Fwd: Readline read_history and write_history addition

[Alexander Dunlap]

On Feb 2, 2008 1:14 PM, Yitzchak Gale <gale at sefer.org> wrote:
> Alexander Dunlap wrote:
> >>> 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:
> >>  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:
> > 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:
> > 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