obscure impossible problem with simplifier

[Simon Peyton-Jones]

OK I've fixed this. Thanks for identifying it

Simon

| -----Original Message-----
| From: Samuel Bronson [mailto:naesten at gmail.com]
| Sent: 04 November 2006 23:55
| To: cvs-ghc at haskell.org; Simon Peyton-Jones
| Subject: obscure impossible problem with simplifier
| 
| I think the simplifier is doing something iffy with types, because
| when I try to call exprType from SimplUtils.interestingArg, I get
| this:
| 
| ghc-6.5: panic! (the 'impossible' happened)
|   (GHC version 6.5 for i386-unknown-linux):
|         applyTypeToArgs
|     a98{v a1eB} [lid] ex{v aqZ} [lid]
|     a{tv a1et} [tv]
| 
| Please report this as a GHC bug:
http://www.haskell.org/ghc/reportabug
| 
| make: *** [GHC/TopHandler.o] Error 1
| 
| I think you only need to add this code:
| 
| interestingArg expr | pprTrace "interestingArg" (ppr expr) $
|                       isUnLiftedType (exprType expr) =
|         -- Unlifted args are only ever interesting if we know what
| they are
|                         case expr of
|                         Lit lit -> True
|                         _       -> False
| 
| at the top of the "interestingArg" function to get this error.
| The call chain looks like:
| completeCall -> interestingArg -> exprType -> applyTypeToArgs
| 
| I think somehow an ill-typed variable has been stuffed into the
| SimplCont? Or perhaps contArgs isn't substituting right? Anyway I
| can't find anything in the immediate area to indicate what the problem
| might be caused by... and I really hope it *is* a bug.
| 
| Btw, here is some relevant trace info:
| 
| completeCall
|     var: base:GHC.IOBase.>>{v rgF} [gid]
|     cont: ApplyTo nodup TYPE base:GHC.Base.(){(w) tc 40}
|           ApplyTo nodup TYPE a{tv a13f} [sk]
|           ApplyTo nodup handler{v ar2} [lid] ex{v aqZ} [lid]
|           ApplyTo nodup lvl_s1dT{v} [lid] @ a{tv a13f} [sk]
|           CoerceIt (base:GHC.IOBase.:CoIO{tc r3v}) a{tv a13f} [sk]
|           ApplyTo nodup new_s{v a1eg} [lid]
|           Stop[arg] (# base:GHC.Prim.State#{(w) tc 32q}
| base:GHC.Prim.RealWorld{(w) tc 31E},
|                        a{tv a13f} [sk] #)
| contArgs
|     -> ([TYPE base:GHC.Base.(){(w) tc 40}, TYPE a{tv a13f} [sk],
|          a98{v a1eh} [lid] ex{v aqZ} [lid],
|          lvl_s1dT{v} [lid] @ a{tv a13f} [sk]],
|         CoerceIt (base:GHC.IOBase.:CoIO{tc r3v}) a{tv a13f} [sk]
|         ApplyTo nodup new_s{v a1eg} [lid]
|         Stop[arg] (# base:GHC.Prim.State#{(w) tc 32q}
| base:GHC.Prim.RealWorld{(w) tc 31E},
|                      a{tv a13f} [sk] #))
| 
| If you can guess what that might mean, great. If not, well, you could
| do your own tracing. Now if you'll excuse me I have a headache :-(.
| Potentially related to trying to follow the simplifier.
| 
| This definately didn't happen before the recent simplifier changes.