promoting Nullable with anything?

[JeffBezanson]

e79e846 added the following definition:

promote_rule{S,T}(::Type{Nullable{S}}, ::Type{T}) = Nullable{promote_type(S, T)}

This might have unintended consequences. For example, we have:

julia> promote(1, "")
(1,"")

julia> promote(Nullable(1), "")
(Nullable{Any}(1),Nullable{Any}(""))

Given a Nullable and something completely unrelated, you get two Nullable{Any}s. I don't think two types should have a non-identity promotion just because one is Nullable. This was not obvious at the time.

I propose this definition instead:

function promote_rule{S,T}(::Type{Nullable{S}}, ::Type{T})
    R = promote_type(S, T)
    if R <: S || R <: T
    else
        if S <: R || T <: R
            return R
        end
    end
    return Nullable{R}
end

This says to leave the values alone if the promoted type (R) is a strict supertype of T or S, and otherwise promote as before. For example

julia> promote(Nullable(1),2.4)
(Nullable{Float64}(1.0),Nullable{Float64}(2.4))

julia> promote(Nullable(1),"")
(Nullable{Int64}(1),"")

This definition can still be type-inferred (since we can now handle constant conditions well).

[nalimilan]

I see your point. Maybe just check that the promoted type is different from Any instead? In practice I'm not sure it's really different, but it could be simpler to explain/remember.

Though there's always a cost in making rules more complex. Do you have a use case in mind where the current simple rule would be problematic?

[JeffBezanson]

There was a piece of code here: JuliaIO/JSON.jl#39 (comment)
that used vcat to combine values before assigning them to fields of another type. Since one was nullable, all values were converted to nullable and didn't match the expected field types.

[nalimilan]

OK. A related issue is that conversion to nullable is allowed, but conversion from nullable is not. Not sure we have other cases like this where promotion gives a type which cannot be converted back to the original type, but that sounds potentially problematic (as in your example).

[KristofferC]

Moved to JuliaAttic/Nullables.jl#6