#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 RyanGlScott): Replying to [comment:27 simonpj]:
An alternative would be to require an instance declaration for DAD, thus {{{ instance C a => C (T a) }}} That's all that is required, provided `C` has suitable generic default methods and `T` is an instance of `Generic`. And if DAC always used this route, there'd be less ambiguity in the `deriving` clause of a data type declaration.
I would strongly object to this! Literally the whole reason why I want this feature in the first place is so I can combine `GeneralizedNewtypeDeriving` with `DeriveAnyClass` in the presence of `deriving` clauses (which very conveniently don't require providing context). And moreover, if type out an instance manually like that, it's no longer `DeriveAnyClass`, it's pure Haskell 98! So imposing this "requirement" for `DeriveAnyClass` is tantamount to completely losing its utility, in my opinion.
I think (but I am not sure) that we don't allow standalone deriving for DAC; thus {{{ deriving instance C a => C (T a) }}} would not work for DAC. Is that right? It seems reasonable to disallow it, because it's one word longer than the ordinary instance declaration. And if it is disallowed, then builtin and GND are already treated differently to DAC. So we'd get * GND/builtin: `deriving` clauses and `deriving instance` declarations * DAC: always a plain `instance` decl
I know that's not backward compatible..
We do currently allow `StandaloneDeriving` to be used with any deriving strategy in existence, including `DeriveAnyClass`. You're right that using `DeriveAnyClass` in this fashion is a tad redundant (you can just as well drop the `deriving` part and get the same thing), but I don't think there's any reason to explicitly disallow it—after all, that would be another breaking change, and it does seem a bit //ad hoc// to disallow one form of `deriving` here when it could easily work with the ideas proposed in this ticket.
On Ryan's synonym thing, like oerjan I'm uncomfortable with pressing synonyms into service like this.
OK, that's two votes against that idea, so I'll stop pursuing it :) In that case, pragmas looks like the path forward. Do the following names sound reasonable for pragmas? * `{-# BUILTIN #-}` * `{-# GND #-}` * `{-# DAC #-}` (goldfire proposed `{-# ANY #-}` above, but I'd argue that is a bit too ambiguous) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10598#comment:28 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler