Re: [Haskell-cafe] instance Monad (Either String) in Prelude or Data.Either?

You could, but then you need overlapping instances to define the one in Control.Monad.Error. -Edward Kmett On Sat, Sep 12, 2009 at 4:26 PM, Henning Thielemann < lemming@henning-thielemann.de> wrote:

Evan Laforge schrieb:

...

On Fri, Sep 11, 2009 at 6:10 AM, Edward Kmett 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