Jesper Stemann Andersen (Aug 14 2023 at 21:02): Jesper Stemann Andersen (Aug 14 2023 at 21:02):I have a (mutable, edit) struct with a field that has type Union{T, Nothing}, and I've been trying to implement Base.isless for this struct, but I cannot figure out how to make JET.jl happy...
A simple function works fine,
julia> function foo(a::Union{Int, Nothing}, b::Union{Int, Nothing})
if a isa Nothing
if b isa Nothing
return false
else
return true
end
else
if b isa Nothing
return false
else
return a < b
end
end
end
foo (generic function with 1 method)
julia> JET.@report_opt foo(1, 2)
No errors detected
julia> JET.@report_call foo(1,2)
No errors detected
But once the a and b are encapsulated in a struct, JET complains:
julia> mutable struct Bar
x::Union{Int, Nothing}
end
julia> function Base.isless(a::Bar, b::Bar)
if a.x isa Nothing
if b.x isa Nothing
return false
else
return true
end
else
if b.x isa Nothing
return false
else
return a.x < b.x
end
end
end
julia> JET.@report_opt Base.isless(Bar(1), Bar(2))
═════ 2 possible errors found ═════
┌ isless(a::Bar, b::Bar) @ Main ./REPL[16]:12
│┌ <(x::Int64, y::Nothing) @ Base ./operators.jl:343
││ runtime dispatch detected: isless(x::Int64, y::Nothing)
│└────────────────────
│┌ <(x::Nothing, y::Int64) @ Base ./operators.jl:343
││ runtime dispatch detected: isless(x::Nothing, y::Int64)
│└────────────────────
julia> JET.@report_call Base.isless(Bar(1), Bar(2))
═════ 3 possible errors found ═════
┌ isless(a::Bar, b::Bar) @ Main ./REPL[16]:12
│┌ <(x::Nothing, y::Nothing) @ Base ./operators.jl:343
││ no matching method found `isless(::Nothing, ::Nothing)`: isless(x::Nothing, y::Nothing)
│└────────────────────
│┌ <(x::Int64, y::Nothing) @ Base ./operators.jl:343
││ no matching method found `isless(::Int64, ::Nothing)`: isless(x::Int64, y::Nothing)
│└────────────────────
│┌ <(x::Nothing, y::Int64) @ Base ./operators.jl:343
││ no matching method found `isless(::Nothing, ::Int64)`: isless(x::Nothing, y::Int64)
│└────────────────────
Using JET v0.8.9 on Julia v1.9.2.
Any ideas?
Jesper Stemann Andersen (Aug 14 2023 at 21:12):... I guess JET.@report_opt is already telling me, quite clearly, that it ends up in dynamic dispatch..., so of course making the Bar field type a type parameter <: Union{Int, Nothing} works...
julia> mutable struct Bar{T<:Union{Int, Nothing}}
x::T
end
julia> function Base.isless(a::Bar, b::Bar)
if a.x isa Nothing
if b.x isa Nothing
return false
else
return true
end
else
if b.x isa Nothing
return false
else
return a.x < b.x
end
end
end
julia> using JET
julia> JET.@report_opt Base.isless(Bar(1), Bar(2))
No errors detected
julia> JET.@report_call Base.isless(Bar(1), Bar(2))
No errors detected
... but that's not what I was looking for - I would like to be able to modify the field - make it a Nothing and vice versa...
Jakob Nybo Nissen (Aug 15 2023 at 04:52):The issue is that currently, the Julia compiler doesn't realise that if you load the same field multiple times from the same struct, the field is of the same type. So, even though you checked that a.x is not nothing and b.x is not nothing, when you do the comparison, Julia has "forgotten" that you checked
Jakob Nybo Nissen (Aug 15 2023 at 04:53):You solve it by storing them in a variable
Jakob Nybo Nissen (Aug 15 2023 at 04:56):See more here: https://aviatesk.github.io/JET.jl/stable/jetanalysis/#no-matching-method-found-(x/y-union-split)
Jesper Stemann Andersen (Aug 15 2023 at 05:20):Right! I should’ve re-read the documentation for JET… Thanks!
Notification Bot (Aug 15 2023 at 06:09):Jesper Stemann Andersen has marked this topic as resolved.
Sebastian Pfitzner (Aug 15 2023 at 07:11):worth noting that JET is correct in this specific case -- Bar is mutable after all, so some other thread could write a nothing to a.x or b.x after your check. Only accessing the field once and storing that in a local variable technically changes the behaviour of your program
Jesper Stemann Andersen (Aug 15 2023 at 07:16):Right ... I was also wondering whether that point should be added as a footnote to the JET documentation...
Last updated: Nov 06 2024 at 04:40 UTC