[GHC] #14748: Infer context for Data instance of (data Foo f = Foo (f Bool) (f Int))

[GHC]

#14748: Infer context for Data instance of (data Foo f = Foo (f Bool) (f Int))
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints wipT2893,
                                     |  deriving
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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Changes (by simonpj):

 * keywords:  QuantifiedConstraints wipT2893 => QuantifiedConstraints
     wipT2893, deriving


Comment:

 I'm not sure.  When doing
 {{{
 data T a = MkT a [a] deriving( Eq )
 }}}
 the deriving mechanism says "oh, we need `Eq` on each field, that is `Eq
 a, Eq [a]`". Then it simplifies that to arrive at the instance context.

 For your example, it'd say "I need `Data (f Bool), Data (f Int)` and then
 get stuck.  There are two ways to get unglued.  One is simply to put those
 in the instance context
 {{{
 instance (Data (f Int), Data (f Bool)) => Data (Foo f) where ...
 }}}
 The other is to use quantified constraints. It's not clear which is best
 in general.   And even figuring out the appropriate quantified-constraint
 generalisation of a set of ground constraints is far from trivial.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14748#comment:1>
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