Loading a typechecked module and then using it immediately as a package

[Sylvain Henry]

Hi,

This part of the API is still awful and a bit in flux to make it less 
so. Modifying the UnitState directly isn't currently supported and seems 
difficult to do correctly (e.g. in your code snippet below you don't 
modify the moduleNameProvidersMap field), so it would probably be better 
to recreate the UnitState from scratch with mkUnitState/initUnitConfig.

You may also have a look to 
GHC.Driver.Backpack.{withBkpSession,buildUnit} in TcSession mode which 
registers virtual units for Backpack's .bkp files similarly to what you 
want to do. If you really don't want to use the filesystem at all, 
however, I think you will have to deal with moving MyLib from the HPT to 
the EPS and I don't know if it is easily feasible (Backpack resets these 
tables via withTempSession so that interface files are read from disk as 
usual instead iiuc).

Good luck :)
Sylvain


On 25/06/2021 11:17, Erdi, Gergo via ghc-devs wrote:
>
> PUBLIC
>
>
> Hi,
>
> I have the following to .hs files:
>
>  1. MyLib.hs:
>
>     module MyLib where
>     …
>
>  2. Test.hs:
>
>     {-# LANGUAGE PackageImports  #-}
>     module Test where
>     import “my-pkg” MyLib
>     …
>
> I would like to parse/typecheck/load MyLib.hs into some Unit 
> “my-unit”, then add that to the package “my-pkg”, and then typecheck 
> Test.hs, all in-proc using the GHC API, without putting any other 
> files on disk. How do I do that?
>
> What I tried is loading MyLib.hs after setting the homeUnitId in the 
> DynFlags to “my-unit”, then editing the packageNameMap in the 
> unitState of the DynFlags to may “my-pkg” to “my-unit”:
>
> setHomeUnit :: (GhcMonad m) => UnitId -> m ()
>
> setHomeUnit unitId = do
>
>     dflags <- getSessionDynFlags
>
> modifySession $ \h -> h{ hsc_dflags = dflags{ homeUnitId = unitId } }
>
> registerUnit :: (GhcMonad m) => PackageName -> UnitId -> m ()
>
> registerUnit pkg unitId = modifySession $ \h -> h{ hsc_dflags = 
> addUnit $ hsc_dflags h }
>
>   where
>
>     addUnit dflags = dflags
>
>         { unitState = let us = unitState dflags in us
>
>             { packageNameMap = M.insert pkg (Indefinite unitId 
> Nothing) $ packageNameMap us
>
>             }
>
>         }
>
> pipeline = do
>
> setHomeUnit myUnit
>
> loadModule =<< typecheckModule =<< parseModule =<< modSumarryFor “MyLib”
>
> registerUnit myPkg myUnit
>
> setHomeUnit mainUnitId
>
> typecheckModule =<< parseModule =<< modSumarryFor “Test”
>
> Alas, this doesn’t work: the import of `MyLib` from `my-pkg` fails with:
>
> input/linking/Test.hs:5:1: error:
>
>     Could not find module ‘MyLib’
>
>     It is not a module in the current program, or in any known package.
>
> TBH I’m not very surprised that it didn’t work – that registerUnit 
> function is doing some pretty deep surgery on the unitState that 
> probably breaks several invariants. Still, I wasn’t able to find a 
> better way – all the functions in GHC.Unit.State seem to be for 
> querying only.
>
> Thanks,
>
> Gergo
>
>
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