#10598: DeriveAnyClass and GND don't work well together -------------------------------------+------------------------------------- Reporter: osa1 | Owner: RyanGlScott Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 7.11 Resolution: | Keywords: Generics Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by oerjan): Replying to [comment:25 RyanGlScott]:
It's not quite backwards-compatible—we'd also have to make a breaking change to `template-haskell` in order to accommodate the presence of these new pragmas. The type synonym approach, however, is only backportable to GHC 7.6.
Ah, there's always something. Which reminds me, Haddock might possibly want to know about the distinctions, too.
In any case, you should also decide how to annotate a `class` definition, which is where I actually suggested a pragma.
I'm not sure what you mean. Are talking about annotating when you should use `DeriveAnyClass` (`DAC`)? Or something else?
DAC or GND, both should be possible to specify. Neither is equivalent to having no annotation. The annotations on the class not only tell what you normally ''should'' use, but also guide the compiler's selection for a newtype if the deriving clause does not itself have an annotation. Also, annotating the class removes the possibility of a warning if GHC must choose between DAC and GND because a module enables both as language extensions. To re-summarize my suggested system, when choosing what mechanism to use for deriving a class for a newtype: 1. Annotations on the deriving clause take top precedence (with the exception of some builtin classes at least including, for safety, `Typeable`). 2. Then builtin derivable status of the class. 3. Then annotations on the class declaration. 4. Only if none of the above exists are the module's enabled relevant language extensions (GND or DAC) used to choose. If both GND and DAC are enabled, a warning is given, and DAC is chosen. 5. No matter what is chosen in the above, the module must have enabled any relevant language extensions. The guiding principles here being backwards compatibility (2,4), the ability to state intended usage at both class declaration and deriving sites (1,2,3), and reducing undetected surprises from irrelevant changes (4,5). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10598#comment:26 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler