On Sun, Feb 13, 2005 at 08:58:29AM -0500, David Roundy wrote:
I've been working on a typeclass that derives from MonadPlus which will encapsulate certain kinds of IO. With MonadPlus, you can write monadic code with exceptions and everything that may not be executed in the IO monad. You just use fail to throw exceptions, and mplus to catch them.
class MonadPlus m => ReadableDirectory m where mInCurrentDirectory :: FilePath -> m a -> m a mGetDirectoryContents :: m [FilePath] mReadFilePS :: FilePath -> m PackedString mReadFilePSs :: FilePath -> m [PackedString] mReadFilePSs f = linesPS `liftM` mReadFilePS f
One instance of this class is IO, but I can also have instances for in-memory data structures (outside the IO monad) or (or example) for reading a tarball from disk--which would be a monad that acts within the IO monad.
According to http://www.haskell.org/hawiki/MonadPlus (see also the recent thread about MonadPlus) a MonadPlus instance should obey m >> mzero === mzero, which IO doesn't. IOW, the MonadPlus instance for IO (defined in Control.Monad.Error) probably shouldn't be there. Groeten, Remi -- Nobody can be exactly like me. Even I have trouble doing it.