続・順列列挙関数 ref: http://qiita.com/nobsun/items/232b7e61d4d0905522af

file0.txt

perms :: [a] -> [[a]]

file1.txt

perms [] = [[]]

file2.txt

perms xs@(_:_) = concat (zipWith prefix xs (map perms (shorts xs)))

file3.txt

perms :: [a] -> [[a]]
perms []       = [[]]
perms xs@(_:_) = concat (zipWiht prefix xs (map perms (shorts xs)))

prefix :: a -> [[a]] -> [[a]]
prefix = undefined

shorts :: [a] -> [[a]]
shorts = undefined

file4.txt

prefix x = map (x:)

file5.txt

shorts xs = unfoldr phi ([],xs)
  where
    phi (_,[])    = Nothing
    phi (ys,z:zs) = Just (ys++zs,(ys++[z],zs))

perm2.hs

perms :: [a] -> [[a]]
perms []       = [[]]
perms xs@(_:_) = concat (zipWiht prefix xs (map perms (shorts xs)))

prefix :: a -> [[a]] -> [[a]]
prefix x = map (x:)

shorts :: [a] -> [[a]]
shorts xs = unfoldr phi ([],xs)
  where
    phi (_,[])    = Nothing
    phi (ys,z:zs) = Just (ys++zs,(ys++[z],zs))