#8599: Deriving in associated data families ignores instance's constraints -------------------------------------+------------------------------------ Reporter: mojojojo | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Documentation | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Changes (by goldfire): * component: Compiler => Documentation Comment: This is by design, though quite non-trivially. The problem is caused by the confluence of two design decisions: 1. Associated data/newtype instances do not interact with any specified context to the enclosing class instance. 2. As the manual says in section 7.5.1, "each constraint in the inferred instance context must consist only of type variables, with no repetitions." In the case at hand, the inferred context would be `Eq (DF' a)`, which does not meet this criterion. The fact in (1) does not seem to be documented anywhere, which is why I'm relabeling this as a documentation bug. Changing this fact seems hard: do we want the context to be inherited by all data constructors of an associated data instance? How can the context interact with an associated type instance (without substantially changing the way type families work)? To me, the only place that the context should possibly be inherited is in derived instances, as the original report suggests. But, even here, its role would be unclear: is the context "fully-specified", like in a standalone-deriving instance? What if a larger context is necessary? What if a smaller one were sufficient? There are consistent points in this design space, but choosing one and articulating it might be hard, and somewhat arbitrary. Thoughts? If someone agrees with relabeling this as a documentation bug, I can update the stuff on data families when I return from holiday (Jan. 6). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8599#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler