Readline read_history and write_history addition

[Judah Jacobson]

On Sat, Feb 2, 2008 at 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.

I've already demonstrated how a library writer can solve that problem in:
http://www.haskell.org/pipermail/libraries/2008-January/009034.html

-Judah