Hi, I can’t figure out how to use a parametric type inside a constructor. Namely, the following single commented line can’t be used: struct Foo{T<:Real, S<:AbstractMatrix{<:T}} <: AbstractArray{T, 4} data::S n::Int function Foo{T, S}(data, n) where {T<:Real, S<:AbstractMatrix{<:T}} Base.requ…

https://github.com/JuliaLang/julia/issues/39280

My best guess is that there is some kind of ambiguity in the various constructors that you’ve defined, but I can’t put my finger on what that would be.

Removing the <: from the type definition makes it work: This works: julia> struct Foo{T<:Real, S<:AbstractMatrix{T}} data::S n::Int end julia> Foo(A::AbstractMatrix{T}, n::Int) where {T<:Real} = Foo{T, typeof(A)}(A, n) Foo julia> Foo{T}(::UndefInitializer,n) …

Awesome finding! That is subtle but definitely solves my issue. But now, two questions: Do you know why this is so? What constitutes the error? Is there a fundamental reason why this AbstractMatrix{<:T} idiom is used in the Julia codebase, e.g., in LinearAlgebra? In fact, I wrote the first part o…

No, I don’t. But those things mean different things. In one you can do: julia> struct Foo{T,S <: Vector{<:T}} x :: S end julia> Foo{Real,Vector{Float64}}(zeros(2)) Foo{Real,Array{Float64,1}}([0.0, 0.0]) In the other you can’t: julia> struct Foo2{T,S<:Vector{T}} x :: S …

Nice! I think I have a better understanding of it now. But that’s still odd that everything “works” when I omit the A = Matrix{T}(undef, d, d) in the last function of my example (that is, Julia doesn’t complain if T is not mentioned, even though the function signature is the same). Of course, I wou…

Thanks, @CameronBieganek ! This seems to be an issue as well, but here I’ve mentioned that there should be more than that in the present case. In fact, by changing a single line in the function body (same signature, same parameters, etc.), everything “works” as expected.

Yes, it has something to do with the undef initialization.

[image] schneiderfelipe: But that’s still odd that everything “works” when I omit the A = Matrix{T}(undef, d, d) in the last function of my example (that is, Julia doesn’t complain if T is not mentioned, even though the function signature is the same). Well, that part I can explain. It’s perf…

The point is that, in my example, running the function does not throw an error. In the context of my example, the following throws an error, function Foo{T}(::UndefInitializer, n::Int) where {T<:Real} d = fld(n * (n + 1), 2) A = Matrix{T}(undef, d, d) return Foo(A, n) end Foo{Float64}(undef, 2)…

Using parametric type inside outer parametric constructor

General Usage

CameronBieganek January 15, 2021, 5:42pm 10

I’m still a bit confused. The suggestion from @lmiq made me wonder about a related question that seems odd to me. I’ve opened a separate thread for that specific question:

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Using parametric type inside outer parametric constructor

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