On Fri, Sep 11, 2009 at 6:10 AM, Edward Kmett
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.
In theory, this is good, because you can then define your own monad instance for Either without that constraint -- if you don't care to exploit the Error instance's ability to get fail to work. Or you want a real Either monad for tricks like Oleg's recent post on the constructive law of excluded middle, or to talk about apomorphisms as a generalized anamorphism, parameterized on the either monad so it can be symmetrical to paramorphisms. In practice, you link in someone's code that links in the MTL, and then you're stuck using the MTL instance anyways.
Right. I know there was some argument a while back, but I thought that position that instances are global period was pretty "official". At least it made sense to me. The more libraries you import the less control you have over the extent of what they may import. But I guess it wouldn't be haskell if every third person didn't have an idea for a better way to implement the mtl... I just want an exception with a message though!