Re: proposal #4095: add Applicative instance for Either

On Tue, 25 May 2010, Bas van Dijk wrote:

+1 for this Applicative instance.

I assume that when this instance is added to Control.Applicative, the one in Control.Monad.Trans.Error from transformers will be removed.

On Tue, May 25, 2010 at 6:22 PM, Yitzchak Gale wrote:

...

-1 for the orphan Monad instance with the silly superclass constraint that this Applicative instance is meant to extend

I assume you are referring to:

instance (Error e) => Monad (Either e) where return = Right Left l >>= _ = Left l Right r >>= k = k r fail msg = Left (strMsg msg)

defined in Control.Monad.Trans.Error from transformers or Control.Monad.Error from mtl.

I think the problem with this instance is that it is orphan. You get a conflict if you import both mtl and transformers. But I assume that it is not possible to add this instance to Prelude, since this may break Haskell 98 programs. Maybe MTL could import that instance from Transformers in order to avoid duplicate definition.