[Haskell-cafe] Review request for my permutations implementation

Hi All, I've written this piece of code to do permutations - perms :: String -> [String] perms []= [] perms (x:[])= [[x]] perms (x:xs)= concat (f [x] (perms xs)) spread :: String -> String -> [String] -- interpolate first string at various positions of second string spread str1 str2 = _spread str1 str2 (length str2) where _spread str1 str2 0= [str1 ++ str2] _spread str1 str2 n= [(take n str2) ++ str1 ++ (drop n str2)] ++ (_spread str1 str2 (n-1)) f xs = map (spread xs) The number of outcomes seem to indicate that correctness of the algo .. however, I'd be very obliged if I could get some feedback on the Haskellness etc of this ... also any performance pointers ... Regards, Kashyap