[GHC] #10749: Boot file instances should imply superclasses

[GHC]

#10749: Boot file instances should imply superclasses
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        Reporter:  ezyang            |                Owner:  ezyang
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:  backpack
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 ezyang):

 * priority:  low => normal


Comment:

 Something that I've noticed is that (1) is not consistent with how
 superclasses work. For example, if I say: `Ord a => a -> a -> Bool`, I DO
 get an `Eq` instance in scope (because Eq is a superclass of Ord). So in
 practice, when I'm writing instances for modules in Backpack, I end up
 having to list a lot of instances, e.g., `Functor`, `Applicative`,
 `Monad`, `MonadFix`, etc. when, in the source code I was looking at, there
 was only `MonadFix`. Food for thought.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10749#comment:2>
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