Where do I put the *definition* of a DFunId?

[Erdi, Gergo]

PUBLIC


PUBLIC

Hi,

I'm trying to make a module out of thin air and register it to GHC so that other modules can import it. So far, I have had success with making a ModIface and a ModDetails, and then registering them using the following function:

registerModule :: (GhcMonad m) => ModIface -> ModDetails -> m ()
registerModule iface details = modifySession $ extendHpt . addModule
  where
    mod_info = HomeModInfo iface details Nothing

    mod = mi_module iface
    modOrig = ModOrigin (Just True) [] [] True

    addModule = modifyUnitState $ \us -> us
        { moduleNameProvidersMap = M.insert (moduleName mod) (M.singleton mod modOrig) $ moduleNameProvidersMap us
        }

    extendHpt env = env
        { hsc_unit_env = let ue = hsc_unit_env env in ue
            { ue_hpt = hpt
            }
        }

      where
        hpt = addToHpt (hsc_HPT env) (moduleName mod) mod_info


This worked when I was only _declaring_ functions and TyCons in these modules. However, now I would also like to add instances. And I'm hitting a problem here, because I don't know where to put the actual definitions of the instance. I'm completely lost here.

Here's what I'm doing in detail. First, I make a fresh DFunId from a fresh Unique:

        let tag = occNameString . getDFunTyKey $ ty
            occ = mkDFunOcc (occNameString (getOccName showClass) <> tag) False emptyOccSet
            name = mkExternalName uniq mod occ loc
            dfun = mkDictFunId name [] [] showClass [ty]

Then, I make a ClsInst that describes my instance:

        return ClsInst
            { is_cls_nm = getName showClass
            , is_tcs = [KnownTc $ getName tycon]
            , is_dfun_name = getName dfun
            , is_tvs = []
            , is_cls = showClass
            , is_tys = [ty]
            , is_dfun = dfun
            , is_flag = OverlapFlag (NoOverlap NoSourceText) False
            , is_orphan = NotOrphan (getOccName $ getName tycon)
            }

And then I add 'AnId dfun' to my ModDetails's type env in 'md_types',, add the instance to the 'md_insts', and fill the 'mi_decls' and 'mi_insts' of the ModIface accordingly. This gives me a ModIface/ModDetails pair just like before - but I never said what the definition of 'dfun' is!

When I try compiling a real source module that imports this synthetic module and tries to use the instance, it gets as far as the "Desugar (after optimization)" step, and then fails with:

ghc-mu-core-to-exp: 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'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?

Thanks,
            Gergo

This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at https: //www.sc.com/en/our-locations

Where you have a Financial Markets relationship with Standard Chartered PLC, Standard Chartered Bank and their subsidiaries (the "Group"), information on the regulatory standards we adhere to and how it may affect you can be found in our Regulatory Compliance Statement at https: //www.sc.com/rcs/ and Regulatory Compliance Disclosures at http: //www.sc.com/rcs/fm

Insofar as this communication is not sent by the Global Research team and contains any market commentary, the market commentary has been prepared by the sales and/or trading desk of Standard Chartered Bank or its affiliate. It is not and does not constitute research material, independent research, recommendation or financial advice. Any market commentary is for information purpose only and shall not be relied on for any other purpose and is subject to the relevant disclaimers available at https: //www.sc.com/en/regulatory-disclosures/#market-disclaimer.

Insofar as this communication is sent by the Global Research team and contains any research materials prepared by members of the team, the research material is for information purpose only and shall not be relied on for any other purpose, and is subject to the relevant disclaimers available at https: //research.sc.com/research/api/application/static/terms-and-conditions. 

Insofar as this e-mail contains the term sheet for a proposed transaction, by responding affirmatively to this e-mail, you agree that you have understood the terms and conditions in the attached term sheet and evaluated the merits and risks of the transaction. We may at times also request you to sign the term sheet to acknowledge the same.

Please visit https: //www.sc.com/en/regulatory-disclosures/dodd-frank/ for important information with respect to derivative products.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20220210/d8687bae/attachment-0001.html>