Parametric type which contains the abstract type it subtypes

General Usage
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1

I’m creating a type that is a matrix with value bounds.

This works:

struct Map{M<:AbstractMatrix{<:Real}}
    data::M
    lb
    ub
end

But I want it to behave like a matrix itself too, so a subtype.

This works:

struct Map{T} <: AbstractMatrix{T}
    data::Matrix{T}
    lb
    ub
end

But this specifies a concrete Matrix inside, which can’t change even if a different form of matrix is passed in. So I’ve tried many variations of where T but they all fail with “T not defined”:

struct Map{M<:AbstractMatrix{T}} <: AbstractMatrix{T} where T<:Real
    data::M
    lb
    ub
end

Is there a way to allow the new type to adapt to any subtype of AbstractMatrix while still subtyping AbstractMatrix? Or is my thought to preserve concrete-type flexibility wrong? I’m trying to get this functionality while avoiding the ambiguity problem discussed in Performance Tips · The Julia Language.

2 
struct Map{T<:Real, M<:AbstractMatrix{T}} <: AbstractMatrix{T}
    data::M
    lb
    ub
end

You might want to give lb and ub a type as well, to avoid a type instability. The default for non-annotated fields in structs is Any after all.

Additionally, this will be helpful:

2 Likes
3

Perfect! Thanks for your help. So parameters within other parameters need to also be listed as parameters first?

4

Yes, if you want to use those parameters in your struct as well or otherwise want to introduce a cross-cutting constraint, like the one spanning both the declared supertype and a field (unfortunately).