Re: [Boost-users] [review] Fusion review begins

From: Joel de Guzman (joel_at_[hidden])
Date: 2006-05-04 11:33:05

Andy Little wrote:
> "Pedro Lamarão" wrote
> 
>> I would like to see some "motivating example", like this one from the
>> MPL documentation:
>>
>> http://www.boost.org/libs/mpl/doc/tutorial/tutorial-metafunctions.html
> 
> If the above is a motivating example for mpl, then I dont think it is the best
> one. The problem is even highlighted in:
> 
> http://www.boost.org/libs/mpl/doc/tutorial/implementing.html
> 
> At the point in the text where it says;
> 
> "
> However, if we try to write:
> 
> quantity<float,force> f = m * a;
> we'll run into a little problem.
> 
> ".
> 
> The problem is that mpl doesnt work as the (naive?) user expects, but worse the
> actual type of the quantity resulting from a multiplication isnt exactly
> defined. All that is known about it is that its dimension is comprised of "some
> sequence". In practise I found this, which was described as a convenience
> feature actually caused a great deal more work in the implementation. It also
> causes problems with trying to figure what to register with Boost.Typeof and I
> suspect that it would continue to throw up similar problems whenever the type is
> required to be known. The fact is that an interface must have exactly
> predictable types. The interface in this case ( The visible part. e.g for use in
> function signatures etc) is in the template signature of the class. IMO mpl isnt
> the best way to implement the dimension of a quantity for this reason.
> 
> I am sorry to go on about this, but I am expecting to have a boost review of my
> pqs library shortly and the issue is bound to come up. I suspect that the fact
> that it doesnt use mpl will be seen by the naive ( in the same sense as used
> above) user as a shortcoming of pqs. I believe that for this useage the
> shortcoming is rather in mpl for this particular useage, which  the above
> example demonstrates.

So MPL is not sequence preserving after all. transform merely
returns a sequence with the same properties as the original. This
makes perfect sense. In Fusion, algorithms are not sequence preserving
by design. However, we do have conversion functions/metafunctions
to get a *true* vector/list/set/map from an arbitrary sequence.
I think that's all that MPL needs to provide to solve the problem
you mention.

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net