Define reverse method for Composite?

[mzgubic]

Should we define a reverse method for Composite?

julia> t = (1, 2, 3.0);

julia> T = typeof(t)
Tuple{Int64,Int64,Float64}

julia> reverse(t)
(3.0, 2, 1)

julia> c = Composite{T}(t)
Composite{Tuple{Int64,Int64,Float64}}((1, 2, 3.0),)

julia> reverse(c)
ERROR: MethodError: no method matching reverse(::Composite{Tuple{Int64,Int64,Float64},Tuple{Tuple{Int64,Int64,Float64}}})

It should be pointed out that reverse does not work on NamedTuples:

julia> nt = (a=1, b=2, c=3.0)
(a = 1, b = 2, c = 3.0)

julia> reverse(nt)
ERROR: MethodError: no method matching reverse(::NamedTuple{(:a, :b, :c),Tuple{Int64,Int64,Float64}})

I guess we could support it for Composites whose primal type is reverse-ible?

[nickrobinson251]

What's the use-case? e.g. when would we need this? Or when would it make something easier/simpler/cleaner?

[mzgubic]

The context is I stumbled upon this when moving Zygote over to use ChainRules differential types.

When defining the adjoint for various map-like functions, the pullbacks are applied in reverse order, which is needed if the mapped function is stateful.

If reverse was defined it would have saved me about 10-15 minutes of debugging + implementation time. I don't foresee any future uses, but to be fair I would not have foreseen this one either 😂

[oxinabox]

Yes,it should.
It is a linear operator.