Nullable with subtype

[andrewcooke]

I am having a problem when I assign a value to a Nullable which it a sub-type of the type contained by the Nullable. This works in 0.4, but not with Compat.

Here is the problem:

julia> abstract Parent

julia> immutable Child <: Parent end

julia> type Container x::Nullable{Parent} end

julia> c = Container(Child())
ERROR: `convert` has no method matching convert(::Type{Nullable{Parent}}, ::Child)
 in Container at no file

However, I have no idea how you solve this because:

julia> convert{T,S<:T}(::Type{Nullable{T}}, s::S) = Nullable{T}(s)
ERROR: T not defined

[andrewcooke]

PS Otherwise, though, thanks for an amazingly useful package.

[johnmyleswhite]

This pattern, convert{T,S<:T}, is what's sometimes called triangular dispatch. To my knowledge, it doesn't work in Julia yet.

I wonder if you want to be working with a parametric type for Container (e.g. Container{T}) rather than something non-concrete like Nullable{Parent}.

[andrewcooke]

but the code i posted works in 0.4 (i'm using a 6 days old checkout).

   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: http://docs.julialang.org
   _ _   _| |_  __ _   |  Type "help()" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.4.0-dev+4995 (2015-05-25 20:43 UTC)
 _/ |\__'_|_|_|\__'_|  |  Commit 96cc970 (6 days old master)
|__/                   |  x86_64-suse-linux

julia> abstract Parent

julia> immutable Child <: Parent end

julia> type Container x::Nullable{Parent} end

julia> c = Container(Child())
Container(Nullable(Child()))

(i can fix my code in various ways, and it may not be great how it is, but i was raising this issue because the code above works in 0.4, but not in 0.3, even when Compat is used).

[andrewcooke]

maybe my hamfisted attempt at guessing how julia works is misleading. ignore the final part of my first post if it helps.

[johnmyleswhite]

I think something has changed that I don't fully understand in either (a) how type parameters work when doing dispatch or (b) the way that convert relates to constructors.

You can raise the same problem you're hitting without involving user types by executing convert(Nullable{Integer}, 1). Contrast Julia 0.3:

julia> @which convert(Nullable{Integer}, 1)
convert(T,x) at base.jl:13

with Julia 0.4:

julia> @which convert(Nullable{Integer}, 1)
convert{T}(::Type{Nullable{T}}, x::T) at nullable.jl:15

[andrewcooke]

huh. well i've been poking through issues and changes and finally realised you wrote this stuff, so i am giving up :)

[johnmyleswhite]

I'm sure someone who understands the issues involved better than I do will chime in. While we wait, I wonder if redesigning things to avoid having a type Nullable{Parent} will solve multiple problems.

[andrewcooke]

JuliaLang/julia#8974 (comment) ?

[johnmyleswhite]

That does seem relevant. And I do find the covariant matching confusing.