Migration guide for multiple home units

[Matthew Pickering]

Gergo, I rewrote your example program for the new API but didn't quite
manage to finish today. I will finish it off and post it tomorrow.

Matt

On Mon, Jun 20, 2022 at 6:16 AM Erdi, Gergo <Gergo.Erdi at sc.com> wrote:
>
> PUBLIC
>
> I managed to get this working, but I would like some feedback from Matt or others on whether this is the intended way of doing this.
>
> 1. The `extendMG` change can be brute-forced just to keep things going, by looking up dependency `ModSummary`s by `ModName` (we don't have full `Module`s at this point yet!):
>
> registerModSummary :: (GhcMonad m) => ModSummary -> m () registerModSummary ms = modifySession \env ->
>     let mg = hsc_mod_graph env
>         -- TODO: of course with a bit more housekeeping we can do better than this...
>         edges = [ NodeKey_Module $ msKey ms' | dep <- deps, ms' <- mgModSummaries mg, ms_mod_name ms' == dep ]
>         mg' = extendMG mg edges ms
>     in env{ hsc_mod_graph = mg' }
>   where
>     deps = map (unLoc . snd) $ ms_textual_imps ms
>
> 2. `registerModule` can be kept mostly as-is, since it only uses `modifyUnitState` to change the active unit:
>
> registerModule :: (GhcMonad m) => ModDetails -> ModIface -> m () registerModule details iface = modifySession $ extendHpt . addModule
>   where
>     hmi = 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 = hscUpdateHUG $ addHomeModInfoToHug hmi
>
> modifyUnitState :: (UnitState -> UnitState) -> HscEnv -> HscEnv modifyUnitState f env = env
>     { hsc_unit_env = let ue = hsc_unit_env env in
>             let units = ue_units ue
>                 units' = f units
>             in ue_setUnits units' ue
>     }
>
> 3. The tricky part is getting `addUnit` right. For this, based on the implementation of `initUnits`, I came up with the following:
>
> modifyUnitEnv :: (UnitEnv -> UnitEnv) -> HscEnv -> HscEnv modifyUnitEnv f env = env
>     { hsc_unit_env = let ue = hsc_unit_env env in f ue
>     }
>
> addUnit :: DynFlags -> UnitId -> HscEnv -> HscEnv addUnit dflags unitId = modifyUnitEnv $ \ue ->
>     let dbs = ue_unit_dbs ue
>         unit_state = ue_units ue
>         home_unit = ue_homeUnit ue
>     in flip updateHug ue $ unitEnv_insert unitId $ HomeUnitEnv
>        { homeUnitEnv_units = unit_state
>        , homeUnitEnv_unit_dbs = dbs
>        , homeUnitEnv_dflags = dflags
>        , homeUnitEnv_hpt = emptyHomePackageTable
>        , homeUnitEnv_home_unit = home_unit
>        }
>
> setCurrentUnit :: UnitId -> HscEnv -> HscEnv setCurrentUnit unitId = modifyUnitEnv $ ue_setActiveUnit unitId
>
>
> So my questions about this:
>
> 1. How does setting the home unit make sense? By doing this, I am effectively setting the home unit to `main` for all units, since that's the initial `ue_homeUnit` of the initial unit environment. Or does it not matter because after `addUnit`, I call `setCurrentUnit` anyway? I've found that I can't use the unit I am just adding as its own home unit, because that then leads to module name resolution problems in `main`: every imported module from `main` is searched for in `main` instead of its correct unit.
>
> 2. Speaking of `main`, why is it that when adding units, I have to skip `mainUnitId`, otherwise module resolution breaks again?
>
> 3. Unlike the previous version, I am no longer creating and putting `UnitInfo`s anywhere. Where is this going to bite me? Where (if anywhere) should I put `UnitInfo`s with the new setup?
>
> Thanks,
>         Gergo
>
>
> -----Original Message-----
> From: ÉRDI Gergő <gergo at erdi.hu>
> Sent: Sunday, June 19, 2022 12:32 PM
> To: Erdi, Gergo <Gergo.Erdi at sc.com>
> Cc: GHC Devs <ghc-devs at haskell.org>; Montelatici, Raphael Laurent <Raphael.Montelatici at sc.com>
> Subject: [External] Re: Migration guide for multiple home units
>
>
> On Thu, 16 Jun 2022, Erdi, Gergo via ghc-devs wrote:
>
> > Is there a migration guide for GHC API clients for the new “multiple home units”
> > feature?
>
> OK so in concrete terms, please see my attached program which is a heavily cut-down, standalone version of my real program. On commit fd42ab5fa1df847a6b595dfe4b63d9c7eecbf400^ (i.e.
> 3219610e3ba6cb6a5cd1f4e32e2b4befea5bd384) it compiles and works as expected. On commit fd42ab5fa1df847a6b595dfe4b63d9c7eecbf400 onwards, two problems pop up:
>
> 1. `extendMG` has changed and now requires manually specifying outgoing dependency edges. I thought the whole point of `summariseFile` was to collect this information? The reason I need to `extendMG` at that point is to get intra-unit orphan instances working.
>
> 2. `modifyUnitState` and its two uses (`addUnit` and `registerModule`) need to be updated to the new API. I think it makes sense that these need changing, since they touch exactly on the issue of which units are being compiled right now. However, I don't know how to update these. Also, I guess `setHomeUnit` should change the `CurrentUnit` instead of the `HomeUnit` now?
>
> Thanks,
>         Gergo
>
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