchetypeIterator[*Ts: ComponentType]:
	alias Element: Archetype[*Ts]

It can be worked around by specifying all variadic parameters as VariadicList[ComponentType], but this requires lots of refactoring and dismisses the whole point of having nice ergonomics around variadic parameters.

Workaround

alias ComponentType = Copyable & Movable

trait ArchetypeIterator:
	alias ComponentTypes: VariadicList[ComponentType]
	alias Element: Archetype[ComponentTypes]

Proposal

I know that (variadic) parameters are not in their final state and are considered sharp edges at the moment?!
I think variadic parameters for traits would make a great addition to the language, especially to enforce that a passed argument is an Iterator for a specific Element type:

# Possible with variadic parameters for Traits:
fn archetype_iter[*Ts: ComponentType, IterType: ArchetypeIterator[*Ts]](iter: IterType):
	element = iter.__next__()
	element.get_archetype_specific_attribute() # all good, because we can deduce that element must be of type Archetype[*Ts]

# Currently possible:
#               V any Iterator allowed
fn any_iter[IterType: Iterator](iter: IterType):
	element = iter.__next__()
	element.get_archetype_specific_attribute() # wouldn't work, because element can be something else then Archetype[*Ts]

If traits got support for all parameter types (which probably would be the way to go to stay consistent), we could define a parametric Iterator trait:

trait Iterator[Element: AnyType]:
	fn __iter__(self, out element: Element): ...
	...

This allows to require iterators for a specific Element type when declaring a function. So the previous archetype_iter function could be rewritten as:

fn archetype_iter[*Ts: ComponentType, IterType: Iterator[Archetype[*Ts]]](iter: IterType):
	element = iter.__next__()
	element.get_archetype_specific_attribute() # all good, because we can deduce that element must be of type Archetype[*Ts]

Question

Therefore I was wondering if you already have any thoughts/ideas regarding using variadic parameters in trait definitions?

2

Variadic parameters on Traits should come with parameters on traits.