[GHC] #8518: panic ghci when probably using type families incorrectly

[GHC]

#8518: panic ghci when probably using type families incorrectly
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        Reporter:  HanStolpo         |            Owner:
            Type:  bug               |           Status:  closed
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:  fixed             |         Keywords:  ghci panic
Operating System:  Unknown/Multiple  |     Architecture:  x86
 Type of failure:  GHCi crash        |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Changes (by simonpj):

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


Comment:

 Thank you!  Happily the original program compiled with HEAD (shortly to be
 7.8.1) gives a sensible error message
 {{{
 T8518.hs:17:78:
     Could not deduce (F a1 ~ (Z a1 -> B a1 -> F a1))
     from the context (Continuation c)
       bound by the type signature for
                  callCont :: Continuation c => c -> Z c -> B c -> Maybe (F
 c)
       at T8518.hs:13:13-64
     Relevant bindings include
       c'' :: a1 (bound at T8518.hs:17:30)
       b' :: B a1 (bound at T8518.hs:17:21)
       z' :: Z a1 (bound at T8518.hs:17:18)
       c' :: a1 (bound at T8518.hs:17:15)
       rpt :: a -> a1 -> Z a1 -> B a1 -> F a1 (bound at T8518.hs:16:9)
     In the expression: rpt (i - 1) c''
     In the expression:
       let c'' = fromJust (snd <$> (continue c' z' b')) in rpt (i - 1) c''
 }}}
 I'll add the example as a regression test though!

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8518#comment:2>
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