順列列挙関数（素朴な実装） ref: http://qiita.com/nobsun/items/babd28fe81ba3b9f304f

file0.txt

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

file1.txt

perms [] = [[]]

file2.txt

perms (x:xs) = concatMap (insert x) (perms xs)

file3.txt

perms :: [a] -> [[a]]
perms []     = [[]]
perms (x:xs) = concatMap (insert x) (perms xs)

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

file4.txt

insert x xs = map (ins x) (divids xs)

file5.txt

perms :: [a] -> [[a]]
perms []     = [[]]
perms (x:xs) = concatMap (insert x) (perms xs)

insert :: a -> [a] -> [[a]]
insert = map (ins x) (divids xs)

ins :: a -> ([a],[a]) -> [a]
ins x (ys,zs) = ys ++ [x] ++ zs

divids :: [a] -> [([a],[a])]
divids = undefined

file6.txt

divids []     = [([],[])]

file7.txt

divids (x:xs) = ([],x:xs) : [ (x:ys, zs) | (ys,zs) <- divids xs ]

file8.txt

perms :: [a] -> [[a]]
perms []     = [[]]
perms (x:xs) = concatMap (insert x) (perms xs)

insert :: a -> [a] -> [[a]]
insert x xs = map (ins x) (divids xs)

ins :: a -> ([a],[a]) -> [a]
ins x (ys,zs) = ys ++ x : zs

divids :: [a] -> [([a],[a])]
divids [] = [([],[])]
divids xxs@(x:xs) = ([],xxs) : [ (x:ys,zs) | (ys,zs) <- divids xs ]