Cannot `convert` an object of type

General Usage
1

What is wrong with this code? Can I fix it by implementing convert somehow?

const Maybe{T} = Union{Some{T},Nothing}

struct Wrapper{F}
    f::F
end

struct WrappedPerson{T}
    name::Wrapper{T}
    age::Wrapper{T}
end

WrappedPerson{Maybe}(Wrapper(Some("Alice")), Wrapper(Some(25)))
# MethodError: Cannot `convert` an object of type
#   Wrapper{Some{String}} to an object of type
#   Wrapper{Union{Nothing, Some{T}} where T}
2

Julia’s type parameters (other than Tuple) are invariant.

This means that

julia> Wrapper(Some("Alice")) isa Wrapper{Maybe}
false

julia> Wrapper(Some("Alice")) isa Wrapper{<:Maybe}
true
3

Can I fix it by defining convert?

4

I think it’d be better fixed at the point where you’re making the wrappers. Rather than write

Wrapper(Some("Alice"))

you could write

Wrapper{Maybe}(Some("Alice"))

e.g.

julia> WrappedPerson{Maybe}(Wrapper{Maybe}(Some("Alice")), Wrapper{Maybe}(Some("Alice")))
WrappedPerson{Union{Nothing, Some{T}} where T}(Wrapper{Union{Nothing, Some{T}} where T}(Some("Alice")), Wrapper{Union{Nothing, Some{T}} where T}(Some("Alice")))

But yeah, I think you could also define some methods to Base.convert that turns a Wrapper{T} into Wrapper{Maybe} I guess. All of this seems pretty unidiomatic and unlikely to work well though. These are like sum types, but not well contained or bounded.