Christian Maeder writes:
Am 28.11.2011 15:30, schrieb Ben Millwood:
It's not exactly a bug, it's a change in behaviour. Beforehand, there was no overlapping instance, the instance was just 'instance (Error e) => Monad (Either e) where' but the Error constraint proved to be more a hindrance than a help, since it prevented the instance being used with Left things that weren't Errors, even though that's often a useful thing to do. So it was agreed that the Error constraint should be removed, breaking 'fail' but making the instance much more generally applicable.
Was there a library proposal that I've missed? Does any release note mention this change?
The library proposal was http://thread.gmane.org/gmane.comp.lang.haskell.libraries/13196 The mtl package hasn't been released with GHC since 6.10, so the GHC 7.0.1 release notes don't mention changes to it, just that instances were added to base.