GHC API panic (evaluated the place holder for a PostTcType)

The same code that works for GHC 7.10.2 blows up on GHC 7.8.4, with errors like

    test/ArgonSpec.hs:104: (best-effort)
      6) Argon.analyze accounts for && operator
           uncaught exception: GhcException (argon-test: panic! (the 'impossible' happened)
             (GHC version 7.8.4 for x86_64-unknown-linux):
           	fixity
           
           Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
           )
    
      test/ArgonSpec.hs:106: (best-effort)
      7) Argon.analyze counts everything in a real example
           uncaught exception: GhcException (argon-test: panic! (the 'impossible' happened)
             (GHC version 7.8.4 for x86_64-unknown-linux):
           	Evaluated the place holder for a PostTcType
           
           Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
           )

The responsible module is https://github.com/rubik/argon/blob/ghc-7.8/src/Argon/Visitor.hs and in particular (I think):

visitExp :: Exp -> Int
visitExp (GHC.HsIf {}) = 1
visitExp (GHC.HsMultiIf _ alts) = length alts - 1
visitExp (GHC.HsLamCase _ alts) = length (GHC.mg_alts alts) - 1
visitExp (GHC.HsCase _ alts)    = length (GHC.mg_alts alts) - 1
visitExp _ = 0

visitOp :: Exp -> Int
visitOp (GHC.OpApp _ (GHC.L _ (GHC.HsVar op)) _ _) =
    case getName op of
      "||" -> 1
      "&&" -> 1
      _    -> 0
visitOp _ = 0

somehow, even if I use wildcards, the first arguments of the type constructors gets evaluated and since it's undefined GHC crashes. What am I supposed to do? The documentation does not say anything on the matter.

changed weight to 5

added GHC API Tbug Trac import labels

I'd ask Alan Zimmerman.

Certainly if you write a wildcard in a pattern match GHC won't evaluate that field in a pattern match:

f (C _ x) = ...

the _ argument of C will not be evaluated by f. (It might be evaluated for some other reason.)

changed the description

I think the problem is the generic traversal on line 27.

getBinds :: (Data from, Typeable from) => from -> [Function]
getBinds = everything (++) $ mkQ [] visit
    where visit fun@(GHC.FunBind {}) = [fun]
          visit _ = []

It is not safe to do generic traversals on the GHC 7.8 AST due to the reason you encountered! There are landmines. You can use ghc-syb-utils if you want to also support 7.8. In future, triggering panics when using the API isn't usually a bug in GHC. Alan and I will be happy to help you on #haskell-refactorer if you have any more questions.

https://hackage.haskell.org/package/ghc-syb-utils-0.2.3/docs/GHC-SYB-Utils.html

Wonderful, thank you! Using everythingStaged solves the problem. I'll check out #haskell-refactorer.

In the future I won't open bugs for these things. But being the first time it happened, and because the error message was telling me to file a bug, I opened a ticket. Now I know better :)

closed

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

reopened

Trac metadata

Trac field	Value
Resolution	ResolvedFixed → Unresolved

closed

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Trac field	Value
Resolution	Unresolved → ResolvedInvalid

added runtime crash label

added Pnormal label