Evan Laforge schrieb: > On Fri, Sep 11, 2009 at 6:10 AM, Edward Kmett <[email protected]> wrote: >> Unfortunately, the instance of Monad for Either a is somewhat misguided in >> Haskell. >> >> There is a spurious restraint that the Left value in your Either be a member >> of some Error class, which was brought about by the deletion of MonadZero >> from Haskell 98 (as part of the elimination of failure free patterns, and an >> attempted simplification of the language). > > I just tried it, and my own instance for Monad (Either String) (all > I'm really interested in anyway) can coexist with the one for Error e > => Monad (Either e). But indeed, with the presence of fail, you can't > write the general Monad (Either e) which "should" work. And it does > require FlexibleInstances. I suppose as long as flexible instances > aren't standard then even the String instance can't go in the Prelude.
Are you sure, that the instance cannot be defined in Haskell 98 using a helper type class for the Char datatype? http://www.haskell.org/haskellwiki/List_instance _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
