On Fri, Oct 12, 2012 at 3:05 PM, Doug Gregor <doug.gregor@gmail.com> wrote:
On Fri, Oct 12, 2012 at 6:25 AM, Andrew Sutton <asutton.list@gmail.com> wrote:
Right now I see two ways forward:
1. I implement N3351 in Boost.Contract and Matt implements N2914 in Boost.Generic. 2. Or, I help Matt implementing N2914 in Boost.Generic (and Boost.Contract's requires clause will use concepts defined using Boost.Generic).
Then we all use the lib(s) to experiment with concepts before (re)proposing concepts (and hopefully contracts) for standardization in C++1x.
Experimenting is great. This is why I have Origin (https://code.google.com/p/origin/). I've been experimenting with concepts-as-a-library in various forms since 2009, and it only gets you so far. It's a very helpful if you want to develop a first pass at concepts for a library, and sometimes it pays off if you need to reason about some language feature interactions.
This is an extremely important point: emulating the concepts language feature with a library has its limits. Most of the hard problems with concepts, including the hard problems of making those concepts that we right actually model what we want---involve the type checking of template definitions. That type checking can be simulated with archetypes, but it's very hard to write archetypes that are as picky as what a compiler would come up with. That means that the concepts we write can't actually be validated against implementations, so it's hard to have any confidence in those concepts.
From the standardization perspective, we'll make zero progress until someone gets working on a real implementation. Just having a concept parser + archetype generator (which then instantiates template definitions based on those archetypes) would be a huge win.
Incidentally, it would be interesting to see if we one can implement a truly complete archetype generator. The following paper, on the Caramel system, hints at a few issues with this alternative: http://www.osl.iu.edu/publications/prints/2001/TMPW01:_Willcock.pdf I'd be interested in learning about experiences using this system (or extensions of it) on various generic libraries. Any idea? Thanks, -- Larisse.
- Doug
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