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Feature #17016

Enumerable#scan_left

Added by parker (Parker Finch) 3 months ago. Updated 25 days ago.

Status:
Open
Priority:
Normal
Assignee:
-
Target version:
-
[ruby-core:99078]

Description

Proposal

Add a #scan_left 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

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

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

I recently described #scan_left, and its relationship to #inject, more thoroughly in this blog post.

Reasoning

We heavily rely on the scan operation. We use an event-sourcing 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 would be more easily answered by #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 and an implementation in C here.

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) scan_left_example.rb Small script showing `scan` usage with other approaches to the same behavior parker (Parker Finch), 07/10/2020 08:52 PM

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