[boost] Oh exhalted MPL gurus...

From: Matthias Schabel (boost_at_[hidden])
Date: 2003-11-08 15:26:40

I've been trying to teach myself some MPL, but have found the  
documentation pretty difficult to follow in places - I  think a bunch  
of practical examples, preferably with STL analogies, would be  
tremendously helpful, especially for those of us who've spent our  
entire lives trapped in the imperative programming ghetto and think  
LISP is a funny way of pronouncing 's'...  A couple of particular  
questions :

How does one implement append? That is, given

typedef mpl::list<t1,t2,t3> l1;
typedef mpl::list<t4,t5> l2;

typedef append<l1,l2> --> mpl::list<t1,t2,t3,t4,t5>

I've tried things like

typedef typename mpl::iter_fold<S2,S1,push_back<_1,deref<_2> > >::type   
a;

(and various permutations), but can't get it to work and suspect that  
I'm missing something fundamental.  Any suggestions?

The more complex problem which I'm trying to resolve is to accomplish  
something like this :

template<class T1,class T2>
struct pair_type
{
        typedef T1	type1;
        typedef T2	type2;
};

template<class T>
struct get_type1
{
        typedef typename T::type1	type;
};

template<class T>
struct get_type1
{
        typedef typename T::type2	type;
};

template<class T1,class T2>
struct equality_op
{
        typedef typename mpl::equal<get_type1<T1>,get_type1<T2> >::type  type;
};

template<class T1,class T2>
struct combine_op
{
        BOOST_STATIC_ASSERT((boost::is_same<get_type1<T1>:: 
type,get_type1<T2>::type>::value == true));

        typedef typename  
pair_type<get_type1<T1>::type,pair_type<get_type2<T1>:: 
type,get_type2<T2>::type> > type;
};

template<class S1>
struct collapse
{
        typedef typename collapsed_type<S1>::type	type;
};

what I would like is for collapsed_type<S1> to return a sequence  
composed of all the unique types in S1, with uniqueness defined by
equality_op, concatenated, with combine_op applied to all duplicate  
types.  For example

typedef  
list<pair_type<T1,V1>,pair_type<T2,V2>,pair_type<T1,V3>,pair_type<T4,V2> 
  > S1;

collapse<S1> --> list<combine_op<pair_type<T1,V1>,pair_type<T1,V3>  
 >::type, pair_type<T2,V2>,pair_type<T4,V2> >

Matthias

------------------------------------------------------------------------ 
---------------------------
Matthias Schabel, Ph.D.                           	
Utah Center for Advanced Imaging Research
729 Arapeen Drive
Salt Lake City, UT  84108
801-587-9413 (work)
801-585-3592 (fax)
801-706-5760 (cell)
801-484-0811 (home)
mschabel at ucair med utah edu