failure.hs

either1 :: IO (Either String Int)
either1 = undefined

either2 :: IO (Either String Int)
either2 = undefined

main1 :: IO Int
main1 = do
  -- to be clear I do not want MonadFail here
  -- i.e. I don't want to use 
  -- Right a <- either1
  a <- fromRight <$> either1
  b <- fromRight <$> either2
  pure (a + b)

{-
    • Couldn't match type ‘Either a0 (Either String Int)
                           -> Either String Int’
                     with ‘Int’
      Expected type: IO Int
        Actual type: IO
                       (Either a0 (Either String Int) -> Either String Int)
    • In a stmt of a 'do' block: pure (a + b)
      In the expression:
        do a <- fromRight <$> either1
           b <- fromRight <$> either2
           pure (a + b)
      In an equation for ‘main1’:
          main1
            = do a <- fromRight <$> either1
                 b <- fromRight <$> either2
                 pure (a + b)
-}