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 <[EMAIL PROTECTED]> wrote: > > 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 >
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