[ruby-core:103615] [Ruby master Feature#17016] Enumerable#accumulate

Issue #17016 has been updated by RubyBugs (A Nonymous).


Thanks everyone continuing to discuss whether to add this method to the Ruby lazy Enumerable!

In case it is helpful, please permit me to clarify that this method (and the functional programming pattern it represents) is of **practical**, rather than theoretical benefit.

This method underlies the system presented in the following conference talk _ETL and Event Sourcing_, which daily rebuilds all system state by re-processing enumerations of the history of data extracted from external systems:
* Part 1: https://www.dropbox.com/s/vibkr2edqmtid9n/047_etl_and_event_sourcing_marc_siegel_panorama_education_part_1.mp4?dl=0
* Part 2: https://www.dropbox.com/s/o6bwxymrkmbepgr/048_etl_and_event_sourcing_marc_siegel_panorama_education_part_2.mp4?dl=0

The implementation of this method we use is published here:
  * Rubygems: https://rubygems.org/gems/scan_left
  * Github: https://github.com/panorama-ed/scan_left/

A blog post discussing presenting the gem and discussing its usage is here: https://medium.com/building-panorama-education/scan-left-a-lazy-incremental-alternative-to-inject-f6e946f74c00

Sincere apologies if this additional context is redundant or unnecessary. My intent in presenting this context, again, is to provide context that this is practical code extracted from a production system, rather than a purely theoretical matter of interest.

Thanks again!

----------------------------------------
Feature #17016: Enumerable#accumulate
https://bugs.ruby-lang.org/issues/17016#change-91710

* 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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