[GHC] #14860: QuantifiedConstraints: Can't quantify constraint involving type family

[GHC]

#14860: QuantifiedConstraints: Can't quantify constraint involving type family
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        Reporter:  RyanGlScott       |                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:  #16123            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I've discovered a way to work around the issue in comment:18. This is the
 instance we are trying to write:

 {{{#!hs
 class (forall b. Show b => Show (T a b)) => C a where
   type family T a :: * -> *
 }}}

 This doesn't work because the head of that quantified constraint mentions
 `T`, which is a type family. The normal trick is to add an extra
 constraint `ta ~ T a` and change the head of the quantified constraint to
 `Show (ta b)`. But where do we quantify `ta`?

 The answer to this question eluded me for the longest time, but it turns
 out the answer is shockingly simple: just quantify `ta` in the quantified
 constraint itself! That is to say, just do this:

 {{{#!hs
 class (forall ta b. (ta ~ T a, Show b) => Show (ta b)) => C a where
   type family T a :: * -> *
 }}}

 That's it! Now the instance typechecks, just like we've always dreamed of.
 I've tried this trick out on a large codebase that makes heavy use of
 quantified superclasses of this form, and it appears to work remarkably
 well.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14860#comment:23>
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