From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-Status: No, score=-4.1 required=3.0 tests=AWL,BAYES_00, MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED,SPF_HELO_NONE,SPF_PASS, UNPARSEABLE_RELAY shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from neon.ruby-lang.org (neon.ruby-lang.org [221.186.184.75]) by dcvr.yhbt.net (Postfix) with ESMTP id 214E71F5AE for ; Sun, 12 Jul 2020 23:51:59 +0000 (UTC) Received: from neon.ruby-lang.org (localhost [IPv6:::1]) by neon.ruby-lang.org (Postfix) with ESMTP id D8A7F120938; Mon, 13 Jul 2020 08:51:27 +0900 (JST) Received: from xtrwkhkc.outbound-mail.sendgrid.net (xtrwkhkc.outbound-mail.sendgrid.net [167.89.16.28]) by neon.ruby-lang.org (Postfix) with ESMTPS id 3C3BF12091D for ; Mon, 13 Jul 2020 08:51:26 +0900 (JST) Received: by filterdrecv-p3mdw1-75c584b9c6-jp7b2 with SMTP id filterdrecv-p3mdw1-75c584b9c6-jp7b2-18-5F0BA213-6 2020-07-12 23:51:47.094789649 +0000 UTC m=+1406527.606194789 Received: from herokuapp.com (unknown) by ismtpd0010p1iad1.sendgrid.net (SG) with ESMTP id 6FseCruXR6e9n9Uci8hdYg for ; Sun, 12 Jul 2020 23:51:47.064 +0000 (UTC) Date: Sun, 12 Jul 2020 23:51:47 +0000 (UTC) From: shyouhei@ruby-lang.org Message-ID: References: Mime-Version: 1.0 X-Redmine-MailingListIntegration-Message-Ids: 74931 X-Redmine-Project: ruby-master X-Redmine-Issue-Tracker: Feature X-Redmine-Issue-Id: 17016 X-Redmine-Issue-Author: parker X-Redmine-Sender: shyouhei X-Mailer: Redmine X-Redmine-Host: bugs.ruby-lang.org X-Redmine-Site: Ruby Issue Tracking System X-Auto-Response-Suppress: All Auto-Submitted: auto-generated X-SG-EID: =?us-ascii?Q?jcfQDMoo=2FMGCmP3uu1SeyLQUxUPXq5PjHpHz3xSFn16JvPtgLef7irdkXEC=2FAJ?= =?us-ascii?Q?3QkH2pMIiW+SU6KWq9brfLIdUYkakd2LDeJmoQB?= =?us-ascii?Q?grlOdHe4HAm61QvzWINznVj1Ac9G1rXQGPRLWTP?= =?us-ascii?Q?oleeF7Z8zY9TByyf3KiYOI5D8=2FO7s4RzCA0vHEi?= =?us-ascii?Q?4Pod7MymRXfy1DHKGaNUZXihLju5k9929HeKwo=2F?= =?us-ascii?Q?mlrY7HkYuEH5Cre8U=3D?= To: ruby-core@ruby-lang.org X-ML-Name: ruby-core X-Mail-Count: 99141 Subject: [ruby-core:99141] [Ruby master Feature#17016] Enumerable#scan_left X-BeenThere: ruby-core@ruby-lang.org X-Mailman-Version: 2.1.15 Precedence: list Reply-To: Ruby developers List-Id: Ruby developers List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: ruby-core-bounces@ruby-lang.org Sender: "ruby-core" Issue #17016 has been updated by shyouhei (Shyouhei Urabe). It might be possible to let inject be lazy if we ignore backwards compatibility. But how do we partially evaluate that lazy enumerator then? ---------------------------------------- Feature #17016: Enumerable#scan_left https://bugs.ruby-lang.org/issues/17016#change-86519 * 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/