Thank you very much ^_^.
What would be a mathematically correct and understandable name for what we
call 'pms' here?
And in what module do foldM, combine, replicate, rest, liftM and so on
reside? How can I import them? o_O
-- Cetin Sert
On 26/01/2008, Ryan Ingram
When you say permuations, I think of reorderings of a list, for example:
permutations [1,2,3] = [ [1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], [3,2,1] ]
Here's an implementation:
-- split [1,2,3] => [ -- ( 1, [2,3] ), -- ( 2, [1,3] ), -- ( 3, [1,2] ) ] split :: [a] -> [(a, [a])] split [] = error "split: empty list" split [a] = [(a, [])] split (a:as) = (a, as) : map prefix (split as) where prefix (x, xs) = (x, a : xs)
permutations :: [a] -> [[a]] permutations [] = return [] permutations xs = do (first, rest) <- split xs rest' <- permutations rest return (first : rest')
The problem you solved can be solved much more elegantly:
pms : [a] -> Int -> [[a]] pms xs n = foldM combine [] (replicate n xs) where combine rest as = liftM (:rest) as
or, for the unreadable version: pms xs n = foldM (map . flip (:)) [] $ replicate n xs
(note that, in the list monad, liftM = map).
-- ryan