[GHC] #8142: Panic on TypeFamilies compile

[GHC]

#8142: Panic on TypeFamilies compile
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        Reporter:  zenzike             |            Owner:
            Type:  bug                 |           Status:  new
        Priority:  normal              |        Milestone:
       Component:  Compiler            |          Version:  7.6.3
      Resolution:                      |         Keywords:
Operating System:  MacOS X             |     Architecture:
 Type of failure:  Compile-time crash  |  Unknown/Multiple
       Test Case:                      |       Difficulty:  Unknown
        Blocking:                      |       Blocked By:
                                       |  Related Tickets:
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Comment (by dreixel):

 Seems to be fixed in HEAD:
 {{{
 Test.hs:6:18:
     Couldn't match type ‛Nu f0’ with ‛Nu f’
     NB: ‛Nu’ is a type function, and may not be injective
     The type variable ‛f0’ is ambiguous
     Expected type: a -> Nu f
       Actual type: a -> Nu f0
     When checking that ‛h’
       has the inferred type ‛forall (f :: * -> *) a. a -> Nu f’
     Probable cause: the inferred type is ambiguous
     In an equation for ‛tracer’:
         tracer
           = h
           where
               h = (\ (_, b) -> ((outI . fmap h) b)) . out

 Test.hs:6:57:
     Occurs check: cannot construct the infinite type: a ~ f1 a
     Expected type: a -> (t0, f1 a)
       Actual type: Nu ((,) t0) -> (t0, Nu ((,) t0))
     Relevant bindings include h :: a -> Nu f1 (bound at Test.hs:6:18)
     In the second argument of ‛(.)’, namely ‛out’
     In the expression: (\ (_, b) -> ((outI . fmap h) b)) . out
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8142#comment:1>
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