#13065: Prohibit user-defined Generic and Generic1 instances -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature | Status: new request | Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.0.1 Keywords: Generics | Operating System: Unknown/Multiple Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- User-defined `Generic` and `Generic1` instances are problematic. === They are susceptible to breakage === Some details of the classes may change between GHC versions, and indeed have done so in the past. User-defined instances are likely to break in the face of various such "internal" changes. This is one reason why `Data.Sequence`, for example, does not have a `Generic` instance. === They require potentially-expensive consistency checks === GHC cannot assume that every type has at most one `Generic` and `Generic1` instance, so it needs to look for possible alternatives at instance resolution time. According to Simon (and maybe also Simon), this may be partly responsible for the performance regressions seen in Phab:D2899. === Downsides === Prohibiting user-defined instances does have some costs. Suppose a type was originally defined concretely, exposing its constructors and a `Generic` instance. The implementer may decide later to make the type abstract, and export pattern synonyms to retain the same interface. But the `Generic` instance will either change or disappear. Someone relying on that instance could be in trouble. If the instance disappears, they'll be forced to write code by hand that they didn't need to before. If it changes, their code may change its behavior unexpectedly. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13065 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler