broken Monad Either instance?

[Paterson, Ross]

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.