On Thu, Jul 01, 2010 at 12:29:57AM +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?
Do you mean an overlapping instance? There are problems with that too.
Yes to both. But it would avoid the problems with the unconstrained Monad instance for Either a, wouldn't it?
It seems disproportionate: Either is a Prelude type, and it seems reasonable to stick to the core language in defining its instances of Prelude classes. One can always use a new type for the Error thing.