do-block-after-a-guard.hs

greet :: Maybe String -> IO ()
greet name
  | Just n <- name
  , not (null n)
  = do putStrLn "Hello,"
       putStrLn n
  | otherwise = pure ()

describe :: Maybe Int -> IO ()
describe x = case x of
  Just n
    | n > 0
    -> do print n
          print (n * 2)
  _ -> pure ()

classify :: Int -> String
classify n
  | n > 0
  = case n of
      1 -> "one"
      _ -> "many"
  | otherwise = "none"

withRemark :: Int -> IO ()
withRemark n
  | n > 0    -- only positive ones
  = do print n
       print n
  | otherwise = pure ()

withNote :: Int -> IO ()
withNote n
  | n > 0
  -- only positive ones
  = do print n
       print n
  | otherwise = pure ()

withTwoRemarks :: Int -> IO ()
withTwoRemarks n
  | n > 0    -- only positive ones
  = do       -- twice
       print n
       print n
  | otherwise = pure ()