220 38782 <7772686a-7e80-4c28-a87b-c5dc11450cc6@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: anonymous.from.applecity@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Apply std::common_type_t to deduce final return type in lambda return type deduction Date: Sat, 23 Jun 2018 13:14:31 -0700 (PDT) Lines: 243 Approved: news@gmane.org Message-ID: <7772686a-7e80-4c28-a87b-c5dc11450cc6@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_21130_1016860082.1529784871278" X-Trace: blaine.gmane.org 1529784746 29317 195.159.176.226 (23 Jun 2018 20:12:26 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 23 Jun 2018 20:12:26 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCLPJI46WUORBKGUXLMQKGQEK4RZ5AY@isocpp.org Sat Jun 23 22:12:22 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f197.google.com ([209.85.161.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fWotS-0007Xj-6M for gclcip-std-proposals@m.gmane.org; Sat, 23 Jun 2018 22:12:22 +0200 Original-Received: by mail-yw0-f197.google.com with SMTP id v80-v6sf7233073ywc.13 for ; Sat, 23 Jun 2018 13:14:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=lHzNkQPI1J4hL5dD5czZAWt1zjwnepZ2K/Nd7syKHIc=; b=DS9YAOkdHc9Tw1jsISCzZYnzLUB88rF5d+YUYwMVlUCk7sUKtoSZYw4CP9OALA6Fq0 4Dua5DzPelltXxtJWn+8GCfhGRoKo+EyuOu5G+VFQBtrzgH+mvP5c4eDES3usL6cpfFC CtKUu/oapet7ZSmJZng/h6BU4lhsLlI8jMgE4mjtyGvV4F+4EkIPwxTZpHNhI0H5j0A/ HesRGf1dIpr51wrPP6UNZ1QmN5AmmxPX6A0sTobmtK1hP5O7N4rMejQYVZp4rx52qYwi lxc0lJ0dW/trLbJUV5279xh1EMdaF0PZ4hmk1UVD6XqmCwjHEntEP57T4dmadTCXOK5y oyTw== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=lHzNkQPI1J4hL5dD5czZAWt1zjwnepZ2K/Nd7syKHIc=; b=akth4JRUI9UkjUEGi/qFlTL5SUvSo/DdD2BiHt8paDXy38lcFjuPMN8+vnQo9WY4OU kksou1WZLhr44fd9X/jjVMpoePuR7J/W51WsWbyImYxKWGdN8B6IByRzIlzoGuaAVmUv 2oiQrkuAaHm0pCs80YudneGZZtsOzTbB1BBjGooAZbfnf9eN6EPaKOONWGA+iB9nQafv TWlr7ibIo+Cfp0D3qJGqi7EY8VhNh06W+t0KaDu+LkH+VZOAH63/IsuGK1vkD2G0shGV JRDhDHXyMr3NQp7zhoKBTcKLcW7HvbBX96OBEQNyPbfyTbSvIVo+DV4DBG9bZugfU9Ob FemA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id: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=lHzNkQPI1J4hL5dD5czZAWt1zjwnepZ2K/Nd7syKHIc=; b=a+Ctjcyl2mwVAPv2RyTv2nbUm8cdIfWmM4CNTrv/3KxIHmw89jPqC+LBBNVd8oLxhs 4ARIexpcrsGwAWKoT7CHtclzjOjdtxmz2vMeyPgMa/zBl4JvWDRkHROy9RVcoCvl5XBh UU/nQsCmaSvYyQ1QATgEdi/QDyjSWXGjPN4fAongbbDz/5hOy/NjMD2c70qGwuSZpKJT sKukjvmQXp0YejGPC4QAODFhmWZ/u5zOFea4JojfaT0jID6BExZdI0KKkPrTwU0LhG0z N0fzdpe1uludFhEwd23OMbelAlSojg+Rjnp/ocxrAmpBZKQ6fGzLTG8Y6BQc5Gyeas4r SLKg== X-Gm-Message-State: APt69E3or6g96CJtemFlPXk4wCocstVPxMF4C4QfWNbR3gUgwau5I+vG RY9fK9d5iQJyjKe5brDBgXiy7g== X-Google-Smtp-Source: ADUXVKLdFFaNWd3pES6Wcwj8GyiR4eGb1itFZ/shiPJ5OHTSfGrQJ8DE98p7HyahAE27TCiKgO/GiQ== X-Received: by 2002:a81:5b89:: with SMTP id p131-v6mr1845696ywb.162.1529784873224; Sat, 23 Jun 2018 13:14:33 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a25:bdce:: with SMTP id g14-v6ls902487ybk.1.gmail; Sat, 23 Jun 2018 13:14:32 -0700 (PDT) X-Received: by 2002:a25:d256:: with SMTP id j83-v6mr495663ybg.3.1529784872012; Sat, 23 Jun 2018 13:14:32 -0700 (PDT) X-Original-Sender: anonymous.from.applecity@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:38782 Archived-At: ------=_Part_21130_1016860082.1529784871278 Content-Type: multipart/alternative; boundary="----=_Part_21131_251306515.1529784871279" ------=_Part_21131_251306515.1529784871279 Content-Type: text/plain; charset="UTF-8" Problem*:* When trying to mix std::optional with lambdas, I run into some rather annoying issue. I need to always use trailing return type syntax to get it deduced correctly in situations like this (e.g. when return std::nullopt comes first): [](auto args) { if (!check(args)) return std::nullopt; return foo(args); //returns some std::optional } The above will not compile, as std::nullopt_t cannot be converted to std::optional. Context*:* I'm trying to implement try_in_sequence, which basically executes functions passed as arguments one by one, until one succeeds (e.g. returns non-empty optional). Writing return type on every one of the 4-5 lambdas per function call is daunting. I understand that lambda's return type is consistent with auto, but it doesn't seem to be practical in this case. Proposal*:* Use std::common_type as return type. E.g. use auto on each return statement *individually*, then deduce common type using `std::common_type`. std::common_type> indeed yields std::optional, as shown in shown in this example : #include #include template struct undefined; int main() { using common_t = std::common_type_t>; static_assert(std::is_same_v, common_t>); //undefined>> foo; } Possible damage: I cannot think of any at the moment. I never encountered a case where compilation error inside of lambda would be of any use. I believe this shouldn't affect existing code, as std::common_type is conversion rules friendly (not 100% sure though). -- 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 email 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/7772686a-7e80-4c28-a87b-c5dc11450cc6%40isocpp.org. ------=_Part_21131_251306515.1529784871279 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Problem:

When trying to mix std::optional w= ith lambdas, I run into some rather annoying issue. I need to always use tr= ailing return type syntax to get it deduced correctly in situations like th= is (e.g. when <= span style=3D"background-color: rgb(238, 238, 238);">return std::nullopt comes first):

[](auto args)
{
=C2=A0 =C2=A0 if (!check(<= /span>args= ))
=C2=A0 =C2=A0 =C2=A0 = =C2=A0 ret= urn std::nullopt;

=C2=A0 =C2=A0 return foo(args); //returns some std::optional<T>
}

The above will no= t compile, as std::nullopt_t cannot be converted to std::optional<T>.

Context:<= /code>

I'm trying to= implement try_in_sequence, which basically executes fun= ctions passed as arguments one by one, until one succeeds (e.g. returns non= -empty optional). Writing return type on every one of the 4-5 lambdas per f= unction call is daunting. I understand that lambda's return type is con= sistent with auto, but it doesn't seem to be practic= al in this case.

Proposal:

=
Use std::common_type<return_stm_types...> as return type. E.g. use = auto on each return statement individually, then deduce common= type using `std::common_type`. std::common_type<std::n= ullopt_t, std::optional<int>> indeed y= ields std::optional<int>, as shown in shown in this example:


= #include <type_traits>
<= span style=3D"color: #800;" class=3D"styled-by-prettify">#include <optional>

template <typename>
struct undefined;

int main()
= {
=C2=A0 =C2=A0 <= /span>using common_t =3D std::common_type_t<std::nullopt_t, std::opt= ional<i= nt>>= ;
=C2= =A0 =C2=A0 static_assert(std::is_same_v<std::optional<int>, common_t>);
=C2=A0 =C2=A0 //undefined<std::common_type_t<std::nullo= pt_t, std::optional<int>>> foo;
}

Possible damage:

I cannot think of any at the moment. I never encountered = a case where compilation error inside of lambda would be of any use. I beli= eve this shouldn't affect existing code, as std:= :common_type is conversion rules friendl= y (not 100% sure though).

=



--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/7772686a-7e80-4c28-a87b-c5dc11450cc6= %40isocpp.org.
------=_Part_21131_251306515.1529784871279-- ------=_Part_21130_1016860082.1529784871278-- .