Stream: helpdesk (published)

Topic: Ref vs Union{T, Nothing}

James Wrigley (Oct 09 2025 at 08:58):

Are there any opinions on whether it's better to use a Ref or Union{T, Nothing} type as a kind of nullable type? I vaguely prefer a union as a matter of style, but maybe Ref's are better for type-stability and error messages. Though one downside of a Ref is that AFAIK there's no way to unassign one.

Mason Protter (Oct 09 2025 at 09:11):

I would use a union personally

Mason Protter (Oct 09 2025 at 09:14):

I doubt there'd be any real type stability benefits to the Ref, since with the union branch you would always do a

if !isnothing(foo.field)
    your_code
else
    some_error_branch()
end

James Wrigley (Oct 09 2025 at 09:23):

I should, but I admittedly I don't always remember to :peep:

Jakob Nybo Nissen (Oct 09 2025 at 11:28):

How can a Ref be nullable?

James Wrigley (Oct 09 2025 at 11:33):

As in you can check whether it has a value with isassigned(). And dereferencing an unassigned Ref will immediately throw an exception, which is nice.

Mason Protter (Oct 09 2025 at 11:35):

note that will only work for non-isbits types

Mason Protter (Oct 09 2025 at 11:35):

julia> r = Ref{Int}()
Base.RefValue{Int64}(140383053485888)

julia> isassigned(r)
true

James Wrigley (Oct 09 2025 at 11:36):

Ugh true, I forgot about that.

Jakob Nybo Nissen (Oct 09 2025 at 11:46):

Also you can't un-assign a Ref (if you need to)

Jakob Nybo Nissen (Oct 09 2025 at 11:46):

Just use Union :thumbs_up:

Sukera (Oct 09 2025 at 15:49):

I usually prefer Union{Some{T}, Nothing}, since that forces me to handle it and I can't forget about it anymore. The downside is that it peppers my code with @something (..) throw(...) to handle the Nothings

John Lapeyre (Oct 09 2025 at 18:16):

Sukera said:

The downside is that it peppers my code

This is the problem with the "make Julia more like Rust" program.

Sukera (Oct 09 2025 at 20:21):

Not sure what this has to do with Rust, this is just the cost of checking my callsites & the lack of syntax support for this in Julia. I personally don't mind this downside at all, because I can rest assured that I have checked every error path

John Lapeyre (Oct 10 2025 at 00:43):

I'm not saying that checking every error path is bad or that doing it with Union{Some{T}, Nothing} is bad. If you went through a Julia program and did this everywhere, you'd probably have a more reliable program. It would also look more like rust code than it did before you made the change.

Last updated: Oct 18 2025 at 04:39 UTC