[Haskell-cafe] Re: Generic permutations
Cetin Sert
cetin.sert at gmail.com
Sat Jan 26 17:17:05 EST 2008
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 <ryani.spam at gmail.com> 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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