220 23910 article Path: news.gmane.org!not-for-mail From: "Arthur O'Dwyer" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: RFC: Unpacking tuples to value sequences Date: Wed, 20 Jan 2016 16:37:06 -0800 Lines: 144 Approved: news@gmane.org Message-ID: References: <6f3ece34-e302-4d95-afd0-4c1924e8a780@isocpp.org> <6ad65a24-8a2e-4053-b9c9-80d296d98e7d@isocpp.org> <72de7daf-93af-4e55-95ea-67898bd2a5f6@isocpp.org> <22792c37-ba68-4a13-af1c-8b8247d3d22a@isocpp.org> <41870a42-b687-4f21-a8b1-d00a239aff01@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=001a1145392c62f0930529cd4da9 X-Trace: ger.gmane.org 1453336633 1845 80.91.229.3 (21 Jan 2016 00:37:13 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 21 Jan 2016 00:37:13 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDLZJYWNDQILHUEAWUCRUBFG4O63M@isocpp.org Thu Jan 21 01:37:12 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lb0-f197.google.com ([209.85.217.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1aM3FN-0000Gh-Ie for gclcip-std-proposals@m.gmane.org; Thu, 21 Jan 2016 01:37:09 +0100 Original-Received: by mail-lb0-f197.google.com with SMTP id p10sf8510198lbo.0 for ; Wed, 20 Jan 2016 16:37:09 -0800 (PST) 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:date:message-id:subject:from:to :content-type: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=SA7hjg4BJcnZ9XFvaGZ7EQ3kPNy3UDxmdKm9Qpjg6dc=; b=lQHQTKJSlAg++YAmQwGPo+9MSVvvw8s45FXusBonZdQ3Rf0IZnSRjqVpigCV5VSli5 73XMVDpR2wCA/bkdmry4I+NWCwuX+Cw2fimCFZZ/1sUK9wyzZIlhALDlCRlmtfLvKoOR a3Fd9SG7veCwjsa3idQjpetTKjGfhubxS1+dCcJNfjLduVqPW9DqA8nQaU2Mi/rhgXjk quFxpXo0HOhL/NEb7FcgTA8xyi3ne8uT76cOcRv/I/ufkC/4L9n0TP+4b0vY2X80rsuI moZn+S2Y3nfwHXx62gXp27tMRy7427orxr3Jg6FH5aQ6xAV/VFIIEkYJLZjnM7HUGDSA AEXg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:date :message-id:subject:from:to:content-type: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=SA7hjg4BJcnZ9XFvaGZ7EQ3kPNy3UDxmdKm9Qpjg6dc=; b=AneYt2qPn8PBv6lBWOCJa7a5uUp3ymqz5giwVW5rBuEg2+M4EE3PP63z7NhMGhQKFx oG1Jbfw3EmMrAEo5gMiMg9SH9t/98tqADMmIHN+6D+5xj/T7p4UCChCaGG3Khk+ehRhW /okfmdu64e2aZR3vtIBX378mVtV6bby2dS/0dZmaR7GJFtu0LjyyHsonAGQUGpgoqkDj rDzLpyDMsYpUR17MpodtN+v7j43NTxI6H/f85M+h5U6kPOcof0JqsQQZiwzSFz0ao4s7 b6KSC9y/kBv7iAD6Qssau8+nEM98kTsI+6eZaWLmVo0BwfR47EEDF2XsIJe3zjvnIl+X GGjQ== X-Gm-Message-State: AG10YOSJQqBNFP8PXY2DQpStOh7co2hEuNQPWTsUCkkOfuEyFYx+PHn5aSSb5jkRPVpSvw== X-Received: by 10.28.142.21 with SMTP id q21mr28269wmd.1.1453336628852; Wed, 20 Jan 2016 16:37:08 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.28.105.136 with SMTP id z8ls264614wmh.46.gmail; Wed, 20 Jan 2016 16:37:07 -0800 (PST) X-Received: by 10.28.223.67 with SMTP id w64mr7133788wmg.28.1453336627135; Wed, 20 Jan 2016 16:37:07 -0800 (PST) Original-Received: from mail-wm0-x22b.google.com (mail-wm0-x22b.google.com. [2a00:1450:400c:c09::22b]) by mx.google.com with ESMTPS id ld8si56808074wjc.77.2016.01.20.16.37.07 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 20 Jan 2016 16:37:07 -0800 (PST) Received-SPF: pass (google.com: domain of arthur.j.odwyer@gmail.com designates 2a00:1450:400c:c09::22b as permitted sender) client-ip=2a00:1450:400c:c09::22b; Original-Received: by mail-wm0-x22b.google.com with SMTP id r129so152374100wmr.0 for ; Wed, 20 Jan 2016 16:37:07 -0800 (PST) X-Received: by 10.28.179.84 with SMTP id c81mr7050023wmf.30.1453336626914; Wed, 20 Jan 2016 16:37:06 -0800 (PST) Original-Received: by 10.27.19.201 with HTTP; Wed, 20 Jan 2016 16:37:06 -0800 (PST) In-Reply-To: <41870a42-b687-4f21-a8b1-d00a239aff01@isocpp.org> X-Original-Sender: arthur.j.odwyer@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of arthur.j.odwyer@gmail.com designates 2a00:1450:400c:c09::22b as permitted sender) smtp.mailfrom=arthur.j.odwyer@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=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-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:23910 Archived-At: --001a1145392c62f0930529cd4da9 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Wed, Jan 20, 2016 at 4:03 PM, T. C. wrote: > On Wednesday, January 20, 2016 at 6:41:58 PM UTC-5, Arthur O'Dwyer wrote: > >> >> template >> void print_to_stream(std::ostream& stream, const TupleLike& x) >> { >> auto commaize =3D [](auto&& x) { >> return [&x](std::ostream& out) -> std::ostream& { return out << = x >> << ", "; }; >> }; >> >> (stream << "begin, " << ... << commaize(x~)) << "end"; >> } >> >> > If you can write stream twice, then a comma fold will do: > > ((stream << "begin, ") , ... , (stream << x~ << ", ")) << "end"; > > Also, I'm fairly sure you need to parenthesize stream << "begin, ". The > grammar only allows a cast-expression there. > I bet you're right. :) The above compiles with GCC 6.0 on Wandbox (if you replace x~ with std::get(x) of course) and fails for unrelated reasons with Clang 3.9. But I can totally believe that GCC 6.0 has bugs and/or helpful extensions in this area. Personally, I was shocked that return (a * b) + ...; failed to compile on either Clang or GCC =E2=80=94 and with a completely un= helpful syntax error! It took me tens of minutes to discover what I was doing wrong (mostly spent trying to figure out if these compilers claimed to support fold-expressions at all). Is there a grammatical reason to *require* parentheses around every single fold-expression, or was it just someone's idea of being "helpful"? =E2=80=93Arthur --=20 ---=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. Visit this group at https://groups.google.com/a/isocpp.org/group/std-propos= als/. --001a1145392c62f0930529cd4da9 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Wed, Jan 20, 2016 at 4:03 PM, T. C. &= lt;rs2740@gmail.com> wrote:
On Wednesday, January 20, 2016 at 6:41:58= PM UTC-5, Arthur O'Dwyer wrote:

template<class TupleLike>
void print_to_stream(std::ostream& strea= m, const TupleLike& x)
{
=C2=A0 =C2=A0 a= uto commaize =3D [](auto&& x) {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 return [&x](std::ostream&= amp; out) -> std::ostream& { return out << x << ", = "; };
=C2=A0 =C2= =A0 };

=C2=A0 =C2=A0 (stream << &= quot;begin, " << ... << commaize(x~)) << "end&q= uot;;
}


If y= ou can write=C2=A0stream=C2=A0twice, then a comma fold= will do:

((stream << <= span style=3D"color:#080">"begin, ") ,= ... ,<= span style=3D"color:#000"> (stream << x~ <<= ", "= ;)) << <= /span>"end";

Also, I'm fairly = sure you need to parenthesize=C2=A0stream << "begin, ". The grammar only allows= a cast-expression there.

I bet y= ou're right. :) =C2=A0The above compiles with GCC 6.0 on Wandbox (if yo= u replace x~ with std::get<I>(x) of course) and fails for = unrelated reasons with Clang 3.9. But I can totally believe that GCC 6.0 ha= s bugs and/or helpful extensions in this area.

Per= sonally, I was shocked that

=C2=A0 =C2=A0 return (a * b) + ...;

failed to compile on either Clang or GCC =E2= =80=94 and with a completely unhelpful syntax error! It took me tens of min= utes to discover what I was doing wrong (mostly spent trying to figure out = if these compilers claimed to support fold-expressions at all).

Is there a gramma= tical reason to require parentheses around every single fold-express= ion, or was it just someone's idea of being "helpful"?
<= div class=3D"gmail_extra">
=E2=80=93Art= hur

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