haskell - 如何从这段代码中删除 `case of`？

Question

如何重写以下代码，使其:

1. 使用较少的字符
2. 最多包含一个案例......

-- parseSQL :: String -> Either ParseError SQL
-- evalSQL :: SQL -> IO (Either EvalError Table)
-- prettyPrintTable :: Table -> IO ()
-- ParseError, EvalError and Table are instances of Show

evalAndPrint :: String -> IO ()
evalAndPrint x =
    case parseSQL x of
      (Left parseErr) ->
        print parseErr
      (Right sql) -> do
        result <- evalSQL sql
        case result of
          (Left err) ->
            print err
          (Right table) -> do
            prettyPrintTable table
            putStrLn $ "(" ++ show (length table) ++ " lines)\n"

Answer 1

目前，我们假设您已经概括了 parseSQL和evalSQL具有这些类型的函数(稍后我们将看到如何将您的专用实现转换为通用实现，即使您无权访问其源代码):

parseSQL :: MonadError ParseError m => String -> m SQL
evalSQL :: (MonadError EvalError m, MonadIO m) => SQL -> m Table

然后我们可以这样写:

-- if we were really doing this the mtl way, we'd introduce a new
-- type class for changing error types instead of specializing to
-- ExceptT, but that's another answer
evalAndThrow :: String -> ExceptT String IO ()
evalAndThrow s = do
    sql   <- withExceptT show (parseSQL s)
    table <- withExceptT show (evalSQL sql)
    liftIO $ prettyPrintTable table
    liftIO . putStrLn $ "(" ++ show (length table) ++ " lines)\n"

顶层函数可以是这样的

evalAndPrint s = do
    v <- runExceptT (evalAndThrow s)
    case v of
        Left err -> putStrLn err
        Right _  -> return ()

这里有一些将现有函数转换为 mtl 风格的多态版本的技巧。您可以直接更改它们的源，也可以使用如下组合器创建适配器:

-- this is a generally useful combinator
liftEither :: MonadError e m => Either e a -> m a
liftEither = either throwError return

-- this is a combinator specific to your question
liftIOEither :: (MonadError e m, MonadIO m) => IO (Either e a) -> m a
liftIOEither = join . liftIO . liftM liftEither

当然还有ExceptT :: IO (Either e a) -> ExceptT e IO a ; ExceptT . evalSQL并不像 liftIOEither . evalSQL 那样多态，但由于我们在 ExceptT 使用它在这种情况下，键入它可能并不重要。