[GHC] #14164: GHC hangs on type family dependency

[GHC]

#14164: GHC hangs on type family dependency
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
                                     |  InjectiveFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  indexed-
                                     |  types/should_compile/T14164
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Ah I see.  So the underlying invariant is

 * Applying a substitution S to a type t should result in a ''well-kinded''
 type S(t).

 What do we need for that to be true?  Perhaps this?  If a is in
 fv(t)\dom(S) then fv(kind(a)) is disjoint from dom(S)?

 That is, if there is free variable in t that we aren't substituting, then
 we aren't substituting in its kind either.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14164#comment:12>
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