On Feb 2, 2008 1:14 PM, Yitzchak Gale wrote:

Alexander Dunlap wrote:

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For instances where an exception would be too intrusive, I don't see how it would be too hard to write a wrapper function

I wrote:

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In a library that does not have direct access to the IO monad, it would be not just hard - it would be impossible. That is because of type restrictions in the current versions of catch, block, and friends.

Judah Jacobson wrote:

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You haven't said why something like the following would not be sufficient:

readHistoryM :: MonadIO m => String -> m Bool readHistoryM file = liftIO $ do result <- try (readHistory file) return (result == Right ())

Because a library - other than readline itself - can't force its users to do that.

OK. Here's a simplified real-world example. Say you want to write a simple library that interfaces the text-to-speech facilities available on multiple platforms. To play nicely with programs written in a monadic style, the interface might be something like:

class MonadIO m => Speech m where sayText :: String -> m () runSpeech :: m a -> IO a

instance Speech SomeSpeechSystem where sayText t = ... runSpeech x = do liftIO startSomeSpeechSystem ret <- x liftIO stopSomeSpeechSystem return ret

Unfortunately, bracket is not available. So if x throws an uncaught IO exception, you may leave around zombies, database corruption, missiles armed for launch, etc.

Well, if your speech system doesn't arm any missiles, you may consider it a reasonable risk to use this library in programs that could throw an IO exception on some rare error condition.

But the proposal here is to raise the exception in a common situation that will definitely occur in regular usage. That may be fine in Java or Python, but it is a bad idea for IO exceptions in Haskell.

Alexander Dunlap wrote:

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So why couldn't you have a Utils.hs file that imports System.IO and provides the wrapper around readHistory? Then you can use the tryReadHistory function in your MonadIO-exporting module _exactly_ the same way as the original readHistory function.

You can. But if you are writing a library, your users might not.

Regards, Yitz

I don't understand. Why don't you compile the functions we are suggesting _into the library_? Alex