Mark Kittisopikul (Apr 19 2021 at 16:46): Mark Kittisopikul (Apr 19 2021 at 16:46):I'm looking at some older code, and I see const None = Union{}. I get the feeling that this originated with some old Julia version. Does anyone know if that was the case?
Mark Kittisopikul (Apr 19 2021 at 16:47):Was this an older version of nothing ?
Mason Protter (Apr 19 2021 at 17:28):Hm, I don't know the history here but I assume not. Union{} can't be instantiated as a value whereas Nothing can. That means that Union{} only exists as a type level construct
Mason Protter (Apr 19 2021 at 17:29):Nowadays, we call Union{} by the name TupeofBottom, meaning it's the 'bottom' of the type hierarchy (where Any is the top)
Mark Kittisopikul (Apr 19 2021 at 17:32): Mark Kittisopikul (Apr 19 2021 at 17:33): Mason Protter (Apr 19 2021 at 17:33):Right, so that shows that None was not an early version of Nothing
Mark Kittisopikul (Apr 19 2021 at 18:10):Perhaps to be concrete, I'm currently looking at https://github.com/jheinen/GR.jl/blob/master/src/GR.jl#L250-L253 . I'm thinking these globals should be converted to const Ref but I would need a fixed type or a perhaps a Union.
Mason Protter (Apr 19 2021 at 20:25):Yeah, that's tricky. Do you know how many possible types this variable can take on?
Mark Kittisopikul (Apr 19 2021 at 22:06):I think most of these could be directly converted to Union{Nothing, String} or Union{Nothing, Int}. In the String case, it looks like we might be able to use empty string. In the Int case, a negative sentinel might work instead.
Perhaps the other question then is how performant is
julia> r = Ref{Union{Nothing,Int}}(5)
Base.RefValue{Union{Nothing, Int64}}(5)
julia> r[] = nothing
julia> r[] = 5
5
It's extremely efficient - usually optimally efficient. Especially when used with heap-allocated structs
Jakob Nybo Nissen (Apr 20 2021 at 06:12):A Union{Nothing, String} takes up only 8 bytes
Jakob Nybo Nissen (Apr 20 2021 at 06:17):And on master, a few important improvements to the compiler means that unions are handled more efficiently. So the only issue is e.g. Union{Int, Nothing} being 16 bytes.
Jakob Nybo Nissen (Apr 20 2021 at 13:37):Edit: Turns out I was wrong - it's only optimized for bitstypes.
Last updated: Oct 02 2023 at 04:34 UTC