Re: [boost] Proposed: a new sequence adapter for MPL (product_view)

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-03-12 18:05:27

Max Motovilov wrote:
>>> I am not sure if this approach can be generalized to an arbitrary number 
>>> of sequences. Perhaps it can be, by making the whole thing a recursive 
>>> template instead of 3 separate ones. I'd try it just to see which 
>>> version results in simpler code (gut feeling: the recursive template) 
>>> and in faster compilation times (gut feeling: my current implementation) 
>>> but customers and management are jointly snapping at my heels...
>>>       
>
> Against my better instincts I got drawn into this exercise... and I am 
> now officially stumped. Since we have this thread, perhaps one of the 
> MPL gurus would chime in. Here's the [entire!] code that uses the 
> accumulator idiom:
>
> ===================================================================
>
> template< typename S, typename A> struct product_view_ex :
> 	public if_<
> 		typename empty<S>::type,
> 		typename push_front< S, A >::type,
> 		transform_view<
> 			typename front<S>::type,
> 			product_view_ex<
> 				typename pop_front<S>::type,
> 		 		push_front<A,_1>
> 			>
> 		>
> 	>::type		
> {
> 	typedef product_view_ex type;
> };
>
> template< typename S> struct product_view :
> 	public if_<
> 		typename empty<S>::type,
> 		S,	
> 		product_view_ex<
> 			S,
> 			typename clear<S>::type
> 		>
> 	>::type
> {
> 	typedef product_view type;
> };
>
> ===================================================================
>
> The problem is that "if_< typename empty<S>::type ...." appears to 
> always evaluate its "else" clause. Or, in other words [I think] 
> empty<S>::type never evaluates as false_, even when the type of S is 
> clearly an empty sequence (vector0<>) according to compiler messages. I 
> am definitely missing something, but what?
>   

The problem is that even when the sequence is empty, the 
pop_front<S>::type will
still be evaluated.  The if_ is working correctly, it's just that the 
compiler doesn't
"know" about mpl::if_ and has to work out the template arguments even if 
they are not used.

The following is still not quite right but it's closer.

#include <boost/mpl/if.hpp>
#include <boost/mpl/range_c.hpp>
#include <boost/mpl/transform_view.hpp>
#include <boost/mpl/back_inserter.hpp>
#include <boost/mpl/push_front.hpp>
#include <boost/mpl/pop_front.hpp>
#include <boost/mpl/clear.hpp>
#include <boost/mpl/vector.hpp>
#include <boost/mpl/empty.hpp>
#include <boost/mpl/front.hpp>
#include <boost/mpl/copy.hpp>

using namespace boost::mpl;
namespace mpl = boost::mpl;

template< typename S, typename A>
struct product_view_ex;

template< typename S, typename A>
struct product_view_ex_impl :
    public
        transform_view<
            typename front<S>::type,
            product_view_ex<
                typename pop_front<S>::type,
                 push_front<A,_1>
            >
        >   
{
};

template< typename S, typename A>
struct product_view_ex :
    public eval_if<empty<S>,
        push_front< S, A >,
        product_view_ex_impl<S, A>
    >
{
};

template< typename S>
struct product_view :
    public if_<
        typename empty<S>::type,
        S,   
        product_view_ex<
            S,
            typename clear<S>::type
        >
    >::type
{
    typedef product_view type;
};

template<class T>
struct print {};

int main() {
    typedef product_view<mpl::vector<mpl::range_c<int, 0, 2>, 
mpl::range_c<int, 3, 5> > > view;
    typedef print<mpl::fold<view, vector0<>, mpl::copy<_2, 
mpl::back_inserter<_1> > >::type>::type type;
}

In Christ,
Steven Watanabe