proposal #4095: add Applicative instance for Either

[Bas van Dijk]

+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 <gale at sefer.org> 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.

Thinking out loudly: since the 'fail' method is considered to be a
mistake, what about not defining it? As in:

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

Regards,

Bas