220 13979 <9F045410-BC6B-492D-B2BD-03AADFA7EDE0@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Comment on N4191 - Folding expressions (Sutton, Smith) Date: Fri, 17 Oct 2014 20:43:05 +0800 Lines: 172 Approved: news@gmane.org Message-ID: <9F045410-BC6B-492D-B2BD-03AADFA7EDE0@gmail.com> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_2EEC6574-5FA2-41CE-A331-A38446865730" X-Trace: ger.gmane.org 1413549816 17332 80.91.229.3 (17 Oct 2014 12:43:36 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 17 Oct 2014 12:43:36 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRB3U5QSRAKGQEDEOKSAA@isocpp.org Fri Oct 17 14:43:29 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yh0-f71.google.com ([209.85.213.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Xf6sS-0000yV-8t for gclcip-std-proposals@m.gmane.org; Fri, 17 Oct 2014 14:43:28 +0200 Original-Received: by mail-yh0-f71.google.com with SMTP id f73sf1565830yha.6 for ; Fri, 17 Oct 2014 05:43:27 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:message-id:mime-version:subject:date :references:to:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=0VznHZOZfae7XiS5qFG9GnNgK6hIC7ArhpDnc2phqKg=; b=Bw1qrV6psJqlsT4rwV29eIWdjq0WgYAwSoAKSwTpT6zYGnfujz6GTUrPN0Y7x9HnQe LI+eIQEzwdEXzI40kuHdhLcXOCVqXgq1X0z6/dq1TsDJcUmgn3ObjO3MO/UsQ81hVMZ+ lIm2wMRdWGKAsMqRK3IeLTNRcTqslBkwinPHEVOxuE4/yG+OqBs5fhLQ8fRqOXwbvFcI s48dqGJWjbG+yVQKZtIyCy6xfTRmDXlDG5vzVzaT1JGfLAwoBKawAastug2Msw3rTp0Q RsqHd/NG+i+UT/+IVna13LkqyyONXP7yYl/fjKrgFi9dollpbyTjF6sfoHbZMcc06WhF g+mQ== X-Gm-Message-State: ALoCoQlC5ofp//25jkxdwFUf8283RZqcp5/aXP18XR/VrP926+DyN0c4m5PkuwnEYXexx4arsJxT X-Received: by 10.236.39.174 with SMTP id d34mr5382179yhb.54.1413549807018; Fri, 17 Oct 2014 05:43:27 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.11.23 with SMTP id v23ls867970ioi.81.gmail; Fri, 17 Oct 2014 05:43:26 -0700 (PDT) X-Received: by 10.70.61.202 with SMTP id s10mr4925461pdr.124.1413549806228; Fri, 17 Oct 2014 05:43:26 -0700 (PDT) Original-Received: from mail-pa0-x236.google.com (mail-pa0-x236.google.com. [2607:f8b0:400e:c03::236]) by mx.google.com with ESMTPS id b2si1165942pde.9.2014.10.17.05.43.26 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 17 Oct 2014 05:43:26 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::236 as permitted sender) client-ip=2607:f8b0:400e:c03::236; Original-Received: by mail-pa0-f54.google.com with SMTP id ey11so782095pad.13 for ; Fri, 17 Oct 2014 05:43:25 -0700 (PDT) X-Received: by 10.68.231.33 with SMTP id td1mr8083736pbc.133.1413549805462; Fri, 17 Oct 2014 05:43:25 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.54.144]) by mx.google.com with ESMTPSA id oq6sm1503777pdb.45.2014.10.17.05.43.18 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 17 Oct 2014 05:43:24 -0700 (PDT) In-Reply-To: X-Mailer: Apple Mail (2.1878.6) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::236 as permitted sender) smtp.mail=potswa@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=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:13979 Archived-At: --Apple-Mail=_2EEC6574-5FA2-41CE-A331-A38446865730 Content-Type: text/plain; charset=ISO-8859-1 On 2014-10-17, at 7:50 PM, Markus Grech wrote: > In functional languages, the initial value of a fold can be specified by the user. Why does the proposal mandate a specific value? Is there anything that prevents the following syntax: > (args + ... 0) // foldl args (+) 0 > (0 ... + args) // foldr args (+) 0 > Would this work? It would be easy to do, with a tuple_fold function. What worries me about this proposal is that its functionality is easily done with a generic template, yet no such template is in popular use, nor does a library proposal exist. We're jumping straight to the core language with no usage experience. Here's some sort-of working library code. Only problem is, I wanted a left-fold but I got a right-fold because I'm a little drunk :P #include #include template< typename tuple_type, typename ftor, typename acc_type > auto && tuple_fold_impl( tuple_type && t, ftor f, acc_type && acc, std::index_sequence<> ) { return std::forward< acc_type >( acc ); } template< typename tuple_type, typename ftor, typename acc_type, std::size_t index, std::size_t ... index_tail > decltype(auto) tuple_fold_impl( tuple_type && t, ftor f, acc_type && acc, std::index_sequence< index, index_tail ... > ) { return f( tuple_fold_impl( std::forward< tuple_type >( t ), f, std::forward< acc_type >( acc ), std::index_sequence< index_tail ... >{} ), std::get< index >( std::forward< tuple_type >( t ) ) ); } template< typename tuple_type, typename ftor, std::size_t ... index_tail > decltype(auto) tuple_fold_impl( tuple_type && t, ftor f, std::index_sequence< 0, index_tail ... > ) { return tuple_fold_impl( std::forward< tuple_type >( t ), f, std::get< 0 >( std::forward< tuple_type >( t ) ), std::index_sequence< index_tail ... >{} ); } template< typename tuple_type, typename ftor, typename seq = std::make_index_sequence< std::tuple_size< tuple_type >::value > > decltype(auto) tuple_fold( tuple_type && t, ftor f ) { return tuple_fold_impl( std::forward< tuple_type >( t ), f, seq{} ); } template< typename tuple_type, typename ftor, typename acc_type, typename seq = std::make_index_sequence< std::tuple_size< tuple_type >::value > > decltype(auto) tuple_fold( tuple_type && t, ftor f, acc_type && acc ) { return tuple_fold_impl( std::forward< tuple_type >( t ), f, std::forward< acc_type >( acc ), seq{} ); } #include int main() { tuple_fold( std::make_tuple( 1, " hello ", false, std::boolalpha, '\n' ), [] ( std::ostream & s, auto && o ) -> std::ostream & { return s << o; }, std::cout ); } -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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On 2014&= ndash;10–17, at 7:50 PM, Markus Grech <markus.grech@gmail.com> wrote:

In functi= onal languages, the initial value of a fold can be specified by the user. W= hy does the proposal mandate a specific value? Is there anything that preve= nts the following syntax:
(args + ... 0) // fold= l args (+) 0
(0 ... = + args) // foldr args (+) 0
Would this work?

It would be easy to do, with a tuple_fold fu= nction.

What worries me about this proposal is tha= t its functionality is easily done with a generic template, yet no such tem= plate is in popular use, nor does a library proposal exist. We’re jum= ping straight to the core language with no usage experience.

=
Here’s some sort-of working library code. Only problem is,= I wanted a left-fold but I got a right-fold because I’m a little dru= nk :P


#inclu= de <tuple>
#include <utility>

template< typename t= uple_type, typename ftor, typename acc_type >
auto && tuple_f= old_impl( tuple_type && t, ftor f, acc_type && acc,
&nbs= p;   std::index_sequence<> )
    { return= std::forward< acc_type >( acc ); }

template< typename tupl= e_type, typename ftor, typename acc_type,
    std::size_t= index, std::size_t ... index_tail >
decltype(auto) tuple_fold_impl( = tuple_type && t, ftor f, acc_type && acc,
   &= nbsp;std::index_sequence< index, index_tail ... > ) {
   = ; return f(
        tuple_fold_impl(
&n= bsp;           std::forward< tuple_type &g= t;( t ),
            f,
  &nb= sp;         std::forward< acc_type >( acc ),=
            std::index_sequence< = index_tail ... >{}
        ),
  &nbs= p;     std::get< index >( std::forward< tuple_type = >( t ) )
    );
}

template< typename tupl= e_type, typename ftor, std::size_t ... index_tail >
decltype(auto) tu= ple_fold_impl( tuple_type && t, ftor f, std::index_sequence< 0, = index_tail ... > ) {
    return tuple_fold_impl(
&n= bsp;       std::forward< tuple_type >( t ),
&n= bsp;       f,
        std::= get< 0 >( std::forward< tuple_type >( t ) ),
    &= nbsp;   std::index_sequence< index_tail ... >{}
  &= nbsp; );
}

template< typename tuple_type, typename ftor,<= br>    typename seq =3D std::make_index_sequence< std::tu= ple_size< tuple_type >::value > >
decltype(auto) tuple_fold(= tuple_type && t, ftor f )
    { return tuple_fol= d_impl( std::forward< tuple_type >( t ), f, seq{} ); }

templat= e< typename tuple_type, typename ftor, typename acc_type,
  &nbs= p; typename seq =3D std::make_index_sequence< std::tuple_size< t= uple_type >::value > >
decltype(auto) tuple_fold( tuple_type &a= mp;& t, ftor f, acc_type && acc )
    { retur= n tuple_fold_impl( std::forward< tuple_type >( t ), f, std::forward&l= t; acc_type >( acc ), seq{} ); }

#include <iostream>
int main() {
tuple_fold( std::make_tuple( 1, " hello ", false, std::boolalpha, '\n' ), [] ( std::ostream & s, auto && o= ) -> std::ostream & { return s << o; }, s= td::cout );
}


--

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