# Comments on "Transducers & Effects – Mike Innes"

**URL:** <https://discourse.julialang.org/t/comments-on-transducers-effects-mike-innes/41353>\
**Category:** Internals & Design\
**Created:** [June 14, 2020, 4:10am UTC](https://discourse.julialang.org/t/comments-on-transducers-effects-mike-innes/41353 "2020-06-14T04:10:50Z")\
**Posts on this page:** 1\
**Showing post:** 3

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**Author:** ![tkf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tkf/32/17635_2.png) [@tkf](https://discourse.julialang.org/u/tkf)\
**Post date:** [June 15, 2020, 9:35pm UTC](https://discourse.julialang.org/t/comments-on-transducers-effects-mike-innes/41353/3 "2020-06-15T21:35:18Z")

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> [@MikeInnes](#):
>
> This is quite a different way of thinking about it, but the core idea is the same: use the same syntax to define a sequence which can either eagerly (and efficiently) produce values upfront, or lazily produce them on demand.

It’s nice to know we arrived at similar solutions from very different angles 🎉

> [@MikeInnes](#):
>
> The main motivation for `into` is that if you know what data structure you’re producing, you can avoid the state machine in some cases (eg building an array via `collect` ).

The corresponding API I use in BangBang.jl (one of the spin-off/dependency libraries of Transducers.jl) is [`append!!(dest, src) -> dest′`](https://juliafolds.github.io/BangBang.jl/dev/#BangBang.append!!). It is driven by the `foldl` of `src` by default. But it’s possible for `dest` to take the control and drive `src` iteration via `iterate`.

> [@MikeInnes](#):
>
> by dispatch (doing the right thing for the data structure)

I’d even say Julia is partially doing this already as of [Transducer as an optimization: map, filter and flatten by tkf · Pull Request #33526 · JuliaLang/julia](https://github.com/JuliaLang/julia/pull/33526). Here, `foldl` dispatches on the lazy transformed iterator types and apply the corresponding transducers to the reducing step function. OK, this still works only with functions calling `foldl` in their call chain. However, it should be possible to lower `for x in xs` to `foldl` [as I demonstrated in FLoops.jl](https://discourse.julialang.org/t/rfc-ann-floops-jl-fast-generic-for-loops-foldl-for-humans/37876) so that the native `for` loop syntax is lowered to appropriate code via the dispatch mechanism.

> [@MikeInnes](#):
>
> Here’s how I’d write `zip` :

What I wasn’t sure was that there might be some magical way to directly do it with `handle`. But the effect handler seems to need a corresponding scope and using `next(xs)` looks like a clean solution to me.

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