Stream: helpdesk (published)

Topic: Dispatch on same type but different type parameters?

peter j. (Jun 16 2021 at 18:11):

I would like to do the following

function foo(a::T{R1}, b::T{R2}) where {R1,R2, T <: SomeAbstractType}

end

but this doesn't compile.

Is there a way to accomplish such a thing? I want foo to dispatch only when a,b have the same subtype of some abstract type but possibly different type parameters.

edit: missing curly bracket, clarification

Max Köhler (Jun 16 2021 at 18:12):

Is your abstract type also parametrized? If so, I think you could do

function foo(a::T, b::T) where {R1,R2, T <: SomeAbstractType{R1,R2}}

end

peter j. (Jun 16 2021 at 18:24):

It is, but only with one type parameter, so that does not work.

mbaz (Jun 16 2021 at 18:35):

This must be possible, I think, since one can do:

julia> f(xx::Vector{Int}, yy::Vector{Float64}) = true
f (generic function with 2 methods)

julia> f([1, 2], [1.0, 2.0])
true

Max Köhler (Jun 16 2021 at 18:48):

function foo(a::T1, b::T2) where {R1,R2, T1 <: SomeAbstractType{R1}, T2 <:SomeAbstractType{R2}}

end

but probably there's a more elegant version

peter j. (Jun 16 2021 at 18:53):

Max Köhler said:

function foo(a::T1, b::T2) where {R1,R2, T1 <: SomeAbstractType{R1}, T2 <:SomeAbstractType{R2}}

end

but probably there's a more elegant version

Thanks, but this doesn't satisfy the question since it dispatches even if T1 and T2 are different subtypes of SomeAbstractType.

e.g.

julia> abstract type Bar{T} end

julia> struct Bar1{T} <: Bar{T} end

julia> struct Bar2{T} <: Bar{T} end

julia> function foo(a::T1, b::T2) where {R1, R2, T1<:Bar{R1}, T2<:Bar{R2}}
       return true
       end
foo (generic function with 1 method)

julia> b = Bar1{Int}()
Bar1{Int64}()

julia> a = Bar2{Float64}()
Bar2{Float64}()

julia> foo(a,b)
true

Max Köhler (Jun 16 2021 at 18:54):

but can't you just do

function foo(a::T1, b::T2) where {R1,R2, T1 <: Bar1{R1}, T2 <:Bar1{R2}}

end

then?

Max Köhler (Jun 16 2021 at 18:55):

and

function foo(a::T1, b::T2) where {R1,R2, T1 <: Bar2{R1}, T2 <:Bar2{R2}}

end

respectively

peter j. (Jun 16 2021 at 18:57):

Yeah. This works for my actual problem, but then I would need to define one of these for each subtype of Bar. It's more academic at this point but I'm wondering if this can work for arbitrary subtypes of Bar.

Max Köhler (Jun 16 2021 at 18:57):

ye sorry if my answer is not satisfying, but I actually think it's a nice problem, so happy to hear other solutions as well

peter j. (Jun 16 2021 at 18:58):

Max Köhler said:

ye sorry if my answer is not satisfying, but I actually think it's a nice problem, so happy to hear other solutions as well

no problem, I appreciate the answers!

Max Köhler (Jun 16 2021 at 19:04):

I think for a general approach, this conversation is essentially the same problem, isn't it? https://julialang.zulipchat.com/#narrow/stream/274208-helpdesk-.28published.29/topic/Functions.20of.20abstract.20parametric.20types

Last updated: Oct 02 2023 at 04:34 UTC