#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 oerjan): Hm. Commenting on that wiki algorithm: * Might it be simpler to treat the bespoke -> newtype optimization as a post-step, independent of the rest? Or is it ever important ''not'' to apply it? Even for step 1. * Is 2(a) missing `Enum`? * Why is `Traversable` in 2(b)? I would have thought that the bespoke -> newtype optimization would apply to it. I guess there's some technical difference. * All the examples in 2(c) are listed in a or b, leaving the strange impression that it can never be triggered. Although I assume `Functor` and `Foldable` belong there. (Which also tells me the last paragraph in my previous comment has been taken care of). * The phrase "can be successfully used with GeneralizedNewtypeDeriving" is needed in 2(c) as well. Should 2(d) apply or not if that check falls? If it does then sometimes standard classes ''could'' get anyclassed. If it does ''not'', then should the classes fall back to bespoke, at least if their extensions are enabled? * It seems that `Functor` and `Foldable` never get newtype derived if that extension is not enabled, even when it would be safe to do so. I think the algorithm can be simplified a bit: remove point 2(c) entirely, and say explicitly in point 2(d) that it ''doesn't'' apply to standard derivable classes. Lastly, while thinking about this, I made a table of the (currently four- way) classification of the "standard/bespoke" classes: ||= GND equivalence =||||= No extension =||||= Extension =|| ||= Always =|| 2(a) || `Eq` `Ord` `Enum` `Ix` `Bounded` |||| || ||= Requires check =|||| || 2(c) || `Functor` `Foldable` || ||= Never =|| 2(b) || `Read` `Show` || 2(b) || `Data` `Generic` `Generic1` `Typeable` `Traversable`(?) `Lift` || -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10598#comment:42 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler