220 4599 article Path: news.gmane.org!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Changing lambda scope Date: Sun, 26 May 2013 17:39:57 +0300 Lines: 115 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=90e6ba308f8290f13404dda00173 X-Trace: ger.gmane.org 1369579202 28148 80.91.229.3 (26 May 2013 14:40:02 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 26 May 2013 14:40:02 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC5JHI7A7ALRBPV5RCGQKGQEVWJNSGQ@isocpp.org Sun May 26 16:40:02 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vc0-f199.google.com ([209.85.220.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Ugc75-0000A0-Kr for gclcip-std-proposals@m.gmane.org; Sun, 26 May 2013 16:39:59 +0200 Original-Received: by mail-vc0-f199.google.com with SMTP id hf12sf7824492vcb.6 for ; Sun, 26 May 2013 07:39:58 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-beenthere:mime-version:in-reply-to:references:date:message-id :subject:from:to:x-original-sender:x-original-authentication-results :reply-to:precedence:mailing-list:list-id:x-google-group-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=qW31wlbb/EwuTbNKLkqBhBmlvc9gq/0+07CJR8YqY4I=; b=aTv3KYIJULsEQcbYgcFHPcOk7btyjypZ2S6FA4AW+Fcn0HgxYnx3HvKTyY11dL1B3t uN2cEiGcUdPLLVnutTi7lUgjvA4YEx7Q7Pmlnuaujpx37FSDOJiXdhtkCc60/WjYNOtj IoPtwkHbRZ/MGEVsVc/aFTd6qjM0wO2OFnYC+ESXHVItiNpESNGWbWK6JqVlb5rsERj7 LL+1touJ5i8xF4wKVkoq5ktZ4LF9Ju/HwF367q+z6pR6iCU0Xmfnrr2daW4+SgDKxn2b ZLsmBg2M8xF93bZMBE7t4UHo1FmT6mgOF+t3WskUD3JwOb+UCs7gIsZQ+7s4ZLIOH/Fn VPOA== X-Received: by 10.236.41.137 with SMTP id h9mr13018905yhb.43.1369579198676; Sun, 26 May 2013 07:39:58 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.129.36 with SMTP id nt4ls2553137qeb.25.gmail; Sun, 26 May 2013 07:39:57 -0700 (PDT) X-Received: by 10.49.0.244 with SMTP id 20mr27930538qeh.50.1369579197755; Sun, 26 May 2013 07:39:57 -0700 (PDT) Original-Received: from mail-qc0-x233.google.com (mail-qc0-x233.google.com [2607:f8b0:400d:c01::233]) by mx.google.com with ESMTPS id ed5si9204341qcb.32.2013.05.26.07.39.57 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sun, 26 May 2013 07:39:57 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400d:c01::233 as permitted sender) client-ip=2607:f8b0:400d:c01::233; Original-Received: by mail-qc0-f179.google.com with SMTP id e1so3189362qcx.10 for ; Sun, 26 May 2013 07:39:57 -0700 (PDT) X-Received: by 10.229.0.141 with SMTP id 13mr9310716qcb.80.1369579197611; Sun, 26 May 2013 07:39:57 -0700 (PDT) Original-Received: by 10.224.174.143 with HTTP; Sun, 26 May 2013 07:39:57 -0700 (PDT) In-Reply-To: X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400d:c01::233 as permitted sender) smtp.mail=ville.voutilainen@gmail.com; dkim=pass header.i=@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:4599 Archived-At: --90e6ba308f8290f13404dda00173 Content-Type: text/plain; charset=ISO-8859-1 On 26 May 2013 16:59, DeadMG wrote: > Pursuant to a recent discussion, I have discovered that lambdas are a > local class to whatever scope they are nested in. This means that ADL can't > find operators for them. For example, > > namespace X { > template L operator+(L l, R r) { > //... > } > auto f() { > return [] {}; > } > }; > int main() { > 1 + X::f(); // Fail > } > > I propose that the wording be changed so that lambdas' associated > namespace should be the most nested namespace containing whatever scope > they are declared in. Having operators available to work on lambdas > considerably simplifies certain functional APIs, as you can just return a > lambda directly. However, the current wording would require using > hand-created types purely for the purpose of ADL. > Seems.. ..reasonable, I guess. I have trouble imagining when I would want to use such a feature, so if you could provide a bit more complete example, that would be nice. -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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On 26 May 2013 16:59, DeadMG <wolfeinstein@gmail.com><= /span> wrote:
Pursuant to a recent discussion, I have disc= overed that lambdas are a local class to whatever scope they are nested in.= This means that ADL can't find operators for them. For example,

namespace X {=
=A0 =A0 template<typename = L, typename R> L operator+(L l, R r)= {<= br> =A0 =A0 =A0 =A0 //...<= br>=A0 =A0 }
=A0 = =A0 auto f() {
=A0 =A0 =A0 =A0 return= [] {};
=A0 =A0 }
};
int main() {
=A0 =A0 1 + X::f(); // Fail
}

I propose that the wording be changed so that lambdas' associated= namespace should be the most nested namespace containing whatever scope th= ey are declared in. Having operators available to work on lambdas considera= bly simplifies certain functional APIs, as you can just return a lambda dir= ectly. However, the current wording would require using hand-created types = purely for the purpose of ADL.

Seems.. ..reasonable, I guess. I have trou= ble imagining when I would want to use such a feature, so if you could
p= rovide a bit more complete example, that would be nice.

--
 
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