[Haskell-beginners] list monad question
jakubuv at gmail.com
Fri Oct 30 08:23:27 EDT 2009
On Fri, Oct 30, 2009 at 12:44:41PM +0100, Matthias Guedemann wrote:
> 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
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]
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