Constructing leaftypes by filling in dependent type parameters

[timholy]

We have some types that have "dependent" type parameters. For instance, in StaticArrays the SArray type is defined

immutable SArray{S <: Tuple, T, N, L} <: StaticArray{S, T, N}
    data::NTuple{L,T}
end

and an example for a 3x2x5 array might be SArray{Tuple{3,2,5},Float32,3,30}. In all "valid" (aka, useful) instances, N = length(S) and L = prod(S). Consequently it might make sense for the package developer to be able to specify that SArray{S, T} constructs a leaftype, filling in the dependent parameters, rather than a UnionAll type.

Ref JuliaArrays/StaticArrays.jl#181.

[timholy]

Alternatively one might want to declare the type

immutable SArray{S <: Tuple, T} where {N = length(S), L = prod(S)}  <: StaticArray{S, T, N}
    data::NTuple{L,T}
end

That might be cleaner. Would be good, though, for a method to be able to extract N and L somehow. Maybe

zerotuple(x::SArray{S, T} where {L = prod(S)}) = ntuple(d->zero(T), Val{L})

[timholy]

xref: this appears to be a potential solution for #5560

[andyferris]

This also targets a different open issue (I'll try find it later) relating to allowing field types which are (more arbitrary) functions of the type parameters, which would be awesome to solve.

[andyferris]

xref: it is #18466

[vtjnash]

already implemented: #18466 (comment)