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

main :: IO ()
main = do
  print (Main.reverse ([1,2,3]::[Int]))
  print (hd []::[String])

reverse :: [a] -> [a]
reverse = reverseAcc []
  where
    reverseAcc = foldl (flip (:))