From: finch.parker@... Date: 2020-07-23T15:39:16+00:00 Subject: [ruby-core:99301] [Ruby master Feature#17016] Enumerable#scan_left Issue #17016 has been updated by parker (Parker Finch). nobu (Nobuyoshi Nakada) wrote in #note-20: > With https://github.com/ruby/ruby/pull/3337, you can > ```ruby > (1..).lazy.inject(0, :+).first(10) #=> [0, 1, 3, 6, 10, 15, 21, 28, 36, 45] > ``` Thank you for writing that nobu! That is exactly the behavior we need. I'm worried about the backwards compatibility of changing the behavior of `#inject` on lazy enumerables. Since right now `[1,2,3].lazy.inject(:+) # => 6` I think it would be breaking to change that behavior to be `[1,2,3].lazy.inject(:+) # => #<Enumerator::Lazy: #<Enumerator::Lazy: [1, 2, 3]>:inject>`. I'm also worried about `#inject` having different behavior for strict and lazy enumerables. I don't think it would be good to have `[1,2,3].inject(:+)` be different than `[1,2,3].lazy.inject(:+).force`. Do you think your implementation could be used for a new method, whether we call it `scan` or `accumulate` or `project` or something else? I think it would be good to have consistent behavior between strict and lazy enumerables. --- **NOTE:** For anyone who is newer to this thread and might have missed it -- you can find this behavior, with some examples, implemented in Ruby [here](https://github.com/panorama-ed/scan_left/). ---------------------------------------- Feature #17016: Enumerable#scan_left https://bugs.ruby-lang.org/issues/17016#change-86689 * Author: parker (Parker Finch) * Status: Open * Priority: Normal ---------------------------------------- ## 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](https://medium.com/building-panorama-education/scan-left-a-lazy-incremental-alternative-to-inject-f6e946f74c00). ## Reasoning We heavily rely on the 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 `#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). ## 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) -- https://bugs.ruby-lang.org/ Unsubscribe: <mailto:ruby-core-request@ruby-lang.org?subject=unsubscribe> <http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>