[boost] fusion concepts

Subject: [boost] fusion concepts
From: Robert Ramey (ramey_at_[hidden])
Date: 2011-06-13 17:41:27

I'm trying to implement concept checking for at least parts of the 
boost::fusion library.  My motivations are several:

a) I'm interested in making another library/program based on fusion and I 
believe that having implemented concepts will make this job easier.

b) I'm interested in the concept checking concept and this is way to 
pracitice.

Attached is the current state of my efforts.  I have a few problems with 
this. I think most of these are specific to fusion but maybe not.

a)
line 29 of test_misc.cpp invokes concept checking on a fusion vector.

lines 28 & 29 of fusion/concepts.hpp are intended to verify that 
result_of::next<I> have been defined for the for the forward iterator 
associated with the fusion vector.  The BOOST_CONCEPT_ASSERT fails with 
element 6 from the sequence.

I conclude that the recurrsive invocation of concept checking on the result 
of next<I> is crapping out at the end of the sequence.  This makes sense in 
a practical way - BUT it would voilate the requirements on the return type 
from result_of::end<vector>.  Sooooo

i) am I right about this?
ii) if so how should it be addressed? fixing the concept implementation 
(how?) would "hide" the violation.  Seems to be that the documentation on 
forward sequence/iterator should be tweaked and my concept checking code 
should be fixed up to match.  Any thoughts on this?

b)
lines 34-38 illustrate a different problem.  I can't figure out what 
distance(i,j) is supposed to return. I would think that 
result_of::distance(i, j) should be a signed integer of some sort.  So I'm 
not understanding why this assertion traps.  I not sure if it's a side 
effect of the problem cited in a) above.

c)
In another file not included here (test_concepts.hpp).  I've attempted to 
implement archtypes for these types.  I managed to do it, but I'm not all 
that confident that i did it right.  After doing this, I came to understand 
the fusion extension mechanism.  I could re-implement the archtypes with 
this and I'm confident that it would be "correct" BUT I'm wonder if this 
just wouldn't be a circular conclusion.  That is would doing this really be 
testing the concepts or would generating archetypes in this way just 
guarentee that things were in some sense internally consistent but not 
necessarily reflection the concepts as stated in the documentation.

Any comments/help would be appreciated.

Robert Ramey 

begin 666 concepts.hpp
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end

begin 666 test_misc.cpp
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end