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-ASN: AS4713 221.184.0.0/13 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 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 366CA1F463 for ; Tue, 24 Sep 2019 13:37:42 +0000 (UTC) Received: from neon.ruby-lang.org (localhost [IPv6:::1]) by neon.ruby-lang.org (Postfix) with ESMTP id 4A4CE1209D8; Tue, 24 Sep 2019 22:37:33 +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 84C131209C5 for ; Tue, 24 Sep 2019 22:37:30 +0900 (JST) Received: by filter0138p3las1.sendgrid.net with SMTP id filter0138p3las1-22165-5D8A1C1A-1A 2019-09-24 13:37:30.211644301 +0000 UTC m=+65337.197499339 Received: from herokuapp.com (unknown [54.157.43.252]) by ismtpd0008p1iad2.sendgrid.net (SG) with ESMTP id E7LHuVrFQlSJQ1CF_RTx9g for ; Tue, 24 Sep 2019 13:37:30.099 +0000 (UTC) Date: Tue, 24 Sep 2019 13:37:30 +0000 (UTC) From: eregontp@gmail.com Message-ID: References: Mime-Version: 1.0 X-Redmine-MailingListIntegration-Message-Ids: 70614 X-Redmine-Project: ruby-trunk X-Redmine-Issue-Id: 16178 X-Redmine-Issue-Author: Eregon X-Redmine-Issue-Assignee: matz X-Redmine-Sender: Eregon 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?KippOI8ZHtTweq7XfQzW93937kJ4QNWwSBuHnaMEcr1LXVHA7F4RFVN09=2FgkPs?= =?us-ascii?Q?NefgrQyc413Vvi2yQRie99tUgD+egp05r2TCsK2?= =?us-ascii?Q?zQFeEAcRxBUMZWMeZqP7SYyKYoGFiWZ1ei9RK2C?= =?us-ascii?Q?QcDm3iMIxHGJUtHiwnq7kf=2FDFl7bplnD48j7+Ab?= =?us-ascii?Q?4vshgv+xKP=2F1DlUW2=2FZp6twGLoyKvB0WnrA=3D=3D?= To: ruby-core@ruby-lang.org X-ML-Name: ruby-core X-Mail-Count: 95059 Subject: [ruby-core:95059] [Ruby master Bug#16178] Numbered parameters: _1 should be the same as |x| and _0 should not exist 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 #16178 has been updated by Eregon (Benoit Daloze). nobu (Nobuyoshi Nakada) wrote: > When `_1` is same as `|x|`, what does `[[1, 2]].map {_1 + _2}` mean? It means `[[1, 2]].map { |a,b| a + b } # => [3]` of course. > The meaning of `_1` changes if `_2` is used or not? Yes, just like `Proc#arity` changes. ```ruby -> { _1 }.arity #=> 1 -> { _2; _1 }.arity #=> 2 ``` It's just consistent. We have to accept the drawback that numbered parameters change arity, there is no way around that. Changing arity with named arguments has the same effect (`[[1, 2]].map { |a| a }` to `[[1, 2]].map { |a,b| a + b }`). My main point is we want `_1` to be the non-dangerous behavior. Changing arity can break things, that is not new, but indeed using numbered parameters makes the change less obvious in the source. That is intrinsically a drawback of numbered parameters. ---------------------------------------- Bug #16178: Numbered parameters: _1 should be the same as |x| and _0 should not exist https://bugs.ruby-lang.org/issues/16178#change-81692 * Author: Eregon (Benoit Daloze) * Status: Open * Priority: Normal * Assignee: matz (Yukihiro Matsumoto) * Target version: * ruby -v: ruby 2.7.0dev (2019-09-24T12:57:54Z master 0e84eecc17) [x86_64-linux] * Backport: 2.5: UNKNOWN, 2.6: UNKNOWN ---------------------------------------- Currently on trunk: ```ruby array = ["string", 42, [1, 2]] array.map { |x| x * 2 } # => ["stringstring", 84, [1, 2, 1, 2]] array.map { _1 * 2 } # => ["stringstring", 84, 2] ``` Oops, this trivial code just lost data and completely ignored the element class! This is clearly contrary to intuition and is very dangerous. Using `_0` instead has the correct behavior but it's clear we use 1-based indexing for numbered parameters, and it doesn't solve that `_1` has dangerous behavior. Basically the current behavior is that `_0` is the same as `|x|` and `_1` is the same as `|x,|`. `|x,|` is almost never used in Ruby, and for good reasons, it just throws away data/information/the class of the object. Such a dangerous operation should only be done when it's explicit, and the trailing comma in `|x,|` shows that, but `_1` does not. So let's make `_1` be `|x|` and remove `_0`. I am going to be harsh, but this discussion has gone too long without any serious written argument for the current behavior: I believe it's irrational and irresponsible to have `_1` be `|x,|`, it's just going to lead to nasty bugs. Try to convince me otherwise. If not, in one week I want to apply this change. >From the discussion in https://bugs.ruby-lang.org/issues/15723#note-127 and in https://bugs.ruby-lang.org/issues/15708 Some reactions to this behavior in https://twitter.com/eregontp/status/1115318993299083265 -- https://bugs.ruby-lang.org/