[GHC] #958: deriving Show, impossible happened

GHC trac at galois.com
Mon Oct 23 13:31:48 EDT 2006 

#958: deriving Show, impossible happened
------------------------------------------+---------------------------------
    Reporter:  greg at sleepingsquirrel.org  |       Owner:       
        Type:  bug                        |      Status:  new  
    Priority:  normal                     |   Milestone:       
   Component:  GHCi                       |     Version:  6.6  
    Severity:  normal                     |    Keywords:       
  Difficulty:  Unknown                    |    Testcase:       
Architecture:  x86                        |          Os:  Linux
------------------------------------------+---------------------------------
Here's the program (note the missing "deriving Show" clause on the Succ
 data type)...

 {{{
 data Succ a = S a
 data Seq' a = Cons' a (Seq' (Succ a)) | Nil' deriving Show
 }}}
 ...and here's the error message for 'ghci -fglasgow-exts -fallow-
 undecidable-instances foo.hs'...

 {{{
    ___         ___ _
   / _ \ /\  /\/ __(_)
  / /_\// /_/ / /  | |      GHC Interactive, version 6.6, for Haskell 98.
 / /_\\/ __  / /___| |      http://www.haskell.org/ghc/
 \____/\/ /_/\____/|_|      Type :? for help.

 Loading package base ... linking ... done.
 ghc-6.6: panic! (the 'impossible' happened)
   (GHC version 6.6 for i386-unknown-linux):
         solveDerivEqns: probable loop
     (main:Main.$f1{v rLa} base:GHC.Show.Show{tc 2h} main:Main.Seq'{tc rFH}
 [a{tv aFR} [tv]] = [base:GHC.Show.Show{tc 2h} a{tv aFR} [tv],
 base:GHC.Show.Show{tc 2h} (main:Main.Seq'{tc rFH} (main:Main.Succ{tc rFN}
 a{tv aFR} [tv]))])
     [[base:GHC.Show.Show{tc 2h} a{tv aFR} [tv],
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} a{tv aFR} [tv]),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} a{tv aFR} [tv])),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} a{tv aFR} [tv]))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} a{tv aFR} [tv])))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} a{tv aFR} [tv]))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} a{tv aFR} [tv])))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} a{tv aFR}
 [tv]))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} a{tv aFR} [tv])))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} a{tv aFR} [tv]))))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} a{tv aFR}
 [tv])))))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} a{tv aFR} [tv]))))))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} a{tv aFR} [tv])))))))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} a{tv aFR}
 [tv]))))))))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} a{tv aFR} [tv])))))))))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} a{tv aFR} [tv]))))))))))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} a{tv aFR}
 [tv])))))))))))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} a{tv aFR} [tv]))))))))))))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} a{tv aFR} [tv])))))))))))))))))),
       base:GHC.Show.Show{tc 2h} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc
 rFN} (main:Main.Succ{tc rFN} (main:Main.Succ{tc rFN} a{tv aFR}
 [tv])))))))))))))))))))]]

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/958>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler

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