I knew there was a tighter way to write the derangement code for my assignment, but my Haskell was really rusty and what I had was fast, so I gave up. This morning it came to me – now I have 5 lines instead of 8. I do not have any idea how one would express this in pseudo code, though and it’s probably really quite difficult to read if you’re not familiar with Haskell.
derange1 :: Int -> [[Int]] derange1 x = perm 0 (makeList x) makeList :: Int -> [Int] makeList 0 =  makeList n = makeList (n-1) ++ [n] perm :: Int -> [Int] -> [[Int]] perm _  = [] perm x list = concat [map (e:) (perm (x+1) (delete e list)) | e <- list, e /= (x+1)]
It’s sometimes surprising how having less can 1) be more work and 2) feel like such an achievement.