Well, this is not so general. It seems to be associated to
the inferred contexts in automatically derived instances (generated with the
"deriving" keyword). I will write a clear example as soon as I make sure
what is different between GHC 6 and 7 about this.
2011/4/18 Alberto G. Corona
By my little experience with GHC 7.* it seems that this version is more "deterministic" when deriving instances. For example if there is a general instance declaration such is:
instance C1 x=> C2 x where ....
then the absence of the C1 instance becomes an error. C1 instance becomes a prerequisite for any C2 instance, where in previous versions, that was just a overlapping instance in collision with any other less general instance of the C2 class, so it could be overcomed with the "OverlappingInstances" flag.
Given that is not possible to hide instances, this is annoying. I´m not saying that this is wrong or even If this is so, because this could be a misinterpretation on my side based on the tests I have done. Where can I read about the rationale of this change?. General Instance derivations rules, as any rule system seem to me naturally flexible and with different degrees of generalization rather tan rigid.