[Pitch] Parameter Packs

[hborla (Holly Borla)]

@gregtitus

I am shocked and appalled that the terminology involved here is "type parameter packs" instead of "typles".

Well, parameter packs are one concept that extend to both type parameter packs and value parameter packs, so we need to have a common term to explain the operations that you can do at the type level and at the value level

@MPLewis

I can't say I love the spelling of length(T...) here, since it adds some further mental overhead to understand a function declaration where the generic requirement isn't immediately different from normal function calls. I don't have any great suggestions here, but even something like #length(T...) to visually pull out that this is something being enforced by the compiler feels like a step in the right direction.

I agree that there's probably a better spelling out there, and I am definitely open to suggestions. I also think that "length" might be the wrong concept to be thinking about, because packs will also have varying degrees of abstract structure depending on the requirements placed on them. Some level of statically known structure might also be useful for destructuring packs, which is mentioned in the future directions. I wonder if the concept in the programming model should be "pack structure" or "pack shape" instead of "pack length" (where same-structure also implies same-length).

@MPLewis

even something like #length(T...) to visually pull out that this is something being enforced by the compiler feels like a step in the right direction.

I worry that # signals "known by the compiler", when the length is still only known at runtime. if #available is similar, and many folks confuse #available with a compile-time constant.

@filip-sakel

Tuple conformances is the most straightforward one,

Tuple conformances will need parameters packs and some of the other future directions in order to express. Modeling packs as tuples was already explored and ruled out due to the fundamental ambiguities between using a tuple as a component parameter and using a tuple as the pack itself, among other disadvantages outlined in the alternatives considered.