power.hs

import System.Random

power :: [a] -> [[a]]
power xs = let
    go [] ys = ys
    go (x:xs) ys = go xs $ map (\a -> a ++ [x]) ys ++ ys
  in
    go xs [[]]

pickRand :: RandomGen r => r -> [a] -> [a]
pickRand gen [] = []
pickRand gen (x:xs) = let
    r :: Double
    (r, genN) = random gen
  in
    if r < 0.5
      then pickRand genN xs
      else x : (pickRand genN xs)

printStrs :: [[Char]] -> IO ()
printStrs xs = let
    pr :: [[Char]] -> IO ()
    pr [] = return ()
    pr (x:[]) = putStr $ "\"" ++ x ++ "\""
    pr (x:xs) = do
      _ <- putStr $ "\"" ++ x ++ "\", "
      pr xs
  in do
    _ <- putStr "["
    _ <- pr xs
    putStrLn "]"

main :: IO ()
main = do
  gen <- newStdGen
  let x = pickRand gen $ power "寝る" in do
    _ <- printStrs x
    putStrLn $ case (length x) of
      4 -> "寝る冪"
      _ -> "寝る冪でない"