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<p class="msipheader7557e74b" style="margin:0in"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#317100">PUBLIC</span><o:p></o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<div>
<div>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">> I'm not surprised that eventually it crashes and burns, because,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">> again, I have only declared my DFunId ('$fShowOrderPolicy' in this<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">> case), but never defined it. Its definition would be a CoreExpr,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">> right? So where would I put the pair of '(dfun,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">> myCoreExprOfTheRightType)' for GHC to pick it up? Or is it the case<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">> that GHC would only need an 'Id's definition if it is trying to<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">> inline/specialize it, i.e. should I just attach the definition to the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">> DFunId as an unfolding? Or is 'registerModule' already incomplete and<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">> it should put the 'CoreProgram' of the module to somewhere deep in the<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">> GHC state?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">I tried adding a reasonable-looking unfolding to the DFun, but I am<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">still getting the same error when compiling code that uses the instance:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">$fShowOrderPolicy [InlPrag=CONLIKE] :: Show OrderPolicy<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">[LclIdX[DFunId],<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">Unf=DFun: \ -><o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"> C:Show \ _ [Occ=Dead] -> unpackCStringUtf8# "foo"#<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"> \ _ [Occ=Dead] _ [Occ=Dead] _ [Occ=Dead] -> unpackCStringUtf8# "bar"#<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"> \ _ [Occ=Dead] _ [Occ=Dead] -> unpackCStringUtf8# "baz"#]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">panic! (the 'impossible' happened)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"> GHC version 9.3.20211130:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"> lookupIdSubst<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">$fShowOrderPolicy<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">InScope {foo mapM_}<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">Call stack:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"> CallStack (from HasCallStack):<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"> callStackDoc, called at compiler/GHC/Utils/Panic.hs:181:37 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Utils.Panic<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"> pprPanic, called at compiler/GHC/Core/Subst.hs:260:17 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Core.Subst<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-GB">I have added a minimal working reproducer to
<a href="https://gitlab.haskell.org/ghc/ghc/-/issues/21080">https://gitlab.haskell.org/ghc/ghc/-/issues/21080</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">So the question remains, where do I need to put the DFun's definition<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">(a CoreExpr of the right type) for it to be usable as an instance?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"> Gergo<o:p></o:p></span></p>
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