220 37648 article Path: news.gmane.org!.POSTED!not-for-mail From: =?UTF-8?B?R2HFoXBlciBBxb5tYW4=?= Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Variable declaration in a ternary conditional Date: Wed, 4 Apr 2018 19:59:05 +0100 Lines: 185 Approved: news@gmane.org Message-ID: References: <8a755089-c146-47ab-8250-c2b3e705cacd@isocpp.org> <3cdd7b2c-3716-4a58-8a13-3f65fb2eae22@isocpp.org> <9fabc312-9500-4b34-9a4a-a2793891c585@isocpp.org> <820d844c-21a7-429c-bf55-17ee12f8ca8e@isocpp.org> <2fe5d081-e13f-4750-87b0-a3b5b47eb252@isocpp.org> <0f06146e-ae61-4c74-b09d-07dae4aa058f@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="94eb2c140702f6efe205690a6bce" X-Trace: blaine.gmane.org 1522868246 28726 195.159.176.226 (4 Apr 2018 18:57:26 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 4 Apr 2018 18:57:26 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDRYVJ6TVYBRBDWBSTLAKGQEFTMRHLQ@isocpp.org Wed Apr 04 20:57:22 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pl0-f69.google.com ([209.85.160.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1f3nav-0007Hm-HL for gclcip-std-proposals@m.gmane.org; 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[209.85.220.65]) by mx.google.com with SMTPS id y189sor1409811pgb.371.2018.04.04.11.59.25 for (Google Transport Security); Wed, 04 Apr 2018 11:59:25 -0700 (PDT) Received-SPF: pass (google.com: domain of gasper.azman@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; X-Received: by 10.99.107.198 with SMTP id g189mr2618426pgc.423.1522868365486; Wed, 04 Apr 2018 11:59:25 -0700 (PDT) Original-Received: by 10.236.173.5 with HTTP; Wed, 4 Apr 2018 11:59:05 -0700 (PDT) In-Reply-To: X-Original-Sender: gasper.azman@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=fpD00A7G; spf=pass (google.com: domain of gasper.azman@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=gasper.azman@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE 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:37648 Archived-At: --94eb2c140702f6efe205690a6bce Content-Type: text/plain; charset="UTF-8" Richard, I think the if-expression has the issue of being a compound statement, and therefore lifetimes get jumbled - basically, the semantics would *have* to be defined as if it were a lambda, and at that point we're just arguing for shorthand immediately-invoked-lambda syntax for a particular purpose. At which point it's just easier to start *actually* arguing about immediately-invoked lambdas, because the lambda introducer is one hell of a way to tell the parser that it needs to do special stuff. G On Wed, Apr 4, 2018 at 7:55 PM, Richard Hodges wrote: > > > On 4 April 2018 at 20:41, Nicol Bolas wrote: > >> >> >> On Wednesday, April 4, 2018 at 2:23:43 PM UTC-4, Richard Hodges wrote: >>> >>> Just a thought, but how badly would it mangle the language if the if >>> keyword, as well as introducing a statement, was also able to introduce an >>> expression which returned: >>> >>> * In the case where there is an else clause: the std::common_type of >>> its true and false clauses >>> * In the case where there is no else clause: a std::optional where T >>> is the expression type of the true case. >>> >> >> I don't like the idea of implicitly tying syntax into a standard library >> type like `optional` in this way. We should just require that >> if-expressions have an `else` clause. >> > > I equivocated on this point myself, feeling that a "value or nothing" > result was useful. Of course that leads directly into, "why should it not > return an outcome<>"... and that I think leads to Gasper's argument for > just using a lambda. > > So I think I am persuaded that the std::common_type<> is sufficient. > > >> >> Otherwise, the idea of an if-expression seems to be a solid one. This >> formulation also solves other problems; since it uses `if` as an >> introducer, it can use `if constexpr` as an introducer as well for >> `constexpr` initializations. >> >> -- >> 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/is >> ocpp.org/d/msgid/std-proposals/0f06146e-ae61-4c74-b09d- >> 07dae4aa058f%40isocpp.org >> >> . >> > > -- > 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/CALvx3haNnehD4fpuzbHr2DeiXA154 > eCgmtbJrP680y4uzBgbCg%40mail.gmail.com > > . > -- 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/CAANG%3DkUHEkkvHpH5OvpmNF025KeFhJqHQCZFO-1gxDNjK5dFOQ%40mail.gmail.com. --94eb2c140702f6efe205690a6bce Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Richard, I think the if-expression has the issue of being = a compound statement, and therefore lifetimes get jumbled - basically, the = semantics would *have* to be defined as if it were a lambda, and at that po= int we're just arguing for shorthand immediately-invoked-lambda syntax = for a particular purpose.

At which point it's just e= asier to start *actually* arguing about immediately-invoked lambdas, becaus= e the lambda introducer is one hell of a way to tell the parser that it nee= ds to do special stuff.

G

On Wed, Apr 4, 2018 at 7:55 PM= , Richard Hodges <hodges.r@gmail.com> wrote:


On 4 April 2018 at 20:41, Nicol Bo= las <jmckesson@gmail.com> wrote:


On Wednesday, April 4, 2018 at 2:23= :43 PM UTC-4, Richard Hodges wrote:
Just a thought, but how badly would it mangle the languag= e if the if keyword, as well as = introducing a statement, was also able to introduce an expression which ret= urned:

* In the case where there is an else clause:=C2=A0 the std::c= ommon_type of its true and false clauses
* In the case where there is no= else clause: a std::optional<T> where T is the expression type of th= e true case.

I don't like the ide= a of implicitly tying syntax into a standard library type like `optional` i= n this way. We should just require that if-expressions have an `else` claus= e.

I equivocated on = this point myself, feeling that a "value or nothing" result was u= seful. Of course that leads directly into, "why should it not return a= n outcome<>"... and that I think leads to Gasper's argument = for just using a lambda.

So I think I am persu= aded that the std::common_type<> is sufficient.
=C2=A0

= Otherwise, the idea of an if-expression seems to be a solid one. This formu= lation also solves other problems; since it uses `if` as an introducer, it = can use `if constexpr` as an introducer as well for `constexpr` initializat= ions.

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 To view this discussion on the web visit = https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/0f06= 146e-ae61-4c74-b09d-07dae4aa058f%40isocpp.org.

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 To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-= proposals/CALvx3haNnehD4fpuzbHr2DeiXA154eCgmtbJrP680y4uzBgbC= g%40mail.gmail.com.

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