[GHC] #4815: Instance constraints should be used when deriving on associated data types

[GHC]

#4815: Instance constraints should be used when deriving on associated data types
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        Reporter:  batterseapower    |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  6.12.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I think it's more than just #11008.

 The question of this ticket is just "what context should GHC infer for a
 'deriving' clause."   But the ticket concerns the specific case of a `data
 instance` inside a `class` instance decl, thus:
 {{{
 instance <context> => C (T a b) where
   data D (T a b) = D1 a | D2 b
 }}}
 Now arguably, whether or not the `deriving Eq` part ''needs'' `<context>`
 it would do no harm to add it, so we get the derived instance
 {{{
 instance <context> => Eq (DT a b) where
  (==) = <blah>
 }}}
 (where `DT` is the representation data type for `D (T a b)`).  The point
 is that `<context>` might help satisfy the `Eq` constraints arising from
 `<blah>`.

 Max argues that `<context>` must already be satisfied (because the class
 instance holds).  But that's not really true
 {{{
 f x y = D1 x == D2 y
 }}}
 Here we need equality on `DT` but we don't need `C (T a b)`.  So
 `<context>` might be over-constraining.

 So I'm unconvinced that this is worth the bother.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4815#comment:17>
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