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Feature #17016

Updated by parker (Parker Finch) 8 months ago

## Proposal 

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

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

 (The name "scan_left" is based on Scala's scanLeft and Haskell's scanl. It seems like "scan_left" would be a ruby-ish name for    this concept, but I'm curious if there are other thoughts on naming here!) 

 ## Background 

 `#accumulate` `#scan_left` 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, 3].scan_left(0, &:+) => [0, 1, 3, 6] 
 ``` 

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

 I recently described `#accumulate`, `#scan_left`, 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 scan 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`. `#scan_left`. 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!

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