[GHC] #15347: QuantifiedConstraints: Implication constraints with type families don't work

[GHC]

#15347: QuantifiedConstraints: Implication constraints with type families don't
work
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        Reporter:  aaronvargo        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > Third, working around this by manually floating out type families can be
 hugely painful, ''as it requires sacrificing one of the major benefits of
 type families, namely the ability to avoid passing around extra class
 parameters which are already determined by other parameters''

 Would it be possible to distil the example into a smaller form that still
 illustrates your key point, in italics.  My example in comment:10 did not
 substantiate this point.  Can you give a small example that does?  The
 `Category` example is so big that I lost the wood for the trees.  That is,
 so far I do not understand why floating out type families (a
 straightforward source-to-source transformation that does not change the
 number of class parameters) would sacrifice a major benefit.  Thanks!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15347#comment:14>
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