[Haskell-cafe] Re: howto tuple fold to do n-ary cross product?

[Larry Evans]

On 11/30/08 12:27, Luke Palmer wrote:
> On Sun, Nov 30, 2008 at 11:04 AM, Larry Evans <cppljevans at suddenlink.net> wrote:
>> The following post:
>>
>>  http://thread.gmane.org/gmane.comp.lib.boost.devel/182797
>>
>> shows at least one person that would find it useful, at least in
>> c++.  Of course maybe it would be less useful in haskell.
> 
> The line:
> 
>   typedef boost::mpl::vector <T1Variants, T2Variants, T3Variants> TT;
> 
> Has the number of lists hard-coded as 3, and does not abstract over
> it.  This corresponds to the "3" in "liftA3", or the number of <*>s in
> the expression.
> 
> Abstracting over the number and types of arguments is something
> neither C++ nor Haskell is very good at.  But in order to be able to
> do any abstraction using such a variable-argument function, the type
> systems of these languages would have to increase in complexity by
> quite a lot.
> 
> Luke

True, but if you look at the cross_nproduct_view template:
{--cut here--
   template
   < class Domains
   >
   struct
cross_nproduct_view
   : fold
     < Domains
     , cross_product_one
     , cross_product_view<arg<2>,arg<1> >
     >::type
{
};

}--cut here--

You'll see Domains can be an mpl::vector of any
length. The cross_nproduct_view_test.cpp tests
with a 3 element Domains:

         typedef
       mpl::vector
       < mpl::range_c<int,0,4>
       , mpl::range_c<int,100,103>
       , mpl::range_c<int,2000,2002>
       >
     domains;


The cross_nproduct_view template  and test driver
are found in the cross_nproduct_view.zip file here:

   http://preview.tinyurl.com/5ar9g4