Issue #17016 has been updated by mame (Yusuke Endoh).


This ticket was discussed in the dev meeting, but no conclusion was reached.

BTW, Mathematica's "Accumulate" is different from this proposal. It is not general but specific to Plus.

https://reference.wolfram.com/language/ref/Accumulate.html

> Accumulate[list] is effectively equivalent to FoldList[Plus,list].

Thus, FoldList is more suitable name than Accumulate in this case.

https://reference.wolfram.com/language/ref/FoldList.html

The use case explained in #18 is not so convincing that it deserves a built-in feature. To make something built-in, one should show that it is so frequently written, and/or that it is difficult to work around. We are not sure that retaining a historical context is a very frequent code pattern (@mrkn said that cumulative sum is actually often used in mathematics, but other use case than cumulative sum was not clear). And Eregon's workaround in #9 looks very simple (maybe even simpler than scan_left).

----------------------------------------
Feature #17016: Enumerable#accumulate
https://bugs.ruby-lang.org/issues/17016#change-91593

* Author: parker (Parker Finch)
* Status: Open
* Priority: Normal
----------------------------------------
## Proposal

UPDATE: Changed proposed method name from `#scan_left` to `#accumulate`.

Add an `#accumulate` method to `Enumerable`.

## Background

`#accumulate` is similar to `#inject`, but it accumulates the partial results that are computed. As a comparison:
```
[1, 2, 3].inject(0, &:+) => 6
[1, 2, 3].accumulate(0, &:+) => [0, 1, 3, 6]
```

Notably, the `accumulate` operation can be done lazily since it doesn't require processing the entire collection before computing a value.

I recently described `#accumulate`, and its relationship to `#inject`, more thoroughly in [this blog post](https://medium.com/building-panorama-education/scan-left-a-lazy-incremental-alternative-to-inject-f6e946f74c00).

## Reasoning
We heavily rely on the accumulate operation. We use an [event-sourcing](https://martinfowler.com/eaaDev/EventSourcing.html) pattern, which means that we are scanning over individual "events" and building up the corresponding state. We rely on the history of states and need to do this lazily (we stream events because they cannot fit in memory). Thus the scan operation is much more applicable than the inject operation.

We suspect that there are many applications that could leverage the scan operation. [This question](https://stackoverflow.com/questions/1475808/cumulative-array-sum-in-ruby) would be more easily answered by `#accumulate`. It is a natural fit for any application that needs to store the incrementally-computed values of an `#inject`, and a requirement for an application that needs to use `#inject` while maintaining laziness.

## Implementation
There is a Ruby implementation of this functionality [here](https://github.com/panorama-ed/scan_left/) and an implementation in C [here](https://github.com/ruby/ruby/pull/3078).

Update: @nobu has provided an alternative implementation [here](https://github.com/ruby/ruby/pull/1972).

## Counterarguments
Introducing a new public method is committing to maintenance going forward and expands the size of the Ruby codebase -- it should not be done lightly. I think that providing the functionality here is worth the tradeoff, but I understand any hesitation to add yet more to Ruby!

---Files--------------------------------
scan_left_example.rb (2.93 KB)


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