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remove rule for identity #64

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merged 1 commit into from Aug 12, 2021
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KristofferC
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This one causes quite a lot of invalidations (SciML/DiffEqBase.jl#698 (comment)). It was added in #54 but it is unclear why it was added.

Is it needed for anything?

@ChrisRackauckas
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We need it in the symbolic stack because identity seems to show up from random things like linear algebra, so I think ForwardDiff.jl needs it too. It would need a lot of downstream tests to remove IMO given how many weird places we've seen this crop up. I would be scared of deleting it, even though it does cause invalidations.

@KristofferC
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KristofferC commented Aug 11, 2021

Ok, we should add some test then. I don't see how the default fallback for identity doesn't work for ForwardDiff or the symbolic stack. Do you have an example?

@ChrisRackauckas
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Oh I see what you're saying.

idenity(f::Dual) = Dual(identity(f.val), identity(f.val) * f.der) == Dual(identity(f.val), identity(f.val)) == identity(f::Any)

For some reason I was thinking the derivative of the identity is 1, so identity(Dual(2,2)) == Dual(2,1)... but no you're right. The standard identity dispatch is fine, so ForwardDiff shouldn't be defining a Dual method on this.

@ChrisRackauckas
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Nabla failures unrelated @oxinabox ?

@oxinabox
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Nabla failures unrelated.
Latest version of Nabla shouldn't even use DiffRules anymore.

@oxinabox
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I agree this is safe to remove.
It is technically breaking.
But it has only been there a month, and Symbolics.jl is the only thing doing anything new in that period that I am aware of.
So if it doesn't hurt Symbolics then it is all good.

@ChrisRackauckas ChrisRackauckas merged commit 8c586af into JuliaDiff:master Aug 12, 2021
@ChrisRackauckas
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If it hurts Symbolics then we'll find out quickly so it's good. Rip off the bandaid.

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3 participants