[GHC] #10141: CUSK mysteries (was: Kind inference regression in closed type families)

[GHC]

#10141: CUSK mysteries
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.1-rc2
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  indexed-
                                     |  types/should_fail/T10141
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Here's an example that I was baffled by.  This works
 {{{
 type family F (a :: k)
 type instance F Maybe = Char
 }}}
 But this does not.
 {{{
 type family F (a :: k) where -- = r  | r -> a where
   F Maybe = Char
 }}}
 The latter is rejected with
 {{{
 Foo.hs:6:5: error:
     * Expecting one more argument to `Maybe'
       Expected kind `k', but `Maybe' has kind `* -> *'
     * In the first argument of `F', namely `Maybe'
       In the type family declaration for `F'
 }}}
 It is bizarre that one works and the other does not, and I was all ready
 to open a ticket when Richard said: This is correct behavior. The former
 has a CUSK, as all open type families have CUSKs with un-annotated kinds
 defaulting to Type. The latter does not have a CUSK, because the result
 kind is unknown. You therefore cannot specialize the k variable in the
 definition of the latter.

 I can't help feeling that our CUSK story is a bit wonky, so I'm recording
 it here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10141#comment:15>
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