#9790: Produce coercion rules for derived Functor instances -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: coercion Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Runtime | Related Tickets: performance bug | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by dfeuer): Replying to [comment:1 goldfire]:
Such a rule is wrong for non-lawful functors, which is why we haven't done it.
It would be nice (perhaps even very nice) if we could give users a way to say that a particular instance is lawful w.r.t. the appropriate laws and would allow magic like the proposed idea. However, I'm not volunteering to design and/or implement such a system...
Right, that's why I only mentioned ''derived'' `Functor` instances. Surely a derived instance that does not depend on any non-derived instances should be lawful! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9790#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler