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

30 Nov 2008

      On 11/30/08 12:27, Luke Palmer wrote:
...
On Sun, Nov 30, 2008 at 11:04 AM, Larry Evans  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  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<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
       , mpl::range_c
       , mpl::range_c
       >
     domains;

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

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