Dispatching on values with constraints

Internals & Design
1

Within a where expression, I can create upper bounds on types with <:

Array{T} where {T<:Real}

I am just wondering if it will ever be possible to use :: or isa within where expressions to dispatch on type parameter values, like

Val{T} where {T::Real}
# or, perhaps
Val{T} where {T isa Real}

so that, e.g., Val(1) <: Val{T isa Real}. I suppose it just might not ever be possible with the type system and I probably need to use WhereTraits.jl / SimpleTraits.jl. (Traits would also be nice for things like f(::Array{T,N}) where {T,N<=2} = ... without needing both Array{T,1} and Array{T,2})

Aside:
Part of the reason I ask is because types are technically instances of a class DataType:

julia> typeof(Float64)
DataType

So, in some ways, T <: Real isn’t too far removed from x < 1.0. Technically, <: is just an operator between two instances T and Real. Obviously on the practical and implementation side, these are very different! But it’s interesting to think about this more generally as where clauses describing a subset of some space, and the dispatch system targetting the method with the smallest volume(/measure) which contains the instance. Right now we are working with a 1D measures along DataType space but perhaps this could be extended to other spaces if there is a hierarchy in place so that smaller measure along one axis takes precedent over a smaller measure along another axis.

Probably also relevant to multiple inheritance: abstract multiple inheritance · Issue #5 · JuliaLang/julia · GitHub

Related:

2

NB: the discussion in that PR is not completely on-topic relatively to the original issue post. The issue was IMO hijacked from the original topic, but in any case the current title and topic corresponds to your question.

3

Cool! Looks like it’s on the road map which is great. Thanks for the link.