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=-2.4 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=no 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 0F9181F5AE for ; Wed, 8 Jul 2020 14:51:24 +0000 (UTC) Received: from neon.ruby-lang.org (localhost [IPv6:::1]) by neon.ruby-lang.org (Postfix) with ESMTP id 082261209EA; Wed, 8 Jul 2020 23:50:52 +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 ADF2D1209E9 for ; Wed, 8 Jul 2020 23:50:49 +0900 (JST) Received: by filterdrecv-p3mdw1-75c584b9c6-f64fx with SMTP id filterdrecv-p3mdw1-75c584b9c6-f64fx-19-5F05DD63-16 2020-07-08 14:51:15.131599689 +0000 UTC m=+1028493.003538431 Received: from herokuapp.com (unknown) by ismtpd0037p1mdw1.sendgrid.net (SG) with ESMTP id kSB4VVAgRp-Z7ie23QsxuQ for ; Wed, 08 Jul 2020 14:51:15.045 +0000 (UTC) Date: Wed, 08 Jul 2020 14:51:15 +0000 (UTC) From: finch.parker@gmail.com Message-ID: References: Mime-Version: 1.0 X-Redmine-MailingListIntegration-Message-Ids: 74879 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+Z5iBHNqHf3ChxeME4JXy?= =?us-ascii?Q?0GuP630Dv3XSOHTjByF92BKiD75B4taI5Tyga9l?= =?us-ascii?Q?WHIkYPSp+5hDls36E6nSHmuxP1Y6UusZ9tglCBi?= =?us-ascii?Q?XL2DxDHyFiPfiwzU4J8BNC7yTOZLpGGC=2FMMWRcV?= =?us-ascii?Q?TD7OKlrnf2Z5BFXnxkT4zvHS+iAVlWC2UvWPiNL?= =?us-ascii?Q?RdkwFNQNf6mRxfXXg=3D?= To: ruby-core@ruby-lang.org X-ML-Name: ruby-core X-Mail-Count: 99089 Subject: [ruby-core:99089] [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). sawa (Tsuyoshi Sawada) wrote in #note-1: > What is wrong with the following? > > ```ruby > [1, 2, 3].inject([0]){|a, e| a << a.last + e} # => [0, 1, 3, 6] > [1, 2, 3].each_with_object([0]){|e, a| a << a.last + e} # => [0, 1, 3, 6] > ``` Good question! Using `#inject` or `#each_with_object` cannot be done lazily. We need to pass a lazy enumerator *after* applying the fold-like operation. As a toy example, instead of `[1, 2, 3]` we can use `(1..).lazy`: ```ruby (1..).lazy.inject([0]){|a, e| a << a.last + e} # => infinite loop (1..).lazy.each_with_object([0]){|e, a| a << a.last + e} # => infinite loop (1..).lazy.scan_left(0, &:+) # => Lazy enumerator ``` ---------------------------------------- Feature #17016: Enumerable#scan_left https://bugs.ruby-lang.org/issues/17016#change-86460 * 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! -- https://bugs.ruby-lang.org/