variation of function choices

gistfile1.txt

import Debug.Trace

subs :: [a] -> [[a]]
subs []     = [[]]
subs (x:xs) = yss ++ map (x:) yss
  where yss = subs xs

interleave :: a -> [a] -> [[a]]
interleave x []     = [[x]]
interleave x (y:ys) = (x:y:ys) : map (y:) (interleave x ys)

perms :: [a] -> [[a]]
perms []     = [[]]
perms (x:xs) = trace "now it terminates" concat (map (interleave x) (perms xs))

choices :: [a] -> [[a]]
choices = concat . map perms . subs

choices' :: [a] -> [[a]]
choices' xs = [ list | yss <- subs xs,
                       list <- perms yss ]