Am 28.11.2011 15:30, schrieb Ben Millwood:
Any comments (or notes) that I missed?
Cheers Christian
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? Previously correct programs can now crash quite unpredictably without warnings!
Overlapping instances could be added for individual types, but overlapping instances are usually considered to cause more problems than they solve – ambiguity can easily arise in polymorphic functions over Either.
Either still works as Maybe with added information, except now you lose the do-pattern-matching magic. Sometimes I use something like this:
maybe (Left "oh no!") Right $ do x:xs<- Just blah [...]
to use little bits of Maybe inside an Either do. Notice I get control of the error message here, too, so I can usually make it more useful.
My use case is (or better was) monadic code like: typeCheck :: Monad m => ... -> m (...) where I used the "Either String" Monad instance to extract the fail messages: case typeCheck ... of Right ... -> ... Left ... -> .... For this I now need another monad! Which one is recommended? Cheers Christian