bsc-leon-vatthauer/slides/code-examples/example.hs

hd :: [a] -> a
hd [] = error "empty list"
hd (x : _) = x

main :: IO ()
main = print (hd []::[String])

reverse :: [a] -> [a]
reverse l = rev l []
  where
    rev []     a = a
    rev (x:xs) a = rev xs (x:a)