Question

我的代码中包含以下内容：

  where
    launch :: MonadIO m => m (Maybe Text)
    launch = do
        line <- launchLine
        return $ lineToText <$> line

    launchLine :: MonadIO m => m (Maybe Line)
    launchLine = fold (inproc "juke" ["launch", "--profile", "jukeplay", pack baseImage] mempty) $
        Fold selectLaunchName Nothing id

上面的行工作正常。我的问题（和问题）是我想在此启动行之前执行另一个命令，所以可能是这样的：

  where
    launch :: MonadIO m => m (Maybe Text)
    launch = do
        line <- launchLine
        return $ lineToText <$> line

    launchLine :: MonadIO m => m (Maybe Line)
    launchLine = fold (inproc "juke" ["image", "copy", "jukebox:" <> pack baseImage, "local:", "--copy-aliases"] mempty) $
        Fold selectLaunchName Nothing id

    launchLine :: MonadIO m => m (Maybe Line)
    launchLine = fold (inproc "juke" ["launch", "--profile", "jukeplay", pack baseImage] mempty) $
        Fold selectLaunchName Nothing id

这显然行不通。我该怎么办？

我需要在执行“ juke启动..”之前完成此“ juke映像副本”

预先感谢您的帮助

Answer 1

您可以将单子动作与(>>)组合在一起：

(>>) :: Monad m => m a -> m b -> m b

或使用do语法，将其简化为(>>)：

\act1 act2 -> do {act1; act2} :: Monad m => m a -> m b -> m b

例如：

launch :: MonadIO m => m (Maybe Text)
launch = do
    preLaunchLine
    line <- launchLine
    return $ lineToText <$> line

preLaunchLine :: MonadIO m => m (Maybe Line)
preLaunchLine = fold (inproc "juke" ["image", "copy", "jukebox:" <> pack baseImage, "local:", "--copy-aliases"] mempty) $
    Fold selectLaunchName Nothing id

launchLine :: MonadIO m => m (Maybe Line)
launchLine = fold (inproc "juke" ["launch", "--profile", "jukeplay", pack baseImage] mempty) $
    Fold selectLaunchName Nothing id

在Haskell中执行多个命令的基本问题

1 个答案: