#9701: GADTs not specialized properly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Resolution: | Keywords: Operating System: | Architecture: Unknown/Multiple Unknown/Multiple | Difficulty: Unknown Type of failure: Runtime | Blocked By: performance bug | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by dfeuer): The reason I set about this whole line of thinking was the fact that `Set` cannot implement an efficient `Data.Foldable.elem` method. Wrapping a `Set` in an `Ord`-carrying GADT would enable such a thing. Unfortunately, if the GADT blocks `Ord` specialization, everything will be rather slow: comparison operators will always be looked up in the dictionary the wrapper points to, and there will be no opportunity for inlining, etc. My position regarding Incoherent Instances (and DysFunctional Dependencies) is that if your program breaks when the compiler hands you a different dictionary than you expected, you get to keep both pieces—humans invented nasal demons for a reason. I'm much more open to arguments against this idea based on implementation complexity. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9701#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler