Reimplementation of the MPL for C++11

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Louis Dionne

18 Oct 2013 18 Oct '13

6:07 p.m.

Hi, I am currently working on a reimplementation of the MPL for C++11. Although the library is not complete yet, I would like to gauge interest for the project from the Boost community. I wanted to wait until I have resolved some core issues before posting here, but reading the [modularization] thread made it unbearable to wait any longer :-). The library is available on GitHub: github.com/ldionne/mpl11. If some people happen to be interested by the project and would like to get involved, please get in touch with me. Regards, Louis Dionne N.B.: Some design decisions will be influenced by the community (if there's interest) and I'm experimenting a lot. Don't be put off if you see something that you don't like: it might change.

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Eric Niebler

18 Oct 18 Oct

7:45 p.m.

On 10/18/2013 11:07 AM, Louis Dionne wrote:

...

Hi,

I am currently working on a reimplementation of the MPL for C++11. Although the library is not complete yet, I would like to gauge interest for the project from the Boost community.

I wanted to wait until I have resolved some core issues before posting here, but reading the [modularization] thread made it unbearable to wait any longer :-). The library is available on GitHub: github.com/ldionne/mpl11.

If some people happen to be interested by the project and would like to get involved, please get in touch with me.

Disclaimer: I haven't looked. But knowing the forward-only nature of parameter packs, I wonder how you can implement a random-access sequence like vector without (a) instantiating O(N) templates, like tuple, or (b) using the preprocessor and living with arbitrary limits. I fear that the mpl simply cannot be implemented with variadics without radically changing the compile-time complexity of things. -- Eric Niebler Boost.org http://www.boost.org

Louis Dionne

8:47 p.m.

Eric Niebler writes: [snip]

...

Disclaimer: I haven't looked. But knowing the forward-only nature of parameter packs, I wonder how you can implement a random-access sequence like vector without (a) instantiating O(N) templates, like tuple, or (b) using the preprocessor and living with arbitrary limits.

I fear that the mpl simply cannot be implemented with variadics without radically changing the compile-time complexity of things.

I think there's a way to do it in O(1), but it's not implemented in the library right now. See [1]. In my opinion, this is only a tiny part of the MPL and even if it can't be improved, there are other areas where we benefit from C++11 in the MPL. Do you feel like that would be a deal breaker if O(1) access was impossible? Regards, Louis Dionne [1]: github.com/dabrahams/mpl11/blob/master/standalone/O1_variadics.cpp

pfultz2

19 Oct 19 Oct

3:45 p.m.

...

But knowing the forward-only nature of parameter packs, I wonder how you can implement a random-access sequence like vector without (a) instantiating O(N) templates, like tuple, or (b) using the preprocessor and living with arbitrary limits.

The O(N) instantiation would happen at creation of the vector, not at lookup. Something like this: template struct vector_base; template struct vector_base : vector_base { using vector_base::item_; static T item_(long_<N>); }; template<long N> struct vector_base<N> { static void item_(); }; template struct vector : vector_base<0, Ts...> { }; Then we have O(1) lookup using decltype: template struct at { typedef decltype(Vector::item_(Index())) type; }; I haven't tested this code yet, but the Boost.MPL works in a similiar way for compilers that suppor typeof. Of course generating the `item_` overloads with preprocessor perhaps could be faster than expaning the parameter pack using base classes, since it requires O(N) instantiations. However, if the prerpocessor version is faster, the `vector` could be specialized up to an arbitary limit to be generated by the preprocessor, and then after that it would use the base classes to generate the overloads. Paul -- View this message in context: http://boost.2283326.n4.nabble.com/Reimplementation-of-the-MPL-for-C-11-tp46... Sent from the Boost - Dev mailing list archive at Nabble.com.

Louis Dionne

5:49 p.m.

pfultz2 writes: [...]

...

Then we have O(1) lookup using decltype:

template struct at { typedef decltype(Vector::item_(Index())) type; };

I haven't tested this code yet, but the Boost.MPL works in a similiar way for compilers that suppor typeof. Of course generating the `item_` overloads

[...] That's how the map is currently implemented, but I don't use the preprocessor to generate overloads. See the top of [1] for details, it's pretty simple. Regards, Louis Dionne [1]: github.com/ldionne/mpl11/blob/master/include/boost/mpl11/container/map.hpp

Larry Evans

20 Oct 20 Oct

11:23 a.m.

On 10/19/13 12:49, Louis Dionne wrote:

...

pfultz2 writes:

[...]

...
Then we have O(1) lookup using decltype:

template struct at { typedef decltype(Vector::item_(Index())) type; };

I haven't tested this code yet, but the Boost.MPL works in a similiar way for compilers that suppor typeof. Of course generating the `item_` overloads

[...]

That's how the map is currently implemented, but I don't use the preprocessor to generate overloads.

See the top of [1] for details, it's pretty simple.

Regards,

Louis Dionne

[1]: github.com/ldionne/mpl11/blob/master/include/boost/mpl11/container/map.hpp

Hi Dionne, I tried variadic templates several years ago. The code is here: http://svn.boost.org/svn/boost/sandbox/variadic_templates/boost/mpl/ Unfortunately, as Eric and Mathias already hinted, the compile times are much slower than with the existing mpl. The existing mpl uses the preprocessor to lessen compile times, a method mentioned by Eric in his (b) item: (b) using the preprocessor and living with arbitrary limits. Christopher Schmidt used the preprocessor to implement a variadic fusion using the help of the preprocessor to lessen compile times but, IIUC, he used some method (I'll call it "chunking") so that the preprocessor didn't have arbitrary limits: http://lists.boost.org/Archives/boost/2012/03/191091.php NOTE: I tried the link in that post to Christopher's code and it no longer works :( However, I'm sure Christopher (if you can find his email address) would be able to supply an update location for the code. You might could use the chunking method to minimize the compile times. HTH. -regards, Larry

Larry Evans

12:04 p.m.

On 10/20/13 06:23, Larry Evans wrote: [snip]

...

Christopher Schmidt used the preprocessor to implement a variadic fusion using the help of the preprocessor to lessen compile times but, IIUC, he used some method (I'll call it "chunking") so that the preprocessor didn't have arbitrary limits:

http://lists.boost.org/Archives/boost/2012/03/191091.php

NOTE: I tried the link in that post to Christopher's code and it no longer works :( However, I'm sure Christopher (if you can find his email address) would be able to supply an update location for the code. You might could use the chunking method to minimize the compile times.

The code here: http://svn.boost.org/svn/boost/sandbox/variadic_templates/sandbox/slim/ might be helpful. It was based on Chrisopher's code but had to be modified slightly as mentioned in the post: http://comments.gmane.org/gmane.comp.compilers.clang.devel/24145 which also mentions the slow compile times when not using the preprocessor. -regards, Larry

Agustín K-ballo Bergé

3:33 p.m.

On 20/10/2013 09:04 a.m., Larry Evans wrote:

...

On 10/20/13 06:23, Larry Evans wrote: [snip]

...
Christopher Schmidt used the preprocessor to implement a variadic fusion using the help of the preprocessor to lessen compile times but, IIUC, he used some method (I'll call it "chunking") so that the preprocessor didn't have arbitrary limits:

http://lists.boost.org/Archives/boost/2012/03/191091.php

NOTE: I tried the link in that post to Christopher's code and it no longer works :( However, I'm sure Christopher (if you can find his email address) would be able to supply an update location for the code. You might could use the chunking method to minimize the compile times.

The code here:

http://svn.boost.org/svn/boost/sandbox/variadic_templates/sandbox/slim/

might be helpful. It was based on Chrisopher's code but had to be modified slightly as mentioned in the post:

http://comments.gmane.org/gmane.comp.compilers.clang.devel/24145

which also mentions the slow compile times when not using the preprocessor.

Compilers appear to have improved since then! I decided to take gcc4.8.1 and fusion::map for a spin, since I happen to have a variadic implementation of map that does not use recursion. The results of this crude exercise can be found here: https://gist.github.com/K-ballo/a44590159133997abcdc What I read from them is that each technique reduces compile-time and memory usage in the order I'd expected (C++98 > C++98 preprocessed > C++11 recursive > C++11 non-recursive), contrary to what this thread seems to be saying. Additionally, instantiating a 255 element variadic non-recursive map and getting to the middle of it takes only slightly more time and just 1.3x the memory it takes doing the same for a 50 element C++03 preprocessed map. There's lots of potential in variadics, as long as they are implemented carefully. Regards, -- Agustín K-ballo Bergé.- http://talesofcpp.fusionfenix.com

Larry Evans

22 Oct 22 Oct

3:28 a.m.

On 10/20/13 10:33, Agustín K-ballo Bergé wrote:

...

On 20/10/2013 09:04 a.m., Larry Evans wrote: [snip]

...
The code here:

http://svn.boost.org/svn/boost/sandbox/variadic_templates/sandbox/slim/

might be helpful. It was based on Chrisopher's code but had to be modified slightly as mentioned in the post:

http://comments.gmane.org/gmane.comp.compilers.clang.devel/24145

which also mentions the slow compile times when not using the preprocessor.

Compilers appear to have improved since then! I decided to take gcc4.8.1 and fusion::map for a spin, since I happen to have a variadic implementation of map that does not use recursion. The results of this crude exercise can be found here: https://gist.github.com/K-ballo/a44590159133997abcdc

This link only has the test driver code and the test results file. Could you please provide a link to the implementation of this non-recursive map? -regards, Larry

Agustín K-ballo Bergé

3:51 a.m.

On 22/10/2013 12:28 a.m., Larry Evans wrote:

...

On 10/20/13 10:33, Agustín K-ballo Bergé wrote:

...
On 20/10/2013 09:04 a.m., Larry Evans wrote: [snip]

...
The code here:

http://svn.boost.org/svn/boost/sandbox/variadic_templates/sandbox/slim/

might be helpful. It was based on Chrisopher's code but had to be modified slightly as mentioned in the post:

http://comments.gmane.org/gmane.comp.compilers.clang.devel/24145

which also mentions the slow compile times when not using the preprocessor.

Compilers appear to have improved since then! I decided to take gcc4.8.1 and fusion::map for a spin, since I happen to have a variadic implementation of map that does not use recursion. The results of this crude exercise can be found here: https://gist.github.com/K-ballo/a44590159133997abcdc

This link only has the test driver code and the test results file. Could you please provide a link to the implementation of this non-recursive map?

Certainly, you can find the proof of concept implementation I used for the tests here: http://fusionfenix.com/ktests/boost.fusion.map.zip It is intended to be used with Boost.Fusion from trunk, you just need to replace fusion's "map" directory with the "map_" one in the zip. Keep in mind that it is just a proof of concept. I am working on a full fledged implementation as time permits, which is not as often as I would like. Regards, -- Agustín K-ballo Bergé.- http://talesofcpp.fusionfenix.com

Larry Evans

1:27 p.m.

On 10/21/13 22:51, Agustín K-ballo Bergé wrote:

...

On 22/10/2013 12:28 a.m., Larry Evans wrote:

...
On 10/20/13 10:33, Agustín K-ballo Bergé wrote:

...
On 20/10/2013 09:04 a.m., Larry Evans wrote: [snip]

...
The code here:

http://svn.boost.org/svn/boost/sandbox/variadic_templates/sandbox/slim/

might be helpful. It was based on Chrisopher's code but had to be modified slightly as mentioned in the post:

http://comments.gmane.org/gmane.comp.compilers.clang.devel/24145

which also mentions the slow compile times when not using the preprocessor.

Compilers appear to have improved since then! I decided to take gcc4.8.1 and fusion::map for a spin, since I happen to have a variadic implementation of map that does not use recursion. The results of this crude exercise can be found here: https://gist.github.com/K-ballo/a44590159133997abcdc

This link only has the test driver code and the test results file. Could you please provide a link to the implementation of this non-recursive map?

Certainly, you can find the proof of concept implementation I used for the tests here: http://fusionfenix.com/ktests/boost.fusion.map.zip It is intended to be used with Boost.Fusion from trunk, you just need to replace fusion's "map" directory with the "map_" one in the zip.

Keep in mind that it is just a proof of concept. I am working on a full fledged implementation as time permits, which is not as often as I would like.

Regards,

The map.hpp in the zip file shows it uses the preprocessor: --{--cut here-- namespace boost { namespace fusion { struct void_; struct fusion_sequence_tag; template struct map : sequence_base > { struct category : random_access_traversal_tag, associative_tag {}; --}--cut here-- with an option to use some preprocessed headers: --{--cut here-- #if !defined(BOOST_FUSION_DONT_USE_PREPROCESSED_FILES) #include #else #if defined(__WAVE__) && defined(BOOST_FUSION_CREATE_PREPROCESSED_FILES) #pragma wave option(preserve: 2, line: 0, output: "detail/preprocessed/map" FUSION_MAX_MAP_SIZE_STR ".hpp") #endif --}--cut here-- The test results contain, for C++98: --{--cut here-- C++98: Execution times (seconds) phase parsing : 5.29 (97%) usr 349702 kB (98%) ggc --}--cut here-- and: --{--cut here-- C++98 (preprocessed): Execution times (seconds) phase setup : 0.01 ( 0%) usr 547 kB ( 0%) ggc phase parsing : 2.64 (93%) usr 112842 kB (95%) ggc --}--cut here-- However, there's no (preprocessed) indicator for the C++11 results. Instead, there's only: --{--cut here-- C++11 (recursive): Execution times (seconds) phase setup : 0.01 ( 1%) usr 653 kB ( 1%) ggc phase parsing : 0.88 (70%) usr 74763 kB (88%) ggc --}--cut here-- and: --{--cut here-- C++11 (non-recursive): Execution times (seconds) phase parsing : 0.79 (95%) usr 70483 kB (96%) ggc --}--cut here-- Was: defined(BOOST_FUSION_DONT_USE_PREPROCESSED_FILES) for the both C++11 tests or not? BTW, the difference between C++11 (recursive) and (non-recursive) was certainly dramatic for the results prefixed with: 255 elements in a fusion::map, accessing the 129th one test (56.93 vs 2.90). Looks promising. -regards, Larry

Agustín K-ballo Bergé

3:34 p.m.

On 22/10/2013 10:27 a.m., Larry Evans wrote:

...

On 10/21/13 22:51, Agustín K-ballo Bergé wrote:

...
On 22/10/2013 12:28 a.m., Larry Evans wrote:

...
On 10/20/13 10:33, Agustín K-ballo Bergé wrote:

...
On 20/10/2013 09:04 a.m., Larry Evans wrote: [snip]

...
The code here:

http://svn.boost.org/svn/boost/sandbox/variadic_templates/sandbox/slim/

might be helpful. It was based on Chrisopher's code but had to be modified slightly as mentioned in the post:

http://comments.gmane.org/gmane.comp.compilers.clang.devel/24145

which also mentions the slow compile times when not using the preprocessor.

Compilers appear to have improved since then! I decided to take gcc4.8.1 and fusion::map for a spin, since I happen to have a variadic implementation of map that does not use recursion. The results of this crude exercise can be found here: https://gist.github.com/K-ballo/a44590159133997abcdc

This link only has the test driver code and the test results file. Could you please provide a link to the implementation of this non-recursive map?

Certainly, you can find the proof of concept implementation I used for the tests here: http://fusionfenix.com/ktests/boost.fusion.map.zip It is intended to be used with Boost.Fusion from trunk, you just need to replace fusion's "map" directory with the "map_" one in the zip.

The map.hpp in the zip file shows it uses the preprocessor: --{--cut here--

namespace boost { namespace fusion { struct void_; struct fusion_sequence_tag;

template struct map : sequence_base > { struct category : random_access_traversal_tag, associative_tag {}; --}--cut here--

You are looking at the C++03 implementation, the one provided by Fusion already, which is only picked #if !defined(BOOST_FUSION_HAS_VARIADIC_MAP). The C++11 declaration is the obvious one: template struct map;

...

Was:

defined(BOOST_FUSION_DONT_USE_PREPROCESSED_FILES)

for the both C++11 tests or not?

I don't think it was, but it might have been... It is irrelevant for the C++11 implementations. Regards, -- Agustín K-ballo Bergé.- http://talesofcpp.fusionfenix.com

Louis Dionne

5:18 p.m.

Agustín K-ballo Bergé writes: [...]

...

Certainly, you can find the proof of concept implementation I used for the tests here: http://fusionfenix.com/ktests/boost.fusion.map.zip It is intended to be used with Boost.Fusion from trunk, you just need to replace fusion's "map" directory with the "map_" one in the zip.

Keep in mind that it is just a proof of concept. I am working on a full fledged implementation as time permits, which is not as often as I would like.

Thanks for sharing. There are some neat tricks I was not aware of in there! I'll keep the ML updated if I come up with something that can be of use to Fusion as well as the MPL. Regards, Louis Dionne

Dave Abrahams

21 Oct 21 Oct

4:32 a.m.

on Sat Oct 19 2013, pfultz2 wrote:

...

...
But knowing the forward-only nature of parameter packs, I wonder how you can implement a random-access sequence like vector without (a) instantiating O(N) templates, like tuple, or (b) using the preprocessor and living with arbitrary limits.

The O(N) instantiation would happen at creation of the vector, not at lookup. Something like this:

template struct vector_base;

template struct vector_base : vector_base { using vector_base::item_; static T item_(long_<N>); };

template<long N> struct vector_base<N> { static void item_(); };

template struct vector : vector_base<0, Ts...> { };

Then we have O(1) lookup using decltype:

template struct at { typedef decltype(Vector::item_(Index())) type; };

I haven't tested this code yet, but the Boost.MPL works in a similiar way for compilers that suppor typeof. Of course generating the `item_` overloads with preprocessor perhaps could be faster than expaning the parameter pack using base classes, since it requires O(N) instantiations. However, if the prerpocessor version is faster, the `vector` could be specialized up to an arbitary limit to be generated by the preprocessor, and then after that it would use the base classes to generate the overloads.

True, but Matt Calabrese and Zach Lane proved that the ways we avoid O(N) instantiation in the existing MPL don't actually result in the expected speedups: http://zao.se/~zao/boostcon/10/2010_presentations/mon/instantiations_must_go... IMO real measurement is a requirement for progress in this area. -- Dave Abrahams

Michael Marcin

6 p.m.

On 10/20/2013 11:32 PM, Dave Abrahams wrote:

...

True, but Matt Calabrese and Zach Lane proved that the ways we avoid O(N) instantiation in the existing MPL don't actually result in the expected speedups:

http://zao.se/~zao/boostcon/10/2010_presentations/mon/instantiations_must_go...

IMO real measurement is a requirement for progress in this area.

Strange the video for this presentation doesn't load for me. http://blip.tv/boostcon/matt-calabrese-zachary-laine-instantiations-must-go-... Is it working for other?

Matt Calabrese

22 Oct 22 Oct

2:45 a.m.

My concepts without concepts video also does not load. I tried contacting Marshall about it, but I never heard back. Certainly there must be backups somewhere so that they can be re-uploaded, or are they just lost now? On Mon, Oct 21, 2013 at 11:00 AM, Michael Marcin wrote:

...

On 10/20/2013 11:32 PM, Dave Abrahams wrote:

...
True, but Matt Calabrese and Zach Lane proved that the ways we avoid O(N) instantiation in the existing MPL don't actually result in the expected speedups:

http://zao.se/~zao/boostcon/**10/2010_presentations/mon/** instantiations_must_go.pdfhttp://zao.se/~zao/boostcon/10/2010_presentations/mon/instantiations_must_go...

IMO real measurement is a requirement for progress in this area.

Strange the video for this presentation doesn't load for me.

http://blip.tv/boostcon/matt-**calabrese-zachary-laine-** instantiations-must-go-4161957http://blip.tv/boostcon/matt-calabrese-zachary-laine-instantiations-must-go-...

Is it working for other?

______________________________**_________________ Unsubscribe & other changes: http://lists.boost.org/** mailman/listinfo.cgi/boosthttp://lists.boost.org/mailman/listinfo.cgi/boost

-- -Matt Calabrese

Marshall Clow

1:18 p.m.

On Oct 21, 2013, at 7:45 PM, Matt Calabrese wrote:

...

My concepts without concepts video also does not load. I tried contacting Marshall about it, but I never heard back. Certainly there must be backups somewhere so that they can be re-uploaded, or are they just lost now?

That's because I'm a bozo. I have the videos (at home), and will try to get them uploaded tomorrow. Sorry.

...

On Mon, Oct 21, 2013 at 11:00 AM, Michael Marcin wrote:

...
On 10/20/2013 11:32 PM, Dave Abrahams wrote:

...
True, but Matt Calabrese and Zach Lane proved that the ways we avoid O(N) instantiation in the existing MPL don't actually result in the expected speedups:

http://zao.se/~zao/boostcon/**10/2010_presentations/mon/** instantiations_must_go.pdfhttp://zao.se/~zao/boostcon/10/2010_presentations/mon/instantiations_must_go...

IMO real measurement is a requirement for progress in this area.

Strange the video for this presentation doesn't load for me.

http://blip.tv/boostcon/matt-**calabrese-zachary-laine-** instantiations-must-go-4161957http://blip.tv/boostcon/matt-calabrese-zachary-laine-instantiations-must-go-...

Is it working for other?

-- Marshall Marshall Clow Idio Software mailto:mclow.lists@gmail.com A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait). -- Yu Suzuki

Lars Viklund

2:30 p.m.

On Tue, Oct 22, 2013 at 06:18:07AM -0700, Marshall Clow wrote:

...

On Oct 21, 2013, at 7:45 PM, Matt Calabrese wrote:

...
My concepts without concepts video also does not load. I tried contacting Marshall about it, but I never heard back. Certainly there must be backups somewhere so that they can be re-uploaded, or are they just lost now?

That's because I'm a bozo.

I have the videos (at home), and will try to get them uploaded tomorrow. Sorry.

...
On Mon, Oct 21, 2013 at 11:00 AM, Michael Marcin wrote:

...
On 10/20/2013 11:32 PM, Dave Abrahams wrote:

...
True, but Matt Calabrese and Zach Lane proved that the ways we avoid O(N) instantiation in the existing MPL don't actually result in the expected speedups:

http://zao.se/~zao/boostcon/**10/2010_presentations/mon/** instantiations_must_go.pdfhttp://zao.se/~zao/boostcon/10/2010_presentations/mon/instantiations_must_go...

IMO real measurement is a requirement for progress in this area.

Strange the video for this presentation doesn't load for me.

http://blip.tv/boostcon/matt-**calabrese-zachary-laine-** instantiations-must-go-4161957http://blip.tv/boostcon/matt-calabrese-zachary-laine-instantiations-must-go-...

Is it working for other?

All the videos I could mirror in 2010 are mirrored at: http://zao.se/~zao/boostcon/10/ and this one in particular is: http://zao.se/~zao/boostcon/10/Boostcon-MattCalabreseZacharyLaineInstantiati... I started out mirroring the videos for personal use back in 2009, and as blip.tv showed to be rather unreliable for actually getting at the videos, I ended up having a semi-public mirror of the videos and presentations. I have most (all?) videos from 2009-2011. Jeroen has a mirror at http://wipkip.nikhef.nl/events/BoostCon/ as well. -- Lars Viklund | zao@acc.umu.se

Michael Marcin

24 Oct 24 Oct

3:27 a.m.

On 10/22/2013 9:30 AM, Lars Viklund wrote:

...

All the videos I could mirror in 2010 are mirrored at: http://zao.se/~zao/boostcon/10/ and this one in particular is: http://zao.se/~zao/boostcon/10/Boostcon-MattCalabreseZacharyLaineInstantiati...

I started out mirroring the videos for personal use back in 2009, and as blip.tv showed to be rather unreliable for actually getting at the videos, I ended up having a semi-public mirror of the videos and presentations.

I have most (all?) videos from 2009-2011.

Jeroen has a mirror at http://wipkip.nikhef.nl/events/BoostCon/ as well.

Thank you!

Marshall Clow

29 Oct 29 Oct

6:58 p.m.

On Oct 21, 2013, at 7:45 PM, Matt Calabrese wrote:

...

My concepts without concepts video also does not load. I tried contacting Marshall about it, but I never heard back. Certainly there must be backups somewhere so that they can be re-uploaded, or are they just lost now?

I finally got some time to work on this. I am uploading the 2011 videos to Youtube as I type, and when those are done, will start on the 2010 and 2009 videos. Sorry that this took so long. -- Marshall Marshall Clow Idio Software mailto:mclow.lists@gmail.com A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait). -- Yu Suzuki

Marshall Clow

31 Oct 31 Oct

2:42 p.m.

On Oct 21, 2013, at 11:00 AM, Michael Marcin wrote:

...

On 10/20/2013 11:32 PM, Dave Abrahams wrote:

...
True, but Matt Calabrese and Zach Lane proved that the ways we avoid O(N) instantiation in the existing MPL don't actually result in the expected speedups:

http://zao.se/~zao/boostcon/10/2010_presentations/mon/instantiations_must_go...

IMO real measurement is a requirement for progress in this area.

Strange the video for this presentation doesn't load for me.

http://blip.tv/boostcon/matt-calabrese-zachary-laine-instantiations-must-go-...

Is it working for other?

All the 2010 videos are now on youtube, on the "BoostCon" channel. Matt and Zach's is here: http://www.youtube.com/watch?v=x7UmrRzKAXU -- Marshall Marshall Clow Idio Software mailto:mclow.lists@gmail.com A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait). -- Yu Suzuki

Mathias Gaunard

19 Oct 19 Oct

4:20 p.m.

On 18/10/13 20:07, Louis Dionne wrote:

...

Hi,

I am currently working on a reimplementation of the MPL for C++11. Although the library is not complete yet, I would like to gauge interest for the project from the Boost community.

I wanted to wait until I have resolved some core issues before posting here, but reading the [modularization] thread made it unbearable to wait any longer :-). The library is available on GitHub: github.com/ldionne/mpl11.

If some people happen to be interested by the project and would like to get involved, please get in touch with me.

It's interesting if it compiles faster and uses less memory. I doubt that will be the case.

Abel Sinkovics

20 Oct 20 Oct

8:06 a.m.

Hi, On 2013-10-18 20:07, Louis Dionne wrote:

...

Hi,

I am currently working on a reimplementation of the MPL for C++11. Although the library is not complete yet, I would like to gauge interest for the project from the Boost community.

I wanted to wait until I have resolved some core issues before posting here, but reading the [modularization] thread made it unbearable to wait any longer :-). The library is available on GitHub: github.com/ldionne/mpl11.

If some people happen to be interested by the project and would like to get involved, please get in touch with me.

I'm developing template metaprogramming libraries and I have built a "base" library (Metamonad) based on the functional nature of template metaprogramming. It is mostly an extension of the current Boost.MPL. It adds algebraic data-types, pattern matching, let expressions, exception handling, etc. I have looked at a few things in your mpl11 (sample programs would be useful). I have noticed the following: 1) I can see that you have this syntax for if_: if_:: else_if:: ... else<ELSE> which is I think a great DSL for the if structure. Can this be used in a lambda expression? For example: sort< vector<....>, lambda< if_, ...>:: else<...>

...

I have built a DSL similar to your if_ for exception handling in template metaprograms: try_<...> ::catch_<....> ::catch_<....> but I could not make it work with lambdas (and a number of things I have built using similar techniques). So I had to change it to: try_<...., catch_<....>, catch_<....>

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There are a few things I would reconsider in a rewrite of MPL: 2) Metafunctions could support lazy evaluation. This makes it possible to use lazy evaluation strategy in metaprograms which makes the code easier to read. (Even for a factorial or Fibonacci there are huge differences) Currently MPL does not support this - for example the following works: boost::mpl::at_c< boost::mpl::push_front< boost::mpl::vector, void

...

::type, 1 ::type

but the following does not: boost::mpl::at_c< boost::mpl::push_front< boost::mpl::vector, void

...

, 1 ::type

The problem here is that at_c expects the caller to pass in push_front<...>::type instead of just push_front<...>. at_c could assume, that it has to use its argument's ::type instead of the argument directly. I have built a template called lazy<...> (see http://abel.web.elte.hu/mpllibs/metamonad/manual.html#lazy-metafunctions) as a workaround, but having the metafunctions of MPL supporting it would be more efficient. Also, I have reimplemented some of the things that are in Boost.MPL (eg. http://abel.web.elte.hu/mpllibs/metamonad/reference.html#pair, http://abel.web.elte.hu/mpllibs/metamonad/if_.html) in a way that they support lazy evaluation and use them instead of the original MPL versions. 3) Naming the lambda arguments _1, _2, ... has limitations. When you use lambda inside lambda, you can not refer to the arguments of the outer lambda from the inner one. I have built a lambda implementation where you can give the arguments names and use that instead of the MPL one. Here is an example for using it: using namespace mpllibs::metamonad; using namespace mpllibs::metamonad::name; boost::mpl::sort< boost::mpl::vector_c, lambda_c>

...

::type

You can read about my lambda implementation here: http://abel.web.elte.hu/mpllibs/metamonad/manual.html#lambda-expressions It is based on syntaxes and variables, which are described earlier on that page. This problem can also be solved by using De Brujin indices, but I find using names more readable so I decided to use this in my lambdas. Regards, Ábel

Louis Dionne

5:19 p.m.

Abel Sinkovics writes:

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1) I can see that you have this syntax for if_:

if_:: else_if:: ... else<ELSE>

which is I think a great DSL for the if structure. Can this be used in a lambda expression? For example:

Yes, you can use it with lambda expressions. This one took a lot of work to get working, though :-).

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2) Metafunctions could support lazy evaluation. This makes it possible to use lazy evaluation strategy in metaprograms which makes the code easier to read. (Even for a factorial or Fibonacci there are huge differences)

I'm aware of this and I'm currently searching for a general solution. I think I might have a very elegant solution for this; I'll provide an update when/if I get it working.

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3) Naming the lambda arguments _1, _2, ... has limitations. When you use lambda inside lambda, you can not refer to the arguments of the outer lambda from the inner one. I have built a lambda implementation where you can give the arguments names and use that instead of the MPL one.

Yes, I'm aware of this. I'm currently working on let-expressions. I'm also working on generalized placeholders: they can perform arbitrary computations and return sequences of types so that it's possible to use LambdaExpressions with variadic parameter packs. Example: mpl11::applympl11::_args, ParameterPack...> The obvious extension to this is to have placeholders like mpl11::_args which returns the arguments in the range [i, j). This is a work in progress.

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You can read about my lambda implementation here: http://abel.web.elte.hu/mpllibs/metamonad/manual.html#lambda-expressions It is based on syntaxes and variables, which are described earlier on that page.

I'll definitely have a look at this, thanks for the pointer!

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This problem can also be solved by using De Brujin indices, but I find using names more readable so I decided to use this in my lambdas.

Same here, I'm not planning on using de Bruijn indices. Regards, Louis Dionne

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Abel Sinkovics
Agustín K-ballo Bergé
Dave Abrahams
Eric Niebler
Larry Evans
Lars Viklund
Louis Dionne
Marshall Clow
Mathias Gaunard
Matt Calabrese
Michael Marcin
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