[GHC] #15793: Type family doesn't reduce with visible kind application

[GHC]

#15793: Type family doesn't reduce with visible kind application
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
                                     |  TypeApplications
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 simonpj):

 * status:  closed => new
 * resolution:  invalid =>


Comment:

 But surely we should reject
 {{{
    F2 @a = Maybe a
 }}}
 on the grounds that, well, `F2` has arity 0, so you can't supply an `@ty`
 arg on the left?

 > We don't yet have the syntax to declare a type family whose argument is
 both invisible and non-dependent.

 Is another way to say this that the final argument of a type family must
 be `Required`; it cannot be `Specified`?

 What about `Inferred`?  I think that is possible, isn't it?

 We should document this somehow in the user manual.

 Re-opening for these reasons.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15793#comment:2>
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