220 34603 article Path: news.gmane.org!.POSTED!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: D0779R0: Proposing operator try() draft 1 paper Date: Wed, 4 Oct 2017 23:55:08 +0300 Lines: 92 Approved: news@gmane.org Message-ID: References: <13df2242-f7dd-405d-b8b9-23b47981c56e@isocpp.org> <59D4D462.9040106@gmx.net> <602e9e20-1d2f-4545-be32-d0234639985c@isocpp.org> <229b9108-ece9-28a1-dbf5-c845e338f557@wanadoo.fr> 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 1507150508 12782 195.159.176.226 (4 Oct 2017 20:55:08 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 4 Oct 2017 20:55:08 +0000 (UTC) Cc: "ISO C++ Standard - Future Proposals" To: "Vicente J. Botet Escriba" Original-X-From: std-proposals+bncBC5JHI7A7ALRBLMV2XHAKGQEVJZTWMQ@isocpp.org Wed Oct 04 22:55:04 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-it0-f70.google.com ([209.85.214.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dzqh5-0002gY-Fh for gclcip-std-proposals@m.gmane.org; Wed, 04 Oct 2017 22:55:03 +0200 Original-Received: by mail-it0-f70.google.com with SMTP id 142sf6871044ite.12 for ; Wed, 04 Oct 2017 13:55:11 -0700 (PDT) ARC-Seal: i=2; a=rsa-sha256; t=1507150510; cv=pass; d=google.com; s=arc-20160816; b=K0Pnyx2xXLuWNWtLCa6rYWi9Sk+hrjRzDX3yfbv1MOnM3JXn8EDLfyrnrbjQSK85is FzxAgr6BOZYP7y3bwUS0GOhdjekjDYBerXFKmsCUAdIMnQrCyHjI8XOh7TNQtm8TEk5E NP9NGjA9hj7QpizmO6V7wviUxCAr7gttViFR527TL49mOHPKA73f6JWtouONqzOPerW7 HX58l6bhhueqcHYMcqvKDNIrYmfnRxXc3zcEUSTNfKuOLNM0+5wpu1ya2VcZBgjoZZdF cgA0VrcMBZjGvHlGmJTz1yd7gDUhKLzdxA0h0fuUvOcOe5gs/xWYFPenutNY27L0nwTP LVCA== 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:reply-to:content-transfer-encoding :cc:to:subject:message-id:date:from:references:in-reply-to :mime-version:arc-authentication-results:arc-message-signature :dkim-signature:arc-authentication-results; bh=ub0JQYjElh8h5apg/jGh25WUsU7/2Z1+l4V/5pSezJI=; b=flSfmGenxNs7N9z5WNT/SDEh8Y0Y0pDf78WeMsGK5GTxdbgcH2+SM2B5dajNZOTp0U IIKyLCoBb1yFIqR0yJX1nZuQhxvQwqjEsvALj4Bvjj0LnbQ9QTlj3jBj7F40zFJka6Bt JsaE2JjQxRMHmsGDSaw/tAI0pLdsWkhu0A15Z+Yq1Vb3kOLkCdduIOogJTcCutIQuqJp 3Xh/KrGXLsQ+FA8tfcN7y+N7bRsbxZAvFN1K99hCsNpWA45VL+O1J+JqOvNTNMGlCuJE p36e+R35xuL6b0Ub7cwObk3+0UKlpxzoqgbiDVOVKTaolk/RamM2qdEjEbf7Q8BUljZC 20PA== ARC-Authentication-Results: i=2; mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=C9Xtzm/t; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=ville.voutilainen@gmail.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=gmail.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc:content-transfer-encoding:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=ub0JQYjElh8h5apg/jGh25WUsU7/2Z1+l4V/5pSezJI=; b=DCgOBhag/6BaCcYLKPGQGhhPWIHp32oYKhau/sdMqt2JTa6wOP5fVZHcnUJ1kTp/si cplcqvno9EXvm9bTP4S9s+41HnC+7jFqMqz03ECIMk7gI+dQYBrOVEy6sK+ouBiJmn/h KqivcqBXXoi+qUVNIbt0lU1AXfIH5ykZ6b5hZWmY67v7p8NmYodmolYe3Ylkap08OzfU 3T29EF77e0TOdgET7nVXKiK4uJ44cUOABPqVaeAl8i88wYySSg5aQK3YnJBNvCLDtQUb 8YIkZofpK9qKUkhzgJj72NVwyUi/KKC6qSAL+9Lnd8Dw/7egTndUU2tpCUJz48w5H5tO +3Pg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:cc:content-transfer-encoding :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=ub0JQYjElh8h5apg/jGh25WUsU7/2Z1+l4V/5pSezJI=; b=L9LWSpiNszXMoGpYt2jRkLqoy0QXfKYefom36XSTq+6EpboX8vIhTA121CfWj2KNVw ohKL1MFmXZZcwEhJa+iUlIiAPtdzxEKS/IlfRtbcu+FAVnrovjazVHl0UgIW2OLveqNN gdj3MhZgp2UiQGeRWQDS74M2liMQPKY1sqnsFzt4FNuvMqR1XovFi7+n3H29iKBmDbcU grYAu64RJ7fQpcy/gLSybUQhaJUOWHHcJfGEoo5iEfZdqaBAQK3lPk1yCTEfmw3QNznj qPrpuBCkiKS5YIKZUCXuMkP+t9niQmO+J36NDHxiUOvBSIxmpJFN2PrOMoBgSx5evviy X-Gm-Message-State: AMCzsaWCa7S2jttBO6uImueo4Vs9NuBpF123DubV2ihazHISfv2X8/Bt vDhAGLQYvA5UekNMue1hKX5rng== X-Google-Smtp-Source: AOwi7QCKdBBQYGWJm3D6ENDnRwDwrtx6fTFM9ws4ioeYXHxedj3XWJvJTp2SOFxtiMwk7DP4SRtYcA== X-Received: by 10.157.51.60 with SMTP id f57mr1077876otc.29.1507150510696; Wed, 04 Oct 2017 13:55:10 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.202.2.21 with SMTP id 21ls4361767oic.12.gmail; Wed, 04 Oct 2017 13:55:09 -0700 (PDT) X-Received: by 10.157.89.133 with SMTP id u5mr5110775oth.56.1507150509574; Wed, 04 Oct 2017 13:55:09 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1507150509; cv=none; d=google.com; s=arc-20160816; b=eqxlmJCKUnzwf7vRYMfqL6jNEgRCIjn9rpVy2QRGoAWJYOwQtyz2c3Tvf3ns4bX67H o9SLXPUYChQax2aNjN35aXcGmOG2P+T/AXQyPsJmj0Xo9zKx6lkeJsKQzxrkr7etLezo WMPG9CB/4kwJc27/kE5Uv/yZTpswFFFylvQ+iMw55HvDl5b46rGHMqhLHoZ6IhN0lHYQ UM5B/KoLa9EnZ4momosbkGZS0EgzCFindYAweEKN1HiPjtwCmmWeoltGNssqPEwj7O2D 2uuccfD3Aj4H2C+tEQSCIPjr6EsiXhGo5rE4+bEkteAaYCq4dovxjpKVD1miFZW3ldtR E5Ag== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=content-transfer-encoding:cc:to:subject:message-id:date:from :references:in-reply-to:mime-version:dkim-signature :arc-authentication-results; bh=WJ44KxcpH+SSL5ZkgHthVL8LjvMZ2dJ4RuqP85gyfL4=; b=YzI39xnIl2MofN0BupT7mQ4iVFWgGJxZmbCV4cF7geqEcRlbF45O5REeuVzSuBmXsz gR+bD0rBiEHQeksTPxV7KwkpIcUBAN5b7sSGZPQAZgRGvo7h2VRcavxXxkbmJNIWRu9w t+YdSvvXnPWrCCzH9u1tXemPT3sSfP9bGDX9q7cPtkMdCwnq8tTS+5tyIXij0hHxKd8i rf8GyWCIljR3LB1ROnAAryDx8ieUqVQW/PMNTN4LFCRfHaZmtE9CPZvLR2S9SnibTJB4 7OXn1XuBTpfkAb97vPoRJUpp3TzyiJxXm2MgL4Zl6qkNk0j0WQz/7jQVJ6tChmK1rjab xdXQ== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=C9Xtzm/t; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=ville.voutilainen@gmail.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=gmail.com Original-Received: from mail-sor-f41.google.com (mail-sor-f41.google.com. [209.85.220.41]) by mx.google.com with SMTPS id p63sor2183881oib.201.2017.10.04.13.55.09 for (Google Transport Security); Wed, 04 Oct 2017 13:55:09 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.202.189.137 with SMTP id n131mr9862845oif.126.1507150509214; Wed, 04 Oct 2017 13:55:09 -0700 (PDT) Original-Received: by 10.157.2.132 with HTTP; Wed, 4 Oct 2017 13:55:08 -0700 (PDT) In-Reply-To: <229b9108-ece9-28a1-dbf5-c845e338f557@wanadoo.fr> X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=C9Xtzm/t; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=ville.voutilainen@gmail.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: 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:34603 Archived-At: On 4 October 2017 at 23:11, Vicente J. Botet Escriba wrote: > Le 04/10/2017 =C3=A0 16:54, Ville Voutilainen a =C3=A9crit : >> >> On 4 October 2017 at 17:48, Nicol Bolas wrote: >>>> >>>> Before we even go there, the paper needs to explain >>>> >>>> a) why this is a problem that needs to be solved > > When we want to propagate the failure or get the value the current code i= s > something like > > auto res =3D expr(); > if (! res) return unexpected(res.error()); // idealy this should be > using the removed get_unexpected. > > > When you have a lot of them nested in a expression , you need to extract = the > rerun logic outside the expression, e.g. if you need to add the result of > two calculations > > auto res1 =3D expr1(); > if (! res1) return unexpected(res1.error()); > > auto res2 =3D expr2(); > if (! res2) return unexpected(res2.error()); > > return *res1+ * res2 > > > Using the try expression we can just do > > return try expr1()+ try expr2(); > > > Clearly the last is much more readable and express better the intent than > the more elaborated code above. > > LESS CODE MORE SOFTWARE > KISS > > Ville, is this the kind of motivation you are asking for? No. The paper needs to explain why such code is so common that it needs a core language facility to help it. >>> If you try to introduce a function that does what `try` does, it would = be >>> unable to do the one thing that makes `try` work: return. A function >>> cannot >>> force the calling function to return. You could make a macro that does >>> what >>> `try` does, but we have no way to do that within the language. >> >> Yes, well, perhaps we should look at adding something more general >> than individual >> keywords that perform specific transformations that then return (or >> not) via a nettle bush >> of traits and hooks. >> > > Yes, maybe there is something generalizing co_await and try. I don't know= .. > For me we need to go from concrete needs to more generic ones. > > Do you have an idea of what this generic thing could be? Well, we could look at providing code transformation facilities powerful enough that you can provide what you need via a specific transformation. As a more limited = idea, there seems to be an occasional need to write operations that can exit the surrounding scope via break/continue/return, but functions can't do that, so perhaps we could add a block facility that can. --=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/CAFk2RUZcpy7Fy2YPf_55zjBdH_3SavqZb_guGwQizp-iUuK= %3DhQ%40mail.gmail.com. .