# Why is objectid not foldable for type arguments?

**URL:** <https://discourse.julialang.org/t/why-is-objectid-not-foldable-for-type-arguments/101928>\
**Category:** Internals & Design\
**Tags:** hash\
**Created:** [July 22, 2023, 2:48pm UTC](https://discourse.julialang.org/t/why-is-objectid-not-foldable-for-type-arguments/101928 "2023-07-22T14:48:35Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![matthias314](https://avatars.discourse-cdn.com/v4/letter/m/a88e4f/32.png) [@matthias314](https://discourse.julialang.org/u/matthias314)\
**Post date:** [July 22, 2023, 2:48pm UTC](https://discourse.julialang.org/t/why-is-objectid-not-foldable-for-type-arguments/101928/1 "2023-07-22T14:48:35Z")

</div>

For some types, `objectid` now folds in Julia 1.10-alpha1:

```julia
julia> f() = objectid(1); @code_typed f()
CodeInfo(
1 ─ return 0x5bca7c69b0d0ebca
) => UInt64

```

However, it doesn’t fold with a type as argument:

```julia
julia> f() = objectid(Int); @code_typed f()
CodeInfo(
1 ─ %1 = Main.Int::Type{Int64}
│ %2 = $(Expr(:foreigncall, :(:jl_object_id), UInt64, svec(Any), 0, :(:ccall), :(%1)))::UInt64
└── return %2
) => UInt64

```

(The reason is that the folding behavior is determined by `Base.isidentityfree`, which returns `false` for types.)

I haven’t gone through the code of `jl_object_id` in detail, but from playing around with `objectid` I get the impression that it is consistent for types, even across different sessions and different machines. (For `Int` I always get `0x0000000007441304`, for instance.) Is this impression wrong, or is there any other reason that `objectid` should not fold for types?

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<div class="post-metadata">

**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [July 22, 2023, 4:52pm UTC](https://discourse.julialang.org/t/why-is-objectid-not-foldable-for-type-arguments/101928/2 "2023-07-22T16:52:42Z")

</div>

this is a very annoying one in that all well formed types have consistent objectids, but it is possible to make horrible abstract types that are not instantiable that don’t have consistent object ids

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<div class="post-metadata">

**Author:** ![matthias314](https://avatars.discourse-cdn.com/v4/letter/m/a88e4f/32.png) [@matthias314](https://discourse.julialang.org/u/matthias314)\
**Post date:** [July 22, 2023, 5:13pm UTC](https://discourse.julialang.org/t/why-is-objectid-not-foldable-for-type-arguments/101928/3 "2023-07-22T17:13:23Z")

</div>

Are there some easily identifiable types for which one could make `objectid` fold? Your answer sounds as if at least concrete types would qualify.

I came across this in the context of writing (my own macro for generating) hash functions. For a type

```julia
struct A{T}
    x::T
end

```

one could use any one of

```julia
hash(a::A, h::UInt) = hash(a.x, h)
hash(a::A, h::UInt) = hash(a.x, hash(A, h))
hash(a::A{T}, h::UInt) where T = hash(a.x, hash(A{T}, h))

```

depending on how much of the type information one wants to see in the hash. Now `hash` of a type is computed via `objectid`. In the cases above where the type does enter the hash, one either has a concrete type or a very specific `UnionAll` type. From a performance point of view, it would be great if `objectid` could fold in these cases.

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<div class="post-metadata">

**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [July 22, 2023, 6:06pm UTC](https://discourse.julialang.org/t/why-is-objectid-not-foldable-for-type-arguments/101928/4 "2023-07-22T18:06:31Z")

</div>

Yeah we probably need to make this smarter It’s just one that’s mildly annoying and no one has bothered to fix it yet.