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. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
