漫画「ムーたち」で、ムー夫のママがドアノブの前で動かなくなってしまうシーンをHaskellで書くとたぶんこう

mu.hs

module Main (main) where

main :: IO ()
main = do
  let fingersPermutation = permutation fingers
  mapM_ (putStrLn . show) fingersPermutation
  putStrLn $ show $ length fingersPermutation

fingers :: [Int]
fingers = [1..5]

permutation :: Eq a => [a] -> [[a]]
permutation fs = go [] fs
  where
    go path []   = [reverse path]
    go path rest = rest >>= \f -> go (f : path) [ x | x <- rest, x /= f]