220 40141 article Path: news.gmane.org!.POSTED!not-for-mail From: Miguel Ojeda Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Parametric Expression Date: Wed, 12 Sep 2018 18:26:16 +0200 Lines: 107 Approved: news@gmane.org Message-ID: References: <95ae303b-329c-4121-aa5f-69edf040d5b5@isocpp.org> <80215dcb-875f-42c0-8f7f-e8030c2145e7@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" X-Trace: blaine.gmane.org 1536769476 28520 195.159.176.226 (12 Sep 2018 16:24:36 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 12 Sep 2018 16:24:36 +0000 (UTC) Cc: swac31@gmail.com To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDRZHGH43YJRBP744TOAKGQEU5AWQ7A@isocpp.org Wed Sep 12 18:24:31 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qt0-f197.google.com ([209.85.216.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1g07wM-0007HW-P5 for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id g58-v6sor512204qtb.110.2018.09.12.09.26.39 for (Google Transport Security); Wed, 12 Sep 2018 09:26:39 -0700 (PDT) Received-SPF: pass (google.com: domain of miguel.ojeda.sandonis@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:aed:3fd3:: with SMTP id w19-v6mr2135891qth.226.1536769597866; Wed, 12 Sep 2018 09:26:37 -0700 (PDT) Original-Received: by 2002:ac8:7494:0:0:0:0:0 with HTTP; Wed, 12 Sep 2018 09:26:16 -0700 (PDT) In-Reply-To: X-Original-Sender: miguel.ojeda.sandonis@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=cX1gevwJ; spf=pass (google.com: domain of miguel.ojeda.sandonis@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=miguel.ojeda.sandonis@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:40141 Archived-At: Hi Jason, On Mon, Sep 10, 2018 at 10:13 PM, Jason Rice wrote: > The feedback on this so far has been greatly appreciated. > > I added a couple of examples and refactored the rules to address some of the > concerns that were brought up. > > https://gist.github.com/ricejasonf/84c11ac6bb093c1ea8c380a9a466d8cb We had a quick discussion a few months ago [1] on how to implement something that would allow you to write things like: x = x || new int; In other words, an operator|| with short-circuit semantics. There were suggestions on changing the language (the topic starter), on standardizing language extensions (the binary ?: GNU operator) and on doing some tricks with a macro and a lambda, e.g: #define LAZY(x) [](){ return (x); } template T operator||(T p, E e) { if (p) return p; return e(); } x = x || LAZY(new int); Half-jokingly, I suggested working with expressions instead would look very nice and would allow to implement these things directly in the language, e.g.: template T operator||(T p, E e) { if (p) return p; return e; } x = x || new int; How would your add() example look using this "expression" syntax? Maybe something like: template E add(T a, T b) { return a + b; } Now, the other way around, let's take a look at the operator||() example using your "using" syntax: using operator||(auto p, ???) { if (p) return p; return ???; } Almost there! I am sure you are seeing what I am getting at here. If we are going to get almost-macro-like entities, we are basically talking about manipulating (or rather, being able to express) expressions. Your approach, if I interpreted it correctly, allows to return a C++ expression given a function-like call, but does not allow to pick expressions themselves as parameters --- which would allow to implement things like those short-circuit operators and replace even more macros. We could implement operators already existing in the language in terms of the language itself: using myOwnTernary(auto c, expression t, expression f) { if (c) return t; return f; } // foo() only called if the condition is true int x = myOwnTernary(something, 1, foo()); Inexpensive logging is another place where it is useful (to avoid evaluating parameters, e.g. Boost's LOG_WARNING): using logWarning(expression... es) { if (level >= warning) doLog(es...); } Think about it! Cheers, Miguel [1] https://groups.google.com/a/isocpp.org/d/msg/std-proposals/jB5TIcRZeic/qZqxtxSDAQAJ -- 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/CANiq72k7K%2BU2Gtjhe8rFk5TjdW8aQLyDO0Qw4pop3G1ftJ-OAQ%40mail.gmail.com. .