[Veedrac]

Fundamentally, there are two ways of representing iteration pipelines: source driven, and drain driven. This almost always maps to the idea of _internal_ iteration and _external_ iteration, because the source is wrapped inside the transforms. Transducers are unusual in being source driven but also external iterators.

Most imperative languages choose one of two things, internal iteration that doesn't support composable flow control, and external iteration that does. This is why you see pause/resume style iteration in Python, Rust, Java, and even Javascript. If that's your experience, transducers are a pretty novel place in the trade-off space: you keep most of the composability, but you get to drive it from things like event sources.

But the gap is a bit smaller than it might appear. Rust's iterators are conceptually external iterators, but they actually do support internal iteration through `try_fold`, and even in languages that don't, you can 'just' convert external to internal iterators.

Then all you have to do to recover what transducers give you is pass the object to the source, let it run `try_fold` whenever it has data, and check for early termination via `size_hint`. There's one more trick for the rare case of iterators with buffering, but you don't have to change the Iterator interface for that, you just need to pass one bit of shared state to the objects on construction.

Not all Iterators are strictly valid to be source-driven, and while most do, not everything works nicely when iterated this way (eg. Skip could but doesn't handle this case correctly, because it's not required to), but I don't think transducers can actually do anything this setup can't. It's just an API difference after that point.

[solomonb]

> If that's your experience, transducers are a pretty novel place in the trade-off space

That is not my experience and TBH I don't know what a lot of your terminology specifically means.

[Veedrac]

I wasn't saying you would have that experience, I was saying that the reason people act like transducers are unique is that transducers are an unconventional place on well worn ground.

Ultimately, yes, everything bottoms out, most special tricks seem less special the more you understand about them, because it's programming and Turing Equivalence is the bedrock the whole field rests on. But the average person learning about transducers is not going to spot how closely related it is to other things that already exist.

I'm happy to elaborate on any part of the terminology if you're curious, but tbh I mostly wrote it for myself because I thought the framing was novel and wanted it noted down somewhere.