On Wed, Jun 30, 2010 at 09:36:00AM +0200, Claus Reinke 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.
One could also wonder whether the instance should be in Data.Either (the instance for Maybe is in Data.Maybe), [...]
The problem with that is that Haskell 98 does not define such an instance, so a legal Haskell98 module that defined an instance itself would be broken by an instance in a module imported by the Prelude.
If you are just concerned about the Error constraint, simply provide a default instance that maps strMsg to error (see example of default instances with user override in base Data.Typable). That way, you'd get 'Monad (Either a)' without Error constraint, but with strictly more defined fail ('Left _|_' instead of '_|_'), and others can add even more defined fail when needed.
Do you mean an overlapping instance? There are problems with that too.