On Thu, Jul 01, 2010 at 10:27:39AM +0200, Claus Reinke wrote:
Anyway, since I want to use Left for fail messages (at least sometimes) and you want to use Left for something else (at least hypothetically), it does not seem to be a good idea to fix one Monad instance in base that is sure to disappoint one of us, right? Once people start importing Control.Monad.Instances in their packages, as they will when mtl and co start depending on that, one of us is stuck with an Either Monad they cannot use.
It will happen long before that: Control.Monad.Instances is imported directly by Control.Applicative and Control.Monad.Fix, and hence by Control.Arrow, Data.Foldable, Data.Traversable, the ST modules and the modules in containers. Anyone straying outside of haskell98 will probably get it. With Haskell as it is, local instances are unworkable. We do have to decide on a single instance (unless no instances at all are possible).
[John Meacham wrote:]
'Either' is a monad with a non-local return, _not_ necessarily an error monad.
Either is just a sum type. There are classes that want to use it for non-local returns (MonadError) and classes that want to use it for sums (MonadPlus). And if I believe you, there are uses that want to use it for something else, or at least the other way round
No, he also wants to use Left for non-local returns, but argues that such returns are not always failures. It seems to me that MonadPlus is not a good fit for Either -- how can it be a monoid?