#13328: Foldable, Functor, and Traversable deriving handle phantom types badly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: deriving-perf Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dfeuer): No, you have to pick efficiency or strictness here. The bottom could show up anywhere down the line. I think derived instances should generally go for what someone's likely to write by hand, rather than trying to imitate the lousy results of the default. The default implementation of `null` will diverge on an infinite snoc-list, but that doesn't mean the derived implementation should. I think the same is true here. If the type is phantom, there's no possible reason to go there, so we shouldn't. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13328#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler