As a practicing economist, I wonder what use cases you're referring
to.  I can't think of any use cases where if one previous value is
useful, having all previous values available (ie, an arbitrary
temporal structure, at the modeler's option) isn't vastly more useful.

Well, see the itertools.accumulate examples yourself then, the ones at We can start with something really simple like interest rates or uniform series, but... before arguing here, please convince other people to update the Wikipedia:

"Recurrence relations, especially linear recurrence relations, are used extensively in both theoretical and empirical economics."

So in the end, even if there are plausible use cases for quick and dirty code, [...]

The proposal isn't about quick and dirty code. The State-space example includes a general linear time-varying MIMO simulation implementation trying to keep the syntax as similar as possible to the control theory engineers are used to. Also, my goal when I was looking for a scan syntax to solve the conditional toggling example was to make it cleaner.

If you aren't seeing the examples I wrote, I wonder what are you writing about.

There was a discussion about this a while ago.

And where's the link?

[...]. From what
I remember, the conclusion reached was that there are too
many degrees of freedom to be able to express reduction
operations in a comprehension-like way that's any clearer

I don't know if that's a conclusion from any other thread, but that's wrong. The only extra "freedom" required a way to access the previous output (or "accumulator", "memory", "state"... you can use the name you prefer, but they're all the same). How many parameters does itertools.scan have? And map/filter?

I can't talk about a discussion I didn't read, it would be unfair, disrespectful. Perhaps that discussion was about an specific proposal and not about the requirements to express a scan/fold.

This proposal should be renamed to "recursive list comprehension". 3 words, and it's a complete description of what I'm talking about. For people from a functional programming background, that's about an alternative syntax to write the scan higher order function.

Forget the word "reduce", some people here seem to have way too much taboo with that word, and I know there are people who would prefer a higher McCabe complexity just to avoid it. Perhaps there are people who prefer masochist rituals instead of using "reduce", who knows? Who cares? I like reduce, but I'm just proposing a cleaner syntax for recursive list comprehensions, and "reduce" isn't the general use case for that. On the contrary, "reduce" is just the specific scenario where only the last value matters.

[...] you better  have a VERY GOOD reason.

I spent months writing PyScanPrev, mainly the examples in several reStructuredText files, not because I was forcing that, but because there are way too many use cases for it, and I know those examples aren't exhaustive.

But talking about "good reasons" reminds me of the "annotations"! A function that uses annotations for one library can't use annotations for another library unless their annotation values are the same, but if one package/framework needs "type information" from your function parameters/result and another package/framework collects "documentation" from it, there's no way to get that working together. Something like "x : int = 2" makes the default assignment seem like an assignment to the annotation, and there's even a new token "->" for annotations. When I saw Python annotations at first I though it was a joke, now I know it's something serious with [mutually incompatible] libraries/packages using them. I strongly agree that everything should need a good reason, but I wrote a lot about the scan use cases and no one here seem to have read what I wrote, and the only reason that matters seem to be a kind of social status, not really "reason". I probably wrote way more reasons for that proposal than annotations could ever have. But if no one seem to care enough to read, then why should I insist? That's like my pprint bugfix patch some months ago, was it applied? AFAIK not even core developers giving +1 was enough for it to be applied.

This maillist isn't very inviting... but I hope some of you at least try to read the rationale and the examples.

Danilo J. S. Bellini
"It is not our business to set up prohibitions, but to arrive at conventions." (R. Carnap)