On Wed, Jun 30, 2010 at 6:29 PM, Claus Reinke <claus.reinke@talk21.com> wrote:

Note that my opposition is against making 'Monad (Either a)' less
defined and less tunable than it is at the moment.

It's a common trade-off: the Error constraint limits the instances that
are available, but gives you a bit more when you have an instance.  One
must weigh the relative value of fail vs the unconstrained instance.

If there is no full definition of 'Monad (Either a)' for all 'a', then
qualifying 'a' by an additional type class seems to be the standard
Haskell solution. 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