Stream: helpdesk (published)

Topic: Equivalent of a `scan` function?

Brian Chen (May 28 2022 at 16:54):

Copied from Slack and hoping for some more expert opinions here :)

Do we have an equivalent to https://jax.readthedocs.io/en/latest/_autosummary/jax.lax.scan.html? accumulate is the closest stdlib I know of, but it doesn't allow for carrying of non-output intermediate states. https://juliafolds.github.io/Transducers.jl/dev/reference/manual/#Transducers.ScanEmit was the best example I could find in the broader ecosystem, but I'm not sure if there's a way to extract uₙ from it.

Mason Protter (May 28 2022 at 20:15):

@Brian Chen yeah, the $u_n$ should be available at least depending on what you're planning to do with them

Mason Protter (May 28 2022 at 20:19):

julia> 1:3 |> ScanEmit(-1) do u, x
           @show u
           (x+1, u-1)
       end |> collect
u = -1
u = -2
u = -3
3-element Vector{Int64}:
 2
 3
 4

Mason Protter (May 28 2022 at 20:22):

The jax readme was not very enlightening to me, maybe you could give a julian pseudoexample of what you want?

Brian Chen (May 28 2022 at 20:33):

It's available but not returned, unfortunately. For comparison JAX's scan would return (-3, [2, 3, 4]).

Brian Chen (May 28 2022 at 20:34):

I could try a local var, but closure boxing makes that difficult. A Ref or other mutable struct is a no-go to because Zygote

Brian Chen (May 28 2022 at 20:38):

Mason Protter said:

maybe you could give a julian pseudoexample of what you want?

function scan(f, init, xs)
  acc = init
  ys = []
  for x in xs
    acc, y = f(acc, x)
    push!(ys, y)
  end
  return acc, ys
end

Brian Chen (May 28 2022 at 20:44):

This is the minimal version. JAX does a bunch of extra stuff with dim handling and SoA inputs, but all I want for Flux/Lux is new_rnn_state, output_seq = scan(rnn_forward, rnn_state, input_seq).

Mason Protter (May 28 2022 at 22:37):

Hm, maybe the onlast option can do that?

Mason Protter (May 29 2022 at 02:14):

@Takafumi Arakaki (tkf) any ideas?

Takafumi Arakaki (tkf) (May 29 2022 at 03:46):

You can do

julia> rf = ScanEmit(-1) do u, x
           @show u
           (x+1, u-1)
       end'(push!!) |> Completing
       acc = Transducers.start(rf, Int[])
       acc = Transducers.foldl_nocomplete(rf, acc, 1:3);
u = -1
u = -2
u = -3

julia> Transducers.unwrap(rf, acc)  # (u, ys)
(-4, [2, 3, 4])

although Transducers.foldl_nocomplete is not documented

Edit: s/append!!/push!!/

Brian Chen (May 29 2022 at 04:14):

That works! Now I need to figure out how to make it Zygote-friendly...

Takafumi Arakaki (tkf) (May 29 2022 at 04:55):

I'd guess that this is not very Zygote friendly, unfortunately... As this is a very generic concept, why not actually write it by hand and also define the chain rule (i.e., make it effectively a "builtin")?

(The foldl part can be optimized by https://github.com/JuliaFolds/FoldsChainRules.jl but we still have push!!)

Brian Chen (May 29 2022 at 04:57):

Was thinking of how to write one :) a function like push!! would make the rule itself trivial (well, trivial for Vectors at least), but nested differentiation makes my head hurt

Brian Chen (May 29 2022 at 04:57):

Any chance you think for something like this to be in Base?

Brian Chen (May 29 2022 at 05:01):

One advantage jax has is that they enforce inputs/outputs to be scalar numbers, arrays or structures of arrays. That allows for tricks like pre-allocating an output buffer for scanning over the first dimension of a multi-dimensional array.

Takafumi Arakaki (tkf) (May 29 2022 at 09:52):

Brian Chen said:

One advantage jax has is that they enforce inputs/outputs to be scalar numbers, arrays or structures of arrays.

Yes, I agree it is an advantage for the compiler/library authors. But it sounds like a flip side of a disadvantage (= limit the kind of program written easily) on the user side to me. In Julia, I think it'd be more natural to let the users express the constraints (output is an array, output shape is known, output element type, etc.) through an API function call. I think it'd be some extension of the "into" PR I opened in julia.

Brian Chen (May 29 2022 at 13:52):

I agree 100%, but the perfect is also kind of the enemy of the good here. Given very limited time as a library author, I'm more likely to leave the status quo of bad performance than to try rolling my own little subsystem for this.

Brian Chen (May 29 2022 at 14:52):

Also double-checked that I am indeed subscribed to that into PR :big_smile:

Last updated: Oct 26 2025 at 04:40 UTC