JDL2 MWE

jdl2mwe1.jl

using JLD2

abstract type AbstractStuffType end
struct Stuff1 <: AbstractStuffType end
struct Stuff2 <: AbstractStuffType end
struct Stuff3 <: AbstractStuffType end

const UnionStuff=Union{Type{Stuff1}, Type{Stuff2}, Type{Stuff3}}

function runtest()
    u0=[Stuff1, Stuff2, Stuff3]
    save("u0.jld2", "u0", u0)
    u1=load("u0.jld2")["u0"]
    @show u0==u1

    v0=Type{Stuff1}[Stuff1, Stuff1, Stuff1]
    save("v0.jld2", "v0", v0)
    v1=load("v0.jld2")["v0"]
    @show v0==v1

    w0=UnionStuff[Stuff1, Stuff2, Stuff3]
    save("w0.jld2", "w0", w0)
    w1=load("w0.jld2")["w0"]
    @show w0==w1

end

runtest()

jdl2mwe2.jl

using JLD2

struct VectorOfConstants{T, Tl} <: AbstractVector{T}
    val::T
    len::Tl
end
Base.IndexStyle(::Type{<:VectorOfConstants}) = IndexLinear()
Base.length(v::VectorOfConstants) = v.len
Base.size(v::VectorOfConstants) = (v.len,)
function Base.getindex(v::VectorOfConstants, i)
    if i > v.len
        throw(BoundsError(v, i))
    end
    return v.val
end
Base.iterate(v::VectorOfConstants) = (v.val, 1)
Base.iterate(v::VectorOfConstants, state) = state >= v.len ? nothing : (v.val, state + 1)
Base.unique(v::VectorOfConstants) = [v.val]

abstract type AbstractStuffType end
struct Stuff1 <: AbstractStuffType end
struct Stuff2 <: AbstractStuffType end
struct Stuff3 <: AbstractStuffType end

const UnionStuff=Union{Type{Stuff1}, Type{Stuff2}, Type{Stuff3}}

function runtest()
    u0=VectorOfConstants(Stuff1, 3)
    save("u0.jld2", "u0", u0)
    u1=load("u0.jld2")["u0"]
    @show u0==u1

    v0=VectorOfConstants{Type{Stuff1}, Int}(Stuff1, 3)
    save("v0.jld2", "v0", v0)
    v1=load("v0.jld2")["v0"]
    @show v0==v1

    w0=VectorOfConstants{UnionStuff, Int}(Stuff1, 3)
    save("w0.jld2", "w0", w0)
    w1=load("w0.jld2")["w0"]
    @show w0==w1

end

runtest()