#10598: DeriveAnyClass and GND don't work well together -------------------------------------+------------------------------------- Reporter: osa1 | Owner: RyanGlScott Type: bug | Status: patch 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): Phab:D2280 Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Replying to [comment:48 oerjan]:
Possibly, it could fit in the middle ''left'' if the table is slightly relabeled, so that the rows tell what to do about GND and the columns tell about general use. Like so:
|||| ||= No extension required =||||= Requires language extension to use =|| ||= GND when possible =|| 2(a) || `Eq`, `Ord`, `Ix`, `Bounded` |||| || ||= GND with extension =|| 2(b) || `Enum` || 2(b) || `Functor`, `Foldable` || ||= Never select GND =|| 2(c) || `Read`, `Show` || 2(c) || `Data`, `Generic`, `Generic1`, `Typeable`, `Traversable`, `Lift` ||
Which reminds me, I think maybe the word "silly" in step 1 should be "impossible", since after all some very silly things ''are'' allowed, like using anyclass on a class that doesn't support it.
Right. (I have no prior relationship with that word myself.) Anyway my
Sounds good to me. Updated with both suggestions. point is that at this point in the algorithm, you ''don't'' want to branch on the (not yet) selected instance, but only on the class. This means you might not want to use a syntax ("bespoke typeclass instance") that could imply that "bespoke" is a property of ''that'' instance.
My quibbling mind says that "when it would otherwise derive a `bespoke`
instance" should be something like "when it supports `bespoke` instances", otherwise there are still corner cases that aren't obviously caught, such as deriving `Foldable` for a newtype with GND, deriving `Enum` for a newtype, or even (when interpreted intuitively) deriving an instance for which bespoke is selected, but the derivation fails for some other reason. I think I can live with the phrase "when deriving a class which supports bespoke instances". I've updated the wiki to use it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10598#comment:49 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler