Demo of Either as a Monad for annotated return types.

test.hs

meow = Right "Meow"

greeting = Right "Hi"

sub_meow input = do
  value <- integer_meow input
  if value > 7 then
    Right "Big Meow"
  else
    Left "Too Small"

integer_meow 42 = Right 10
integer_meow value = Left "Wrong input"

someThing = do
  meow
  greeting
  sub_meow 42
  sub_meow 17 -- Remove this to make things succeed

main = case someThing of
  (Left error) -> putStrLn ("Woah baby, error: " ++ error)
  (Right result) -> putStrLn ("Success! " ++ result)