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=-2.8 required=3.0 tests=AWL,BAYES_00, DKIM_ADSP_CUSTOM_MED,FORGED_GMAIL_RCVD,FREEMAIL_FORGED_FROMDOMAIN, FREEMAIL_FROM,HEADER_FROM_DIFFERENT_DOMAINS,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 957491F5AE for ; Tue, 14 Jul 2020 17:51:00 +0000 (UTC) Received: from neon.ruby-lang.org (localhost [IPv6:::1]) by neon.ruby-lang.org (Postfix) with ESMTP id 337CA120A2E; Wed, 15 Jul 2020 02:50:28 +0900 (JST) Received: from o1678948x4.outbound-mail.sendgrid.net (o1678948x4.outbound-mail.sendgrid.net [167.89.48.4]) by neon.ruby-lang.org (Postfix) with ESMTPS id E4327120A2C for ; Wed, 15 Jul 2020 02:50:25 +0900 (JST) Received: by filterdrecv-p3las1-7754f7d4cc-x8q2r with SMTP id filterdrecv-p3las1-7754f7d4cc-x8q2r-21-5F0DF07A-A9 2020-07-14 17:50:50.957593522 +0000 UTC m=+1640227.288469235 Received: from herokuapp.com (unknown) by geopod-ismtpd-3-1 (SG) with ESMTP id YWHfAmjfT2q9xlkbHgTChw for ; Tue, 14 Jul 2020 17:50:50.740 +0000 (UTC) Date: Tue, 14 Jul 2020 17:50:50 +0000 (UTC) From: finch.parker@gmail.com Message-ID: References: Mime-Version: 1.0 X-Redmine-MailingListIntegration-Message-Ids: 74957 X-Redmine-Project: ruby-master X-Redmine-Issue-Tracker: Feature X-Redmine-Issue-Id: 17016 X-Redmine-Issue-Author: parker X-Redmine-Sender: parker 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?pwrarsM0T2iL4aPgLngF2+VbEmD0Mx5cfa5PXbdWp+YgZWnh1yrPimdJFbLb7b?= =?us-ascii?Q?O11ey8LpxD0RNjY+Ha0FMKGYqFMvNx6IyXb89mr?= =?us-ascii?Q?c0nCLu6+0U5atcvCcspWhpCu2kaGaBgtmQJNMDw?= =?us-ascii?Q?gKQyG4kGoKz4K1r=2FxGfItOtPzT1Bc9LxofRgQiN?= =?us-ascii?Q?fjEamu8A42Mis=2F439dpJ5S6aucHYlKL9sMAfZEk?= =?us-ascii?Q?TePAPg59GqKpmDudM=3D?= To: ruby-core@ruby-lang.org X-ML-Name: ruby-core X-Mail-Count: 99167 Subject: [ruby-core:99167] [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 parker (Parker Finch). shyouhei (Shyouhei Urabe) wrote in #note-10: > It might be possible to let inject be lazy if we ignore backwards compatibility. But how do we partially evaluate that lazy enumerator then? I think this is the crux of the issue. Because `#inject` *needs* to evaluate every element in order to return a meaningful value, it can't be partially evaluated. The "scan" operation allows for partial evaluation. ---------------------------------------- Feature #17016: Enumerable#scan_left https://bugs.ruby-lang.org/issues/17016#change-86545 * 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/