#7624: Handling ImplicitParams in Instance Declaration --------------------------------------+------------------------------------ Reporter: philipjf | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.1 Component: Documentation | Version: 7.6.1 Resolution: | Keywords: ImplicitParams Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: Documentation bug | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: --------------------------------------+------------------------------------ Description changed by simonpj: Old description:

The GHC documentation states that ImplicitParam's may not be used in class or instance declaration constraints. see for example http://www.haskell.org/ghc/docs/7.6.1/html/users_guide/other-type- extensions.html In practice though, at least on the Mac, all recent version of GHC have permitted such instances. At the very least the documentation should make clear that the compiler might permit such instances, but they have undefined behaviour. Ideally, these would simply be permitted.

[http://joyoftypes.blogspot.com/2013/01/using-compiler-bugs-for-fun-and- profit.html Why I think the (undocumented) current behavior is correct]

New description: The GHC documentation states that `ImplicitParam`'s may not be used in class or instance declaration constraints. see for example http://www.haskell.org/ghc/docs/7.6.1/html/users_guide/other-type- extensions.html In practice though, at least on the Mac, all recent version of GHC have permitted such instances. At the very least the documentation should make clear that the compiler might permit such instances, but they have undefined behaviour. Ideally, these would simply be permitted. [http://joyoftypes.blogspot.com/2013/01/using-compiler-bugs-for-fun-and- profit.html Why I think the (undocumented) current behavior is correct] [[BR]] [http://www.reddit.com/r/haskell/comments/178w9u/using_compiler_bugs_for_fun_... reddit discussion thereof] -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7624#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler