220 29634 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Use stateless lambdas without capturing Date: Sat, 3 Dec 2016 07:46:04 -0800 (PST) Lines: 113 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1045_194412753.1480779964130" X-Trace: blaine.gmane.org 1480779968 10136 195.159.176.226 (3 Dec 2016 15:46:08 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 3 Dec 2016 15:46:08 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBPGRRPBAKGQEH27SYZA@isocpp.org Sat Dec 03 16:46:03 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pg0-f69.google.com ([74.125.83.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cDCVm-0001JX-D2 for gclcip-std-proposals@m.gmane.org; Sat, 03 Dec 2016 16:46:02 +0100 Original-Received: by mail-pg0-f69.google.com with SMTP id f188sf243553504pgc.1 for ; Sat, 03 Dec 2016 07:46:06 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=W9m2QwiIep96L/Fr35v/XpHzuPV0mvub5W+PtxwR41A=; b=v+AUaTkL633iDDyZ1ziiXXW8495tVsd219YwKfVO53mMQUd6ADGuqpydq30oGXoy8x zx3nRBkHLbiO6l4Kl+25Im0KVQ5SHA4vsP00FVMpflOvoY38i2xNTay23WvXXE9iGLWN bVwNSNH309/m/hkXaOEpX2xpbzbGVLpi64Yo5h7gtCP+/cIV2ruX2oE9H+lI9xmQL+nP KHr1CRhDonBHzkN97vWLeNyZOAtk8xmUjtnSM9SEbP8JD//WLPTDEFBVHIL/0kkpfC8e oe60gcV9wFhGi2VBgoBt9UaYmb8cb5B9Xi5v0/Tzx5bXGNAY+PE9z29Mv3n2PdlurZgS ytOg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=W9m2QwiIep96L/Fr35v/XpHzuPV0mvub5W+PtxwR41A=; b=c/1PmEBQb5nb9oU1UDhsFdhGzpiWw2quV77/tTy+Oyvn9kwT5+E3kxE3OkJfn8N5vR Y+wtf/Zbe9TZ/4zvP/2a0O7whqrgsxgxseJaMrDcW0dZkAs6mUkSi0OHckUwv9wJ2e9B 6/MHw8CWvVjKcBhcFmHHyNUiaso6N2tXVsfSiDPZbs0Wzpo2+mntAS7MMgyy8kdhx12r lHFx0kYRL5QbagZriWTOaJc6iG8BaDBFL8SFT7hMXFtkEw+Ncjha3vYDvnwe+fZ7f/1J oZM6UhNCvMWAxgVnUZBA0K26QbaF3V9r+kBgwJt1laOkLBDucLwpImlmdHi0PnpMKdlK fmpQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=W9m2QwiIep96L/Fr35v/XpHzuPV0mvub5W+PtxwR41A=; b=N5L64rR+uDcnmUi16EUU8mTLPpankPksYBCICoXFriq1FTA0EvC+1T2ClAxNtSnVOC k1/nFEBzQpmqgZBJVL0xVfxILQIqi6vlum9DOIeGHcBIkTee8L8WZu6jDgFvFyFU884w mB6UecArPWGZzdxqMFdcr7GbW8UrSRar936yw/Ld4oJopD6gqDXsFPebtnva36X42zvJ DAvjCcDVgZPo2ya0pILE7FyvMMupeVpFwyOfd3pkRX7RJGwBts48oC2ugO/7kKMztKBj /KvW43LCuKO7PNJxAbWNqH/UUrC3Mh49vLwyQSZx4oOY+72VDTS6bI3MFAMHdKMXzTU9 vcKA== X-Gm-Message-State: AKaTC03+moeG875BdCec9hcr78pjQCIsmrRFk/PDHHAZaw6xni22uwrNX04IK26hu1ZquA== X-Received: by 10.99.168.74 with SMTP id i10mr22459396pgp.90.1480779965786; Sat, 03 Dec 2016 07:46:05 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.46.13 with SMTP id q13ls21558213otb.20.gmail; Sat, 03 Dec 2016 07:46:04 -0800 (PST) X-Received: by 10.157.8.134 with SMTP id 6mr3379904otf.17.1480779964809; Sat, 03 Dec 2016 07:46:04 -0800 (PST) In-Reply-To: X-Original-Sender: jmckesson@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:29634 Archived-At: ------=_Part_1045_194412753.1480779964130 Content-Type: multipart/alternative; boundary="----=_Part_1046_1489039637.1480779964130" ------=_Part_1046_1489039637.1480779964130 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Saturday, December 3, 2016 at 6:47:13 AM UTC-5, David Krauss wrote: > > int main() { > auto a =3D []{ return 5; }; > auto b =3D []{ +a; a(); }; // Error, =E2=80=9Ca=E2=80=9D not captured= .. > } > > A stateless lambda object is empty, so it=E2=80=99s unfortunate that they= must be=20 > captured for use by another lambda. In this case, b cannot be stateless. > > C++17 constexpr lambdas offer a partial workaround: > > int main() { > constexpr auto a =3D +[]{ return 5; }; > auto b =3D []{ +a; a(); }; // OK, =E2=80=9Ca=E2=80=9D is a constant i= d-expression used=20 > with L-to-R conversion. > } > > This is a bit esoteric, and it can possibly cost some performance. > > Perhaps it should be permissible to call, convert, and even copy a=20 > stateless lambda from an enclosing scope. (Binding to a reference or taki= ng=20 > an address are genuinely problematic, though. Pass-by-reference would be= =20 > broken, but fortunately and such pass by value.) > Why is this important? If it is a stateless lambda, then there's really no= =20 reason not to just make them named functions outside of the function's=20 scope. I fail to see why encouraging people to write a plethora of=20 functions within another function is a good idea. Lambdas are fine things, when taken in moderation. But I don't like us=20 adding features that encourage people to use them more often than is=20 strictly necessary for code-clarity and functionality purposes. --=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/f2e7975d-3c7e-411e-8237-b60041603378%40isocpp.or= g. ------=_Part_1046_1489039637.1480779964130 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Saturday, December 3, 2016 at 6:47:13 AM UTC-5, David K= rauss wrote:
int main() {
=C2=A0 =C2=A0 auto a =3D []{ return 5; };
=C2=A0 =C2=A0 auto b =3D []{ +a; a(); }; // = Error, =E2=80=9Ca=E2=80=9D not captured.
}

A stateless lambda object is empty, so i= t=E2=80=99s unfortunate that they must be captured for use by another lambd= a. In this case, b cannot be stateless.
<= br>
C++17 constexpr lambdas offer a partial workaround:

int main() {
<= font face=3D"Courier">=C2=A0 =C2=A0 constexpr auto a =3D +[]{ return 5; };<= /font>
=C2=A0 =C2=A0 auto b =3D []{ +a; a(= ); }; // OK, =E2=80=9Ca=E2=80=9D is a constant id-expression used with L-to= -R conversion.
}
=

This is a bit esoteric, and it can possibly cost some p= erformance.

Perhaps it should be permissible to ca= ll, convert, and even copy a stateless lambda from an enclosing scope. (Bin= ding to a reference or taking an address are genuinely problematic, though.= Pass-by-reference would be broken, but fortunately=C2=A0<algorithm>=C2=A0and such pass by value.)

Why is this important? If it is a stateless lambda, then = there's really no reason not to just make them named functions outside = of the function's scope. I fail to see why encouraging people to write = a plethora of functions within another function is a good idea.

Lamb= das are fine things, when taken in moderation. But I don't like us addi= ng features that encourage people to use them more often than is strictly n= ecessary for code-clarity and functionality purposes.

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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/f2e7975d-3c7e-411e-8237-b60041603378= %40isocpp.org.
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