On Thu, Jul 01, 2010 at 11:18:22AM +0200, Claus Reinke wrote:
As I said, even if you just want to drop 'Error', you could define 'fail s = Left (error s)'. That would still be less defined than the current instance, but more defined than the proposed instance. That didn't occur to me -- it seems harmless enough, but it wouldn't be enough to support pattern binding with the Either monad, would it?
Only partially - sufficient to use it for MonadPlus mzero composition or for MonadError early exit, but not sufficient to make use of the error information (essentially, we model 'Either Void b' on 'Maybe b').
I thought so -- that functionality is already available in the Monad instance for Maybe.
those who want Either with different uses should use a different Either-like type.
I think we all agree on that. (We only differ on whose are the "different uses".)