3 ms·
Here's me copying your second example as closely as possible in julia: using MLStyle mutable struct TypeStable o::Union{Some{Int}, Nothing}
by eigenspace 5y ago
Here's me copying your second example as closely as possible in julia:
using MLStyle
mutable struct TypeStable
o::Union{Some{Int}, Nothing}
end
MLStyle.@as_record TypeStable
MLStyle.@as_record Nothing
function type_stable()
o = returns_option()
@match o.o begin
Some(_) => 1
Nothing() => 0
end
end
returns_option() = TypeStable(rand(Bool) ? Some(rand(1:10)) : nothing)
And what does the compiler have to say about it?
julia> Core.Compiler.return_type(type_stable, Tuple{})
Int64
and
julia> @code_warntype type_stable()
Variables
#self#::Core.Const(type_stable)
o::TypeStable
259::Union{Nothing, Some{Int64}}
return#257::Union{Nothing, Int64}
Body::Int64
1 ─ (o = Main.returns_option())
│ (return#257 = Main.nothing)
│ (259 = Base.getproperty(o, :o))
│ %4 = (259 isa Nothing)::Bool
└── goto #3 if not %4
2 ─ (return#257 = 0)
└── goto #5
3 ─ %8 = (259::Some{Int64} isa Some)::Core.Const(true)
│ %8
│ %10 = (259::Some{Int64} !== Main.nothing)::Core.Const(true)
│ %10
│ (return#257 = 1)
└── goto #5
4 ─ Core.Const(:((error)("matching non-exhaustive, at #= REPL[9]:3 =#")))
5 ┄ return return#257::Int64
This is on version 1.6.1 for me, but should work fine on earlier releases. I agree that having MLStyle.jl pattern matching bundled into julia would be great!
- hexane360 5y agoDoesn't the TypeStable struct just hide the unknown type, not remove it? The type of o.o still needs to be looked up at runtime. Furthermore, TypeStable itself has to be on the heap, which I think means this causes two pointer lookups rather than just one.
- eigenspace 5y ago> Doesn't the TypeStable struct just hide the unknown type, not remove it? No, of you look at the typed IR I posted, it says 259::Union{Nothing, Some{Int64}} and (259 = Base.getproperty(o, :o)) So it's inferred properly to be a small union which julia handles efficiently at runtime: julia> @btime type_stable() 18.988 ns (1 allocation: 32 bytes) 1 > Furthermore, TypeStable itself has to be on the heap, which I think means this causes two pointer lookups rather than just one. It's on the heap because you wrote mutable struct in your example and I was just mimicking you as close as possible. If you don't actually want it to be mutable, then you can just remove the keyword mutable and it'll be even faster and have no heap allocations: julia> @btime type_stable() 18.426 ns (0 allocations: 0 bytes) 1
- hexane360 5y ago>small union which julia handles efficiently at runtime Oh! I didn't realize Julia already optimized small unions. https://julialang.org/blog/2018/08/union-splitting/ https://julialang.org/blog/2018/08/union-splitting/ https://docs.julialang.org/en/v1/devdocs/isbitsunionarrays/ https://docs.julialang.org/en/v1/devdocs/isbitsunionarrays/ I wasn't quite sure whether something like this was possible given the semantics of Julia. Personally, I'd still prefer something more explicit, so it would be more obvious when a union is being handled efficiently. But given that small unions are optimized, I can see why Julia would make the opposite decision. This makes me feel a lot better about writing Julia code with union types. Thanks
- eigenspace 5y agoYep, small union optimizations are a godsend!