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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

(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
type,get_type1<T2>::type>::value == true));

        typedef typename
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

> S1;

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


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

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