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 Iceland_jack):

 Yeah this is something odd

 Replying to [comment:1 RyanGlScott]:
 > Perhaps this is my inexperience with this feature bleeding through.
 After all, I didn't even know "type constraints" were a thing until I
 stumbled upon this ticket.

 Richard has a paper on
 [http://cs.brynmawr.edu/~rae/papers/2017/partiality/partiality.pdf
 Constrained Type Families], which as I understand it might express this
 with closed type classes who knows

 {{{#!hs
 class TypeLit (a :: Type) where
   instance TypeLit Nat
   instance TypeLit Symbol

 data T :: forall a. TypeLit a => a -> Type where
   MkNat    :: T 42
   MkSymbol :: T "Don't panic!"
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12102#comment:3>
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