[GHC] #7624: Handling ImplicitParams in Instance Declaration

[GHC]

#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_and_profit/
 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