Allow use of a function/complex object to pick the `subtype` of an `AbstractType`?

[kshyatt]

Suppose I have a situation like this:

abstract type MyAbs end

struct A <: MyAbs
    id_field::Tuple{String, Int}
    field_a
end

struct B <: MyAbs
    id_field::Tuple{String, Int}
    field_b
end

And I want to dispatch constructing using id_field[1]. I think currently this isn't supported by the NamedTuple approach of StructTypes.subtypes and StructTypes.subtypekey, but would it make sense to extend them to support such cases?

[quinnj]

Yeah, I think we can make this work. It seems like we would keep StructTypes.subtypekey as the same, but we need do allow something like StructTypes.getsubtype with a default definition like:

StructTypes.getsubtype(::Type{T}, val) where {T} = StructTypes.subtypes(T)[Symbol(val)]

Then you could overload StructTypes.getsubtype however you want to take the val of the subtypekey we found and you just have to return the right subtype to dispatch to.

[kshyatt]

Actually I think all you need to do is define a new struct with get and haskey and friends -- the subtypes doesn't actually have to be a NamedTuple, it just needs to have the ability to look up values from keys. One could imagine a SubtypeClosure or something that ingests some function the user defines to transform raw input -> subtype.

[quinnj]

Yeah good point