Re: [GHC] #12102: “Constraints in kinds” illegal family application in instance (+ documentation issues?)

[GHC]

#12102: “Constraints in kinds” illegal family application in instance (+
documentation issues?)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeInType
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 didn't even know we allowed constraints in kinds.  We certainly should
 not allow lifted equality constraints, like `T :: forall k. (t1 ~ t2) =>
 blah`.  Because `(t1 ~ t2)` is represented by lifted, heap-allocated,
 possibly-bottom value, and we don't have a `case` expression in types to
 unpack it.

 Possibly we should allow unlifted equality `T :: forall k. (t1 ~# t2) =>
 blah`.  I'm not sure.  But what you have is definitely wrong and should be
 rejected with a decent error message.  (And the user manual should be
 fixed!)

 Richard what do you think?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12102#comment:5>
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