Fwd: Readline read_history and write_history addition

2 Feb 2008


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