[GHC] #11008: GND with Type Families

22 Oct 2015

      #11008: GND with Type Families
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           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In the following code:
 {{{
 {-# LANGUAGE TypeFamilies, FlexibleContexts, UndecidableInstances #-}

 class C r
 type family F r

 newtype Foo r = Foo r
 instance (C (F r), Eq r) => Eq (Foo r)

 newtype Bar r = Bar (Foo r) deriving (Eq)
 }}}

 GHC produces the error

 {{{
     No instance for (C (F r))
       arising from the 'deriving' clause of a data type declaration
     Possible fix:
       use a standalone 'deriving instance' declaration,
         so you can specify the instance context yourself
     When deriving the instance for (Eq (Bar r))
 }}}

 If I enable `StandaloneDeriving`, I am able to derive the instance:
 `deriving instance (Eq (Foo r)) => Eq (Bar r)`

 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/deriving.htm...
 #newtype-deriving The docs] indicate that this is exactly the instance
 that should be generated by GND for

   `newtype T v1..vn = MkT (t vk+1..vn) deriving (C t1..tj)`,

 where my code uses the following parameters:
 {{{
   T = Bar
   v1 = r
   n = 1
   MkT = Bar
   t = Foo r
   k= 1
   C = Eq
   j = 0
 }}}

 and all of the conditions enumerated in the docs seem to be met.
 Furthermore, `Bar` seems to fit squarely into the format of `data T1` in
 section 7.5.1 of the same documentation, which indicates it should not
 need `StandaloneDeriving`. It seems that the type family constraint on the
 `Eq` instance for `Foo` is causing the problem, but it's not clear why I'm
 forced to write a standalone instance that is identical to the one that
 should be generated by GND.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11008
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