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.
Thanks. The recursive method worked with:
-{--cross.hs--
crossr::[[a]] -> [[a]]
crossr lls = case lls of
{ [] -> []
; [hd] -> map return hd
; hd:tail -> concat (map (\h ->map (\t -> h:t) (crossr tail)) hd)
}
-}--cross.hs--
However, I'm not sure fold will work because fold (or rather foldr1)
from:
http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#12
has signature:
(a->a->a)->[a]->a
and in the cross product case, a is [a1]; so, the signature would be
([a1]->[a1]->[a1]->[[a1]]->[a1]
but what's needed as the final result is [[a1]].
Am I missing something?
-Larry
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