220 39102 article Path: news.gmane.org!.POSTED!not-for-mail From: Zhihao Yuan Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: RE: Concept-defined placeholder type Date: Thu, 12 Jul 2018 03:40:56 -0400 Lines: 94 Approved: news@gmane.org Message-ID: References: <0fe92570-1bc7-4a8b-aaf4-b272950f20af@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1531381139 32381 195.159.176.226 (12 Jul 2018 07:38:59 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 12 Jul 2018 07:38:59 +0000 (UTC) To: 'Nicol Bolas' , 'ISO C++ Standard - Future Proposals' Original-X-From: std-proposals+bncBCSKRWMD4EHBBD4MTTNAKGQEIJQ4VNQ@isocpp.org Thu Jul 12 09:38:54 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f72.google.com ([74.125.82.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fdWBh-0008KG-BO for gclcip-std-proposals@m.gmane.org; Thu, 12 Jul 2018 09:38:53 +0200 Original-Received: by mail-wm0-f72.google.com with SMTP id f13-v6sf3491251wmb.4 for ; Thu, 12 Jul 2018 00:41:04 -0700 (PDT) ARC-Seal: i=2; a=rsa-sha256; t=1531381264; cv=pass; d=google.com; s=arc-20160816; b=bOWCAlOERz2vtI9UfoM8wmCzxA9BZAE2pybtzuXfXwKUO8qp00BzwKTghmeTIiKH7/ 1kD8XjRVJMr7AQAR7JXhYFRNd4ouYRPlMRqxgibcPH88rUXItAbZTGg9zcaDnPaN0RUu A3l/xP/dFjx1tOs5xrgRv8xy8DdBFphKBF32Kl1+G7xj2WkfMq0zUs5udfgb3oVDSsuB MgMSn2mmZueVM/EO7fSktFIsCsUBTH8XuiNNx70aSvf4ed+Wzw76SEss43blCXtWRYwE lJoDriwTx5+1p4FoJ9RRjY29rExN4YKF23oZ9vs7dxjyZPzRTIwzoVvnyD7Neh5GeHWF 1aDQ== ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-unsubscribe:list-subscribe:list-archive:list-help:list-post :list-id:mailing-list:precedence:content-transfer-encoding :mime-version:feedback-id:references:in-reply-to:message-id:subject :reply-to:from:to:date:arc-authentication-results :arc-message-signature:dkim-signature:arc-authentication-results; bh=l6QBFiYjRXL05mqb6O5Z4/Dqp3BztNJ2FMk80i6wNYA=; b=s6v3Wh+iQKNE13R2/64bi7vI+2plYNS/uz/atCnhlYK0Z2P0g3B6V3qKidJsbGMBjQ 5EKfk0oA0aeC5tZTV+aFnpCVtXw0mPaaVh48iBSXq5cAwGNFX14f5e1thq9SH9MKGvZS Uv9EQjUiOvRdOOk20brOW6mzQPeCIdGVRXo3/EKPXIC0wlYgtiVHn/V4IcgVWLxC+o6F 5Ln6h+1HvecrNuq43PfWx4TB4u4ptMiTHWMinW21TT9LqrFB+s9UAU5d6HIAp/GkrbHk Mdn1HK0EMHQtUoPvt4Ed/00LnRof3i5ovue8T5yciBPziikbTxFmye9Ihp5i/BttKUp6 ARC-Authentication-Results: i=2; mx.google.com; dkim=pass header.i=@miator.net header.s=protonmail header.b=dQqY4MEG; spf=pass (google.com: domain of zy@miator.net designates 185.70.40.132 as permitted sender) smtp.mailfrom=zy@miator.net DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:to:from:reply-to:subject:message-id:in-reply-to:references :feedback-id:mime-version:content-transfer-encoding :x-original-sender:x-original-authentication-results:precedence :mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=l6QBFiYjRXL05mqb6O5Z4/Dqp3BztNJ2FMk80i6wNYA=; b=osWOg9SfmPrxwwD1kSWFTI+AZjXhlmlBkbDJ00SYA7qbwFnh/942lBNYLtcRHxNyeL 1H7X2biJfRNy6HkkmiwJMnvWXbOXGEtpdTyzed1pbbwnJi0Yg6uRXjx7iaBKW7ZpZg7W wA7alxqJuLLr9gJLD8ci8LyTUmgIVDOYKUCsN2gerXW3UfT0x+wAWAyR+PllVJvCqN1w pT0G6kZIODpURjrNwcHYuhGIvZ36bai33f+q64E/Tv6DYARquzQfgAY2SoKxQZ8EqFUS VX1dCY7pxx/qTHPFTe2SnERBf8K6iWqMP4MMXkXuGxUmtR+VZqIxP5ym5xLBAfQiGSWU 6wVA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:to:from:reply-to:subject:message-id :in-reply-to:references:feedback-id:mime-version :content-transfer-encoding:x-original-sender :x-original-authentication-results:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=l6QBFiYjRXL05mqb6O5Z4/Dqp3BztNJ2FMk80i6wNYA=; b=fjrDFcr0bEQwc7OsIQvQMjVnBNFVm+e9/otcIpuobTRgfizrOxwqA3vM187tbVwkee vjp/ApoDdBtv1RItgmT+i+JRBwaqQVUtSL+vJJIvJKNgH/hFfYeqhU9hQwClEgfzmJ/p MOgMLeZQboKzma1unnAGML7aKW4WiezUGFvbeP3uRauot7+ToQFuxQf4fz2iRRTvBGix DQ6gxRabRzF+YWXXSUkHwAOPao/LKXsNcUDw3pHH/nRbjUrV7JT/XgjM2miyNoNmoyei mM4Q95zlX5XuoLkIzel4dZ8HtGFU9uSvG1bEoqqzVEWQahSJYUfRvEhrr0j X-Gm-Message-State: AOUpUlE4mCeLCTn8DAA9HpSChIMS6sjnNEH4DETfk1Yp0e2+xbPl9SxR Hyudk8ajy7vyBVqFVdRN1xzm2A== X-Google-Smtp-Source: AAOMgpegBTQc0eZmlTkjrvWYlBUEmJmciZkHzYn30UifzVecRnxsBXQBrOcRPdOQwBh02NEs328R2Q== X-Received: by 2002:adf:c58d:: with SMTP id m13-v6mr95194wrg.9.1531381264353; Thu, 12 Jul 2018 00:41:04 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:adf:e60f:: with SMTP id p15-v6ls3236299wrm.14.gmail; Thu, 12 Jul 2018 00:41:03 -0700 (PDT) X-Received: by 2002:adf:c354:: with SMTP id e20-v6mr782783wrg.257.1531381263318; Thu, 12 Jul 2018 00:41:03 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1531381263; cv=none; d=google.com; s=arc-20160816; b=nLy/h6XLlr0kSpXZPkaMk40C9ZCkLm5fS2qlvqcIBAW2iWJZuXEiJEIyyyj6/+pz55 e4NjfY22YbOLqiArikDBuDAi0VQzJ0c23tX53s7FBZKQKwE812anC1Cf4AHWnZV+JL52 twe2t/ai7Dh2z252l7W0EavgQ9PDGOriW4XgjoKVM+G1xUuSGoehi90Um1nDoodrBNF4 8VaIx2m1cPldIqy8c5GUPzVecN9SFH9WA8LlbrjdkCIuo2yZrndPpL5Or/l3Rd6+G1AM 0zJzTY0XMKsAzxYRORJnBFUTBsy4SGlADbHaFtMqS+1All75u7+K0ySWIEUp8RpfxViH 9+qQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=content-transfer-encoding:mime-version:feedback-id:references :in-reply-to:message-id:subject:reply-to:from:to:dkim-signature:date :arc-authentication-results; bh=b6W+e7kjNcUCmFQiEtt5AeJsN3DCMTdFqxUOG/iL/eM=; b=Lh4llqKqjpEAFR5Ck1NQo+w4x5+LFgtczR4dVmQ61oNPq49MYVavfrCck2bK1sjSVN jL/uc+ExB8CD5JZWkkSRNRmCFWRy7+vlKt5FjcjTE8D4MgT+ExtIJ6naBQD0xF1O8Uru z3j/pLDGj7t67VPyxs4dQucqbDJq1SvQU4ivV+3TOpXQpPYfhmZZFJbB+SLxFTMouZHb Ia/2JEqVta+5XO7tfYsVHTe3mhm1TV0DWGXKft9vEWWtVAHkFbppHMMBGJ3iZ/P1BoHJ E9fdEZ3aRB7N/w6JH+JFJmyGgGkrLYMeMdIqNSZQj5e86wNdmqBfGeJVn8nq4nD65fnr fihg== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@miator.net header.s=protonmail header.b=dQqY4MEG; spf=pass (google.com: domain of zy@miator.net designates 185.70.40.132 as permitted sender) smtp.mailfrom=zy@miator.net Original-Received: from mail-1857040132.protonmail.ch (mail-1857040132.protonmail.ch. [185.70.40.132]) by mx.google.com with ESMTPS id i10-v6si19199216wrh.196.2018.07.12.00.41.03 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 12 Jul 2018 00:41:03 -0700 (PDT) Received-SPF: pass (google.com: domain of zy@miator.net designates 185.70.40.132 as permitted sender) client-ip=185.70.40.132; In-Reply-To: <0fe92570-1bc7-4a8b-aaf4-b272950f20af@isocpp.org> Feedback-ID: xu1i2ADzKwGyrUl8KbXpsa1PQSI0Bc21qlX1V7gpMylRDQPR42YBpWcLHW4brubvCmGM18xDo5tgsD-vWFu_cQ==:Ext:ProtonMail X-Original-Sender: zy@miator.net X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@miator.net header.s=protonmail header.b=dQqY4MEG; spf=pass (google.com: domain of zy@miator.net designates 185.70.40.132 as permitted sender) smtp.mailfrom=zy@miator.net Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:39102 Archived-At: From: Nicol Bolas =20 Sent: Thursday, July 12, 2018 12:25 AM > This construct isn't even "conceptually" a placeholder type. Why? Because= you don't have to deduce its type via template argument deduction. You can= just call the function with an explicit template parameter. No deduction w= ill take place, since you explicitly provided the type. >=20 > I know you're trying to make an analogy between template parameters and y= our feature. But that analogy simply doesn't match reality. Your feature is= , in the end, quite distinct from a template parameter. Based on your logic, does the analog matches reality if we allow Copyable T; using T =3D char; assigning a type directly to the constrained-type-name without deduction? I would say these are all additional functionalities regarding "something" in place of a type which can be deduced. These functionalities do not change the entities' nature. > Despite the fact that your proposal references P0915 directly, your propo= sal is written as if in a vacuum. That is, it's written as if there are no = competing proposals within this design space at all. We are not actually competing. Take Concept TS for example, Copyable T; T x =3D foo(); Copyable x =3D foo(); strictly speaking, they can coexist. Adding an 'auto' can also be justified. I can mention some of those papers, but I haven't decided which form... Thanks for the review. > Retroactive compile errors: >=20 > SomeConcept T; > static_assert(sizeof(T) > 4); > T t =3D 'c'; >=20 > I'm not saying its unimplementable. [...] If implementors says no then it's no, I just feel this model is easier to understand (you declared something, then its interpretation follows the same rule on every line that can lookup the thing you declared). > Your syntax forces me to create a typename even if I don't want one. We don't have enough C++20 code to tell whether this is a real issue. I can raise a hypophysis saying "repeated constraints happens frequently:" Iterator T; T a =3D begin(...); // ... T ed =3D find(...); and we don't have data to disprove that either. After we have the data, it's not too late to adjust designs. Meanwhile, other designs cannot extend to support constraining two variables that are "EqualityComparable" afaict. > Even worse, if I don't want to constrain a deduced variable, template concept Any =3D true; Any T; T a =3D ...; -- Zhihao Yuan, ID lichray The best way to predict the future is to invent it. _______________________________________________ --=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/HuI7FbT9Wv5a-_FWIr0Qhv9zmtfCrxYpONZ4G93vbE_K0pyk= o8Cri_ItEQhSio6bj6XfcssQc2UnmMTb93BpZaB1Ksw7q0GN0vS14warHAc%3D%40miator.net= .. .