Hi Matthias, On Fri, Oct 30, 2009 at 12:44:41PM +0100, Matthias Guedemann wrote:
Hi,
a friend of mine wanted to write function (in Perl) that creates all tuples of length 3 of the elements of a given list...
all3 ls = do a <- ls b <- ls c <- ls return [a,b,c]
Now I want to make it capable to create all combinations of length n instead of fixed length 3 (that's why list instead of tuple), but I don't really see how.
How about a recursive function like this: alln 1 ls = map (:[]) ls alln n ls = do a <- ls as <- alln (n-1) ls return (a:as) Note that `ls :: [t]` and `all (n-1) ls :: [[t]]` has different types but it's okay because both are in the list monad. Also, it can be done with list comprehensions: alln' 1 ls = [[a] | a<-ls] alln' n ls = [a:as | a<-ls, as<-alln' (n-1) ls] Sincerely, jan. -- Heriot-Watt University is a Scottish charity registered under charity number SC000278.