On Sun, Aug 17, 2008 at 11:27 AM, Yitzchak Gale
There, Twan van Laarhoven designs the implementation of the permutations function that is slated to be included in GHC 6.10.
I look forward to Twan's design. I found the Haskell 1.3 definition.
-- permutations xs returns the list of all permutations of xs. -- e.g., permutations "abc" == ["abc","bac","bca","acb","cab","cba"] permutations :: [a] -> [[a]] permutations [] = [[]] permutations (x:xs) = [zs | ys <- permutations xs, zs <- interleave x ys ] where interleave :: a -> [a] -> [[a]] interleave x [] = [[x]] interleave x (y:ys) = [x:y:ys] ++ map (y:) (interleave x ys)
I like the use of list comprehension, but I was surprised the last line was not:
interleave x (y:ys) = (x:y:ys) : map (y:) (interleave x ys)
John