PUBLIC

 

> I'm not surprised that eventually it crashes and burns, because,

> again, I have only declared my DFunId ('$fShowOrderPolicy' in this

> case), but never defined it. Its definition would be a CoreExpr,

> right? So where would I put the pair of '(dfun,

> myCoreExprOfTheRightType)' for GHC to pick it up? Or is it the case

> that GHC would only need an 'Id's definition if it is trying to

> inline/specialize it, i.e. should I just attach the definition to the

> DFunId as an unfolding? Or is 'registerModule' already incomplete and

> it should put the 'CoreProgram' of the module to somewhere deep in the

> GHC state?

 

I tried adding a reasonable-looking unfolding to the DFun, but I am

still getting the same error when compiling code that uses the instance:

 

$fShowOrderPolicy [InlPrag=CONLIKE] :: Show OrderPolicy

[LclIdX[DFunId],

Unf=DFun: \ ->

       C:Show \ _ [Occ=Dead] -> unpackCStringUtf8# "foo"#

              \ _ [Occ=Dead] _ [Occ=Dead] _ [Occ=Dead] -> unpackCStringUtf8# "bar"#

              \ _ [Occ=Dead] _ [Occ=Dead] -> unpackCStringUtf8# "baz"#]

 

panic! (the 'impossible' happened)

  GHC version 9.3.20211130:

        lookupIdSubst

 

$fShowOrderPolicy

InScope {foo mapM_}

Call stack:

    CallStack (from HasCallStack):

      callStackDoc, called at compiler/GHC/Utils/Panic.hs:181:37 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Utils.Panic

      pprPanic, called at compiler/GHC/Core/Subst.hs:260:17 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Core.Subst

 

I have added a minimal working reproducer to https://gitlab.haskell.org/ghc/ghc/-/issues/21080

 

So the question remains, where do I need to put the DFun's definition

(a CoreExpr of the right type) for it to be usable as an instance?

 

Thanks,

        Gergo


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