[GHC] #10831: Document conditions on deriving Functor (was: DeriveFunctor not capable of deriving when wrapping Either)

[GHC]

#10831: Document conditions on deriving Functor
-------------------------------------+-------------------------------------
        Reporter:  htebalaka         |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  low               |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.10.2
  checker)                           |
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Documentation     |  Unknown/Multiple
  bug                                |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------
Changes (by rwbarton):

 * failure:  GHC rejects valid program => Documentation bug


Comment:

 Right, this is expected behavior. The compiler doesn't "look inside" the
 definitions of other data types when deriving Functor, it relies on
 Functor instances for those types. But that is not enough to handle the
 occurrence of `b` inside the first argument to `Either`. One would need
 another class
 {{{
 class Functor1 f where
   fmap1 :: (a -> b) -> f a x -> f b x
 instance Functor1 Either
 }}}

 I note that there is no documentation in the User's Guide about when
 deriving Functor will succeed. That at least should be fixed, so I'm
 repurposing this ticket. Feel free to also submit a feature request if you
 like.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10831#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler