# Type stability of destructure / re-build in Optimisers.jl
**URL:** https://discourse.julialang.org/t/type-stability-of-destructure-re-build-in-optimisers-jl/128783
**Category:** Optimization (Mathematical)
**Created:** [May 7, 2025, 10:14am UTC](https://discourse.julialang.org/t/type-stability-of-destructure-re-build-in-optimisers-jl/128783 "2025-05-07T10:14:25Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@FerreolS](https://discourse.julialang.org/u/FerreolS)
#### Post date: [May 7, 2025, 10:14am UTC](https://discourse.julialang.org/t/type-stability-of-destructure-re-build-in-optimisers-jl/128783/1 "2025-05-07T10:14:25Z")
Hi,
Destructuring struct with `destructure` is very handy. Thank you.
However I’m a bit surprise that the restructuring is not Type stable. Am I missing something? Does it have some impact on speed when `re(flat)` (and its gradient) is called very often ?
```julia-repl
julia> using Optimisers
julia> struct Bar; a::Float64;b::Vector{Float64};end
julia> b = Bar(2.0, randn(10))
Bar(2.0, [0.42917489814746823, 1.266826972405323, 0.6992954064945902, -0.965529236441083, -0.6069994307534123, -0.5668205590293701, -0.231523643263828, 1.9408140168456018, 0.6864706994403353, 0.6929670854487652])
julia> flat, re = destructure(b)
([0.42917489814746823, 1.266826972405323, 0.6992954064945902, -0.965529236441083, -0.6069994307534123, -0.5668205590293701, -0.231523643263828, 1.9408140168456018, 0.6864706994403353, 0.6929670854487652], Restructure(Bar, ..., 10))
julia> @code_warntype re(flat)
MethodInstance for (::Optimisers.Restructure{Bar, @NamedTuple{a::Tuple{}, b::Int64}})(::Vector{Float64})
from (re::Optimisers.Restructure)(flat::AbstractVector) @ Optimisers ~/.julia/packages/Optimisers/W5seC/src/destructure.jl:59
Arguments
re::Optimisers.Restructure{Bar, @NamedTuple{a::Tuple{}, b::Int64}}
flat::Vector{Float64}
Body::Any
1 ─ %1 = Optimisers._rebuild::Core.Const(Optimisers._rebuild)
│ %2 = Base.getproperty(re, :model)::Bar
│ %3 = Base.getproperty(re, :offsets)::@NamedTuple{a::Tuple{}, b::Int64}
│ %4 = Base.getproperty(re, :length)::Int64
│ %5 = (%1)(%2, %3, flat, %4)::Any
└── return %5
```
---
### Author:  [@mcabbott](https://discourse.julialang.org/u/mcabbott)
#### Post date: [May 7, 2025, 12:36pm UTC](https://discourse.julialang.org/t/type-stability-of-destructure-re-build-in-optimisers-jl/128783/2 "2025-05-07T12:36:18Z")
Glad it’s useful. I agree it would be nice if this were type-stable.
I think the core problem is that Optimisers.jl cares about object identity. If the same mutable object appears more than once in a struct, this `===` relation is preserved. But this property isn’t visible to the type system. The reconstruction chooses whether to make a new `Vector{Float64}`, or to retrieve one from an `IdDict{Any}`, based on `objectid`. (This is all outsourced to Functors.jl.)
That may not be insurmountable – the two choices should have the same type. There are examples of ID-aware recursion which are type-stable, e.g. `Base.deepcopy`. In fact `Functors.fmap` has been upgraded to often be type-stable. Maybe the same could be done for `destructure`?
```julia
julia> struct Tri; a::Float64; b::Vector{Float64}; c::Vector{Float64}; end
julia> bi = Tri(1.11, [2.22, 3.33], [2.22, 3.33]);
julia> un = let twice = [2.22, 3.33]
Tri(1.11, twice, twice)
end;
julia> typeof(bi) == typeof(un)
true
julia> v1, re1 = Optimisers.destructure(un)
([2.22, 3.33], Restructure(Tri, ..., 2))
julia> v2, re2 = Optimisers.destructure(bi)
([2.22, 3.33, 2.22, 3.33], Restructure(Tri, ..., 4))
julia> typeof(re2) == typeof(re1)
true
julia> @code_warntype re1([4.4, 5.5]) # bad
julia> @code_warntype Base.deepcopy(un) # fine
julia> @code_warntype Functors.fmapstructure(complex, un) # also fine
```
See also [issue 177](https://github.com/FluxML/Optimisers.jl/issues/177).
Another approach is to make `destructure!` which mutates the original object, instead of reconstructing. Then the return type ought to be trivial to infer. This was [PR 165](https://github.com/FluxML/Optimisers.jl/pull/165), it got stuck but could perhaps be revived.
---
### Author:  [@FerreolS](https://discourse.julialang.org/u/FerreolS)
#### Post date: [May 7, 2025, 3:20pm UTC](https://discourse.julialang.org/t/type-stability-of-destructure-re-build-in-optimisers-jl/128783/3 "2025-05-07T15:20:49Z")
Thank you very much. I didn’t saw this issue 177 that similar to my issue. For the moment, I will explicitly set the return type of the reconstruct expression as suggested there. I’m not guru enough to submit a PR on that.
Anyway, I find this package very useful and it has a broader use case than just optimization (in my case I don’t use the optimization part). May I suggest to put the parameter part in a separate package with a much clearer name for users. It will be easier to find.
Thanks again