Re: [boost] typelists: MPL tutorial + partial review

From: David A. Greene (greened_at_[hidden])
Date: 2001-12-11 15:18:06

David Abrahams wrote:

 > I think the "quoted" name hurts the understandability of the idiom for
 > anyone but a lisp programmer. There are technical reasons apart from
 > broken compilers to prefer this idiom, but I'll let Aleksey address
 > them.

I'm certainly not a Lisp programmer, so that absolutely could
be the case.  As I said, I'm approaching this from a novice
standpoint.  I'd like to hear about the other technical reasons.
I have _Generative_Programming_ but haven't got to that point
in the book yet.  :)

 >>By this point I was almost totally dumbfounded.  That's a lot of
 >>ugly code to do a conceptually very simple thing. Andrei is right.
 >>Template metaprogramming is more naturally expressed using recursion
 >>because templates define a functional language.  I'm reminded of the
 >>ugly transformations from recursion to iteration, or explicity
 >>coding a parser value stack rather than using inherited and
 >>synthesized attributes.  Yes, it's possible, but why do it when it
 >>is often much simpler to let things "fall out" naturally?  Less
 >>code, fewer bugs.
 >
 > I think you're drawing conclusions from things unrelated to the
 > conclusions themselves.

Err..urm...huh?  What things?

 > People have the same complaints about the STL. We all know that the
 > use of lots of composers and binders can make code hard to read, and
 > many of us wouldn't write that way for runtime code.

That's exactly what I was getting at, though I didn't state it
explicitly.  We don't like STL code that looks messy, so why
should MPL code be messy?

 > Let's see if we can rewrite Mat's example more simply, if in more
 > characters:

Good, this is a useful exercise.

 > // Now we can define GenScatterHierarchy
 >
 >    template <class TList, class Transformation>
 >    struct GenScatterHierarchy {
 >         typedef boost::mpl::for_each<
 >             TList, EmptyType, inherit_transformed<Transformation>
 >         >::state type;
 >    };

This is certainly much more readable.  "Aggregating" the
pieces-parts into inherit_transformed improved my understanding
greatly.  Still, a looping construct to generate trees seems
a little "out of place."  For me, recursion is just much more
natural.  Perhaps this isn't true for most.

 > I find the analogy between the use of the STL and the use of MPL makes
 > things much easier.

I think it is somewhat deceptive.  The names and arguments look
familiar, but the STL algorithms work smoothly because iteration
is a natural interface to containers/sequences.  I suppose one
could argue that a typelist is just another sequence, which it
certainly is, but due to the limited interface we have for
C++ metaprogramming (no vararg templates, etc.), recursion
is a natural way to represent them.  The simple observation
that (partial) specialization is used to provide a terminator/
sentinel points to the natural use of recursion in
metaprogramming.

The STL algorithms break down somewhat with the associative
containers.  Their recursive structure makes std::find (as
an example) a non-ideal tool.  Thus we have the member find
which (in my implementation) does not mention iterators until
setting up the return value (pity that it is not implemented
with recursion :)).  Use the right tool for the job, as
always.  I'm not arguing that iteration  isn't useful for
metaprogramming.  I just take issue with it being presented
as the universal way of doing things.

This is all just IMHO, of course.  These are first impressions.

 > Composers and binders become really useful when you have lots of
 > little metafunctions lying around for doing important jobs in your
 > domain. At that point, you want to be able to re-use and combine
 > them.

Of course composers and binders are useful.  But they have their
limitations.  Perhaps a few more typedefs in Mat's example would
have helped.

 > Of course, a compile-time lambda facility (which has already been
 > started in MPL) could be much nicer.

It's tough to disagree with that!  :)

                                 -Dave

-- 
"Some little people have music in them, but Fats, he was all music,
   and you know how big he was."  --  James P. Johnson