220 40110 article Path: news.gmane.org!.POSTED!not-for-mail From: Jason Rice Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Parametric Expression Date: Fri, 31 Aug 2018 01:00:23 -0700 Lines: 188 Approved: news@gmane.org Message-ID: References: <95ae303b-329c-4121-aa5f-69edf040d5b5@isocpp.org> <53248610-d4ed-4a28-b5f8-bf7c61247eb9@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="0000000000001efe020574b6962d" X-Trace: blaine.gmane.org 1535702311 20989 195.159.176.226 (31 Aug 2018 07:58:31 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 31 Aug 2018 07:58:31 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCBKPROT3IBBBI7LUPOAKGQEWNIEQXY@isocpp.org Fri Aug 31 09:58:27 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oi0-f70.google.com ([209.85.218.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fveK2-0005MT-DV for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id j125-v6sor7665413oif.114.2018.08.31.01.00.35 for (Google Transport Security); Fri, 31 Aug 2018 01:00:35 -0700 (PDT) Received-SPF: pass (google.com: domain of ricejasonf@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:aca:4342:: with SMTP id q63-v6mr6501576oia.309.1535702435035; Fri, 31 Aug 2018 01:00:35 -0700 (PDT) In-Reply-To: <53248610-d4ed-4a28-b5f8-bf7c61247eb9@isocpp.org> X-Original-Sender: ricejasonf@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=g6CGk5WI; spf=pass (google.com: domain of ricejasonf@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=ricejasonf@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:40110 Archived-At: --0000000000001efe020574b6962d Content-Type: text/plain; charset="UTF-8" Admittedly, compile time performance is the main motivation here. Think of this as having the same utility that a type alias template has over using a class template. However, there are a couple of things that this can do that normal functions cannot because we are just generating AST in the same context/scope as where it is invoked. Here is an example of constexpr parameters: ``` #include template struct unpack_string_t; template <> struct unpack_string_t { using operator()(auto str, auto ...chars, auto fn) { if constexpr(str[sizeof...(chars)] == 0) { return fn(chars...); } else { return unpack_string 9000>{}(str, chars..., str[sizeof...(chars)], fn); } } }; inline constexpr unpack_string_t unpack_string; template struct ct_string { }; struct make_ct_string_t { using operator()(auto ...chars) { return ct_string{}; } }; inline constexpr make_ct_string_t make_ct_string; int main() { ct_string<'f', 'o', 'o'> x = unpack_string("foo", make_ct_string); } ``` To me it would be more interesting if we could replace a larger subset of > macros, maybe 'using' as a parameter type which substitutes in the body at > an early parsing phase or something like that... I'm not sure what you mean by "replace a larger subset of macros". Having any parameter in a normal function have the ability to be a "using" parameter sounds interesting, but function scope is isolated so arbitrary expressions would be out. If "using" parameters were just constexpr values then that function would have a different type for every value it was invoked with. On Thu, Aug 30, 2018 at 9:54 PM Bengt Gustafsson < bengt.gustafsson@beamways.com> wrote: > I faail to understand how this differs from doing the same but replacing > the using with auto. If you worry about compile times there is nothing that > hinders current clmpikers from taking a shortcut if all parameter types are > plain auto... > > Please provide an example of code that can be written using this feature > but not with auto return type. Preferably understandable without requiring > too much knowledge of higher math. > > To me it would be more interesting if we could replace a larger subset of > macros, maybe 'using' as a parameter type which substitutes in the body at > an early parsing phase or something like that... > > -- > 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/53248610-d4ed-4a28-b5f8-bf7c61247eb9%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/CANfsYuwwU0MHV%2BSaDZQZetwTzJpvjJ%2BptFzK4A4CQHWLaHecYg%40mail.gmail.com. --0000000000001efe020574b6962d Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Admittedly, compile time performance is t= he main motivation here. Think of this as having the same utility that a ty= pe alias template has over using a class template.

However, there ar= e a couple of things that this can do that normal functions cannot because = we are just generating AST in the same context/scope as where it is invoked= ..

Here is an example of constexpr parameters:

```
#include= <type_traits>

template <bool>
struct unpack_string_t= ;

template <>
struct unpack_string_t<false> {
=C2= =A0 using operator()(auto str, auto ...chars, auto fn) {
=C2=A0=C2=A0=C2= =A0 if constexpr(str[sizeof...(chars)] =3D=3D 0) {
=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 return fn(chars...);
=C2=A0=C2=A0=C2=A0 }
=C2=A0=C2=A0=C2= =A0 else {
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return unpack_string<sizeof= ....(chars) > 9000>{}(str, chars...,
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 str[sizeof.= ...(chars)], fn);
=C2=A0=C2=A0=C2=A0 }
=C2=A0 }
};

inline co= nstexpr unpack_string_t unpack_string;

template <char...>
s= truct ct_string { };

struct make_ct_string_t {
=C2=A0 using opera= tor()(auto ...chars) {
=C2=A0=C2=A0=C2=A0 return ct_string<chars...&g= t;{};
=C2=A0 }
};

inline constexpr make_ct_string_t make_ct_st= ring;


int main() {
=C2=A0 ct_string<'f', 'o= 9;, 'o'> x =3D unpack_string("foo", make_ct_string);}
```

To me i= t would be more interesting if we could replace a larger subset=20 of macros, maybe 'using' as a parameter type which substitutes in t= he=20 body at an early parsing phase or something like that...
=
I'm not sure what you mean by "replace a larger subset of macr= os".

Having any parameter in a normal function have = the ability to be a "using" parameter sounds interesting, but fun= ction scope is isolated so arbitrary expressions would be out. If "usi= ng" parameters were just constexpr values then that function would hav= e a different type for every value it was invoked with.

On Thu, Aug 30, 2018 at 9= :54 PM Bengt Gustafsson <bengt.gustafsson@beamways.com> wrote:
I faail to understand how this differs from doing the same bu= t replacing the using with auto. If you worry about compile times there is = nothing that hinders current clmpikers from taking a shortcut if all parame= ter types are plain auto...

Please provide an example of code that can be written using this feature bu= t not with auto return type. Preferably understandable without requiring to= o much knowledge of higher math.

To me it would be more interesting if we could replace a larger subset of m= acros, maybe 'using' as a parameter type which substitutes in the b= ody at an early parsing phase or something like that...

--
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-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.co= m/a/isocpp.org/d/msgid/std-proposals/53248610-d4ed-4a28-b5f8-bf7c61247eb9%4= 0isocpp.org.

--
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