On 11/23/08 13:52, Luke Palmer wrote:
2008/11/23 Larry Evans <[EMAIL PROTECTED]>:
http://www.muitovar.com/monad/moncow.xhtml#list
contains a cross function which calculates the cross product
of two lists. That attached does the same but then
used cross on 3 lists. Naturally, I thought use of
fold could generalize that to n lists; however,
I'm getting error:
You should try writing this yourself, it would be a good exercise. To
begin with, you can mimic the structure of cross in that tutorial, but
make it recursive. After you have a recursive version, you might try
switching to fold or foldM.
The type of the function will not involve tuples, since they can be
arbitrary length (dynamic-length tuples are not supported in Haskell;
we use lists for that).
cross :: [[a]] -> [[a]]
However, list's contain elements all of the same type. What the
following boost post:
http://thread.gmane.org/gmane.comp.lib.boost.devel/182797/focus=182915
demonstrated was, AFAICT, the c++ template metaprogramming counterpart
to the moncow haskell cross. Now, AFAICT, the boost vault directory:
http://www.boostpro.com/vault/index.php?PHPSESSID=ab51206c9d980155d142f5bcef8e00ee&direction=0&order=&directory=Template%20Metaprogramming
in the cross_nproduct_view_test.zip, contains what I'm looking for
in haskell. I'm guessing that:
template<class Row, class Column>struct row_view;
corresponds to the haskell tuple type
(row,column)
I'm trying to confirm that by printing out the typename in
a formated form, but I'm having trouble doing that at the
moment:
http://preview.tinyurl.com/66x4nx
Is there some version of haskell, maybe template haskell,
that can do that, i.e. instead of:
cross::[[a]] -> [[a]]
have:
crossn::[a0]->[a1]->...->[an] -> [(a0,a1,...,an)]
?
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