[Git][ghc/ghc][wip/fendor/ghci-multiple-home-units] 2 commits: Make GHCi commands compatible with multiple home units

9 May 2025

Hannes Siebenhandl pushed to branch wip/fendor/ghci-multiple-home-units at Glasgow Haskell Compiler / GHC


Commits:
8e4120d5 by fendor at 2025-05-09T17:24:50+02:00
Make GHCi commands compatible with multiple home units

=== Design

We enable all GHCi features that were previously guarded by the `inMulti`
option.

GHCi supported multiple home units up to a certain degree for quite a while now.
The supported feature set was limited, due to a design impasse:
One of the home units must be "active", e.g., there must be one `HomeUnit`
whose `UnitId` is "active" which is returned when calling

```haskell
do
  hscActiveUnitId <$> getSession
```

This makes sense in a GHC session, since you are always compiling a particular
Module, but it makes less intuitive sense in an interactive session.
Given an expression to evaluate, we can`t easily tell in which "context" the expression
should be parsed, typechecked and evaluated.
That`s why initially, most of GHCi features, except for `:reload`ing were disabled
if the GHCi session had more than one `HomeUnitEnv`.

We lift this restriction, enabling all features of GHCi for the multiple home unit case.
To do this, we fundamentally change the `HomeUnitEnv` graph to be multiple home unit first.
Instead of differentiating the case were we have a single home unit and multiple,
we now always set up a multiple home unit session that scales seamlessly to an arbitrary
amount of home units.

We introduce two new `HomeUnitEnv`s that are always added to the `HomeUnitGraph`.
They are:

The "interactive-ghci", called the `interactiveGhciUnit`, contains the same
`DynFlags` that are used by the `InteractiveContext` for interactive evaluation
of expressions.
This `HomeUnitEnv` is only used on the prompt of GHCi, so we may refer to it as
"interactive-prompt" unit.
See Note [Relation between the `InteractiveContext` and `interactiveGhciUnitId`]
for discussing its role.

And the `interactive-session`, called `interactiveSessionUnit` or
`interactiveSessionUnitId`, which is used for loading Scripts into
GHCi that are not `Target`s of any home unit, via `:load` or `:add`.

Both of these "interactive" home units depend on all other `HomeUnitEnv`s that
are passed as arguments on the cli.
Additionally, the "interactive-ghci" unit depends on `interactive-session`.

We always evaluate expressions in the context of the
"interactive-ghci" session.
Since "interactive-ghci" depends on all home units, we can import any `Module`
from the other home units with ease.

As we have a clear `HomeUnitGraph` hierarchy, we can set `interactiveGhciUnitId`
as the active home unit for the full duration of the GHCi session.
In GHCi, we always set `interactiveGhciUnitId` to be the currently active home unit.

=== Implementation Details

Given this design idea, the implementation is relatively straight
forward.
The core insight is that a `ModuleName` is not sufficient to identify a
`Module` in the `HomeUnitGraph`. Thus, large parts of the PR is simply
about refactoring usages of `ModuleName` to prefer `Module`, which has a
`Unit` attached and is unique over the `HomeUnitGraph`.

Consequentially, most usages of `lookupHPT` are likely to be incorrect and have
been replaced by `lookupHugByModule` which is keyed by a `Module`.

In GHCi/UI.hs, we make sure there is only one location where we are
actually translating `ModuleName` to a `Module`:

* `lookupQualifiedModuleName`

If a `ModuleName` is ambiguous, we detect this and report it to the
user.

To avoid repeated lookups of `ModuleName`s, we store the `Module` in the
`InteractiveImport`, which additionally simplifies the interface
loading.

A subtle detail is that the `DynFlags` of the `InteractiveContext` are
now stored both in the `HomeUnitGraph` and in the `InteractiveContext`.
In UI.hs, there are multiple code paths where we are careful to update
the `DynFlags` in both locations.
Most importantly in `addToProgramDynFlags`.

---

There is one metric increase in this commit:

-------------------------
Metric Increase:
    T4029
-------------------------

It is an increase from 14.4 MB to 16.1 MB (+11.8%) which sounds like a
pretty big regression at first.
However, we argue this increase is solely caused by using more data
structures for managing multiple home units in the GHCi session.
In particular, due to the design decision of using three home units, the
base memory usage increases... but by how much?

A big contributor is the `UnitState`, of which we have three now, which
on its own 260 KB per instance. That makes an additional memory usage of
520 KB, already explaining a third of the overall memory usage increase.
Then we store more elements in the `HomeUnitGraph`, we have more
`HomeUnitEnv` entries, etc...

While we didn't chase down each byte, we looked at the memory usage over time
for both `-hi` and `-hT` profiles and can say with confidence while the memory
usage increased slightly, we did not introduce any space leak, as
the graph looks almost identical as the memory usage graph of GHC HEAD.

- - - - -
798408f3 by fendor at 2025-05-09T17:24:50+02:00
FIXUP: Multiple Home Units is no longer a special case

- - - - -


20 changed files:

- compiler/GHC.hs
- compiler/GHC/Driver/Downsweep.hs
- compiler/GHC/Driver/Session.hs
- compiler/GHC/Iface/Load.hs
- compiler/GHC/Rename/Unbound.hs
- compiler/GHC/Runtime/Context.hs
- compiler/GHC/Runtime/Eval.hs
- compiler/GHC/StgToByteCode.hs
- compiler/GHC/StgToJS/Linker/Linker.hs
- compiler/GHC/Tc/Module.hs
- compiler/GHC/Types/Name/Ppr.hs
- compiler/GHC/Unit/Env.hs
- compiler/GHC/Unit/Home/Graph.hs
- compiler/GHC/Unit/Types.hs
- ghc/GHCi/UI.hs
- ghc/GHCi/UI/Exception.hs
- ghc/GHCi/UI/Info.hs
- ghc/GHCi/UI/Monad.hs
- ghc/Main.hs
- testsuite/tests/linters/notes.stdout


Changes:

=====================================
compiler/GHC.hs
=====================================
@@ -38,7 +38,9 @@ module GHC (
         setSessionDynFlags,
         setUnitDynFlags,
         getProgramDynFlags, setProgramDynFlags,
+        setProgramHUG, setProgramHUG_,
         getInteractiveDynFlags, setInteractiveDynFlags,
+        normaliseInteractiveDynFlags, initialiseInteractiveDynFlags,
         interpretPackageEnv,
 
         -- * Logging
@@ -55,6 +57,7 @@ module GHC (
         addTarget,
         removeTarget,
         guessTarget,
+        guessTargetId,
 
         -- * Loading\/compiling the program
         depanal, depanalE,
@@ -83,6 +86,7 @@ module GHC (
         getModuleGraph,
         isLoaded,
         isLoadedModule,
+        isLoadedHomeModule,
         topSortModuleGraph,
 
         -- * Inspecting modules
@@ -155,6 +159,7 @@ module GHC (
         getBindings, getInsts, getNamePprCtx,
         findModule, lookupModule,
         findQualifiedModule, lookupQualifiedModule,
+        lookupLoadedHomeModuleByModuleName, lookupAllQualifiedModuleNames,
         renamePkgQualM, renameRawPkgQualM,
         isModuleTrusted, moduleTrustReqs,
         getNamesInScope,
@@ -443,6 +448,7 @@ import Control.Concurrent
 import Control.Monad
 import Control.Monad.Catch as MC
 import Data.Foldable
+import Data.Function ((&))
 import Data.IORef
 import Data.List (isPrefixOf)
 import Data.Typeable    ( Typeable )
@@ -458,7 +464,7 @@ import System.Environment ( getEnv, getProgName )
 import System.Exit      ( exitWith, ExitCode(..) )
 import System.FilePath
 import System.IO.Error  ( isDoesNotExistError )
-import GHC.Unit.Home.PackageTable
+
 
 -- %************************************************************************
 -- %*                                                                      *
@@ -861,6 +867,113 @@ setProgramDynFlags_ invalidate_needed dflags = do
   when invalidate_needed $ invalidateModSummaryCache
   return changed
 
+-- | Sets the program 'HomeUnitGraph'.
+--
+-- Sets the given 'HomeUnitGraph' as the 'HomeUnitGraph' of the current
+-- session. If the package flags change, we reinitialise the 'UnitState'
+-- of all 'HomeUnitEnv's in the current session.
+--
+-- This function unconditionally invalidates the module graph cache.
+--
+-- Precondition: the given 'HomeUnitGraph' must have the same keys as the 'HomeUnitGraph'
+-- of the current session. I.e., assuming the new 'HomeUnitGraph' is called
+-- 'new_hug', then:
+--
+-- @
+--  do
+--    hug <- hsc_HUG \<$\> getSession
+--    pure $ unitEnv_keys new_hug == unitEnv_keys hug
+-- @
+--
+-- If this precondition is violated, the function will crash.
+--
+-- Conceptually, similar to 'setProgramDynFlags', but performs the same check
+-- for all 'HomeUnitEnv's.
+setProgramHUG :: GhcMonad m => HomeUnitGraph -> m Bool
+setProgramHUG =
+  setProgramHUG_ True
+
+-- | Same as 'setProgramHUG', but gives you control over whether you want to
+-- invalidate the module graph cache.
+setProgramHUG_ :: GhcMonad m => Bool -> HomeUnitGraph -> m Bool
+setProgramHUG_ invalidate_needed new_hug0 = do
+  logger <- getLogger
+
+  hug0 <- hsc_HUG <$> getSession
+  (changed, new_hug1) <- checkNewHugDynFlags logger hug0 new_hug0
+
+  if changed
+    then do
+      unit_env0 <- hsc_unit_env <$> getSession
+      home_unit_graph <- HUG.unitEnv_traverseWithKey
+        (updateHomeUnit logger unit_env0 new_hug1)
+        (ue_home_unit_graph unit_env0)
+
+      let dflags1 = homeUnitEnv_dflags $ HUG.unitEnv_lookup (ue_currentUnit unit_env0) home_unit_graph
+      let unit_env = UnitEnv
+            { ue_platform        = targetPlatform dflags1
+            , ue_namever         = ghcNameVersion dflags1
+            , ue_home_unit_graph = home_unit_graph
+            , ue_current_unit    = ue_currentUnit unit_env0
+            , ue_eps             = ue_eps unit_env0
+            }
+      modifySession $ \h ->
+        -- hscSetFlags takes care of updating the logger as well.
+        hscSetFlags dflags1 h{ hsc_unit_env = unit_env }
+    else do
+      modifySession (\env ->
+        env
+          -- Set the new 'HomeUnitGraph'.
+          & hscUpdateHUG (const new_hug1)
+          -- hscSetActiveUnitId makes sure that the 'hsc_dflags'
+          -- are up-to-date.
+          & hscSetActiveUnitId (hscActiveUnitId env)
+          -- Make sure the logger is also updated.
+          & hscUpdateLoggerFlags)
+
+  when invalidate_needed $ invalidateModSummaryCache
+  pure changed
+  where
+    checkNewHugDynFlags :: GhcMonad m => Logger -> HomeUnitGraph -> HomeUnitGraph -> m (Bool, HomeUnitGraph)
+    checkNewHugDynFlags logger old_hug new_hug = do
+      -- Traverse the new HUG and check its 'DynFlags'.
+      -- The old 'HUG' is used to check whether package flags have changed.
+      hugWithCheck <- HUG.unitEnv_traverseWithKey
+        (\unitId homeUnit -> do
+          let newFlags = homeUnitEnv_dflags homeUnit
+              oldFlags = homeUnitEnv_dflags (HUG.unitEnv_lookup unitId old_hug)
+          checkedFlags <- checkNewDynFlags logger newFlags
+          pure
+            ( packageFlagsChanged oldFlags checkedFlags
+            , homeUnit { homeUnitEnv_dflags = checkedFlags }
+            )
+        )
+        new_hug
+      let
+        -- Did any of the package flags change?
+        changed = or $ fmap fst hugWithCheck
+        hug = fmap snd hugWithCheck
+      pure (changed, hug)
+
+    updateHomeUnit :: GhcMonad m => Logger -> UnitEnv -> HomeUnitGraph -> (UnitId -> HomeUnitEnv -> m HomeUnitEnv)
+    updateHomeUnit logger unit_env updates = \uid homeUnitEnv -> do
+      let cached_unit_dbs = homeUnitEnv_unit_dbs homeUnitEnv
+          dflags = case HUG.unitEnv_lookup_maybe uid updates of
+            Nothing -> homeUnitEnv_dflags homeUnitEnv
+            Just env -> homeUnitEnv_dflags env
+          old_hpt = homeUnitEnv_hpt homeUnitEnv
+          home_units = HUG.allUnits (ue_home_unit_graph unit_env)
+
+      (dbs,unit_state,home_unit,mconstants) <- liftIO $ initUnits logger dflags cached_unit_dbs home_units
+
+      updated_dflags <- liftIO $ updatePlatformConstants dflags mconstants
+      pure HomeUnitEnv
+        { homeUnitEnv_units = unit_state
+        , homeUnitEnv_unit_dbs = Just dbs
+        , homeUnitEnv_dflags = updated_dflags
+        , homeUnitEnv_hpt = old_hpt
+        , homeUnitEnv_home_unit = Just home_unit
+        }
 
 -- When changing the DynFlags, we want the changes to apply to future
 -- loads, but without completely discarding the program.  But the
@@ -900,24 +1013,8 @@ getProgramDynFlags = getSessionDynFlags
 setInteractiveDynFlags :: GhcMonad m => DynFlags -> m ()
 setInteractiveDynFlags dflags = do
   logger <- getLogger
-  dflags' <- checkNewDynFlags logger dflags
-  dflags'' <- checkNewInteractiveDynFlags logger dflags'
-  modifySessionM $ \hsc_env0 -> do
-    let ic0 = hsc_IC hsc_env0
-
-    -- Initialise (load) plugins in the interactive environment with the new
-    -- DynFlags
-    plugin_env <- liftIO $ initializePlugins $ mkInteractiveHscEnv $
-                    hsc_env0 { hsc_IC = ic0 { ic_dflags = dflags'' }}
-
-    -- Update both plugins cache and DynFlags in the interactive context.
-    return $ hsc_env0
-                { hsc_IC = ic0
-                    { ic_plugins = hsc_plugins plugin_env
-                    , ic_dflags  = hsc_dflags  plugin_env
-                    }
-                }
-
+  icdflags <- normaliseInteractiveDynFlags logger dflags
+  modifySessionM (initialiseInteractiveDynFlags icdflags)
 
 -- | Get the 'DynFlags' used to evaluate interactive expressions.
 getInteractiveDynFlags :: GhcMonad m => m DynFlags
@@ -1022,6 +1119,36 @@ normalise_hyp fp
 
 -----------------------------------------------------------------------------
 
+-- | Normalise the 'DynFlags' for us in an interactive context.
+--
+-- Makes sure unsupported Flags and other incosistencies are reported and removed.
+normaliseInteractiveDynFlags :: MonadIO m => Logger -> DynFlags -> m DynFlags
+normaliseInteractiveDynFlags logger dflags = do
+  dflags' <- checkNewDynFlags logger dflags
+  checkNewInteractiveDynFlags logger dflags'
+
+-- | Given a set of normalised 'DynFlags' (see 'normaliseInteractiveDynFlags')
+-- for the interactive context, initialize the 'InteractiveContext'.
+--
+-- Initialized plugins and sets the 'DynFlags' as the 'ic_dflags' of the
+-- 'InteractiveContext'.
+initialiseInteractiveDynFlags :: GhcMonad m => DynFlags -> HscEnv -> m HscEnv
+initialiseInteractiveDynFlags dflags hsc_env0 = do
+  let ic0 = hsc_IC hsc_env0
+
+  -- Initialise (load) plugins in the interactive environment with the new
+  -- DynFlags
+  plugin_env <- liftIO $ initializePlugins $ mkInteractiveHscEnv $
+                  hsc_env0 { hsc_IC = ic0 { ic_dflags = dflags }}
+
+  -- Update both plugins cache and DynFlags in the interactive context.
+  return $ hsc_env0
+              { hsc_IC = ic0
+                  { ic_plugins = hsc_plugins plugin_env
+                  , ic_dflags  = hsc_dflags  plugin_env
+                  }
+              }
+
 -- | Checks the set of new DynFlags for possibly erroneous option
 -- combinations when invoking 'setSessionDynFlags' and friends, and if
 -- found, returns a fixed copy (if possible).
@@ -1084,7 +1211,7 @@ removeTarget target_id
   where
    filter targets = [ t | t@Target { targetId = id } <- targets, id /= target_id ]

--- | Attempts to guess what Target a string refers to.  This function
+-- | Attempts to guess what 'Target' a string refers to.  This function
 -- implements the @--make@/GHCi command-line syntax for filenames:
 --
 --   - if the string looks like a Haskell source filename, then interpret it
@@ -1093,27 +1220,52 @@ removeTarget target_id
 --   - if adding a .hs or .lhs suffix yields the name of an existing file,
 --     then use that
 --
---   - otherwise interpret the string as a module name
+--   - If it looks like a module name, interpret it as such
 --
+--   - otherwise, this function throws a 'GhcException'.
 guessTarget :: GhcMonad m => String -> Maybe UnitId -> Maybe Phase -> m Target
 guessTarget str mUnitId (Just phase)
    = do
      tuid <- unitIdOrHomeUnit mUnitId
      return (Target (TargetFile str (Just phase)) True tuid Nothing)
-guessTarget str mUnitId Nothing
+guessTarget str mUnitId Nothing = do
+  targetId <- guessTargetId str
+  toTarget targetId
+     where
+         obj_allowed
+                | '*':_ <- str = False
+                | otherwise    = True
+         toTarget tid = do
+           tuid <- unitIdOrHomeUnit mUnitId
+           pure $ Target tid obj_allowed tuid Nothing
+
+-- | Attempts to guess what 'TargetId' a string refers to.  This function
+-- implements the @--make@/GHCi command-line syntax for filenames:
+--
+--   - if the string looks like a Haskell source filename, then interpret it
+--     as such
+--
+--   - if adding a .hs or .lhs suffix yields the name of an existing file,
+--     then use that
+--
+--   - If it looks like a module name, interpret it as such
+--
+--   - otherwise, this function throws a 'GhcException'.
+guessTargetId :: GhcMonad m => String -> m TargetId
+guessTargetId str
    | isHaskellSrcFilename file
-   = target (TargetFile file Nothing)
+   = pure (TargetFile file Nothing)
    | otherwise
    = do exists <- liftIO $ doesFileExist hs_file
         if exists
-           then target (TargetFile hs_file Nothing)
+           then pure (TargetFile hs_file Nothing)
            else do
         exists <- liftIO $ doesFileExist lhs_file
         if exists
-           then target (TargetFile lhs_file Nothing)
+           then pure (TargetFile lhs_file Nothing)
            else do
         if looksLikeModuleName file
-           then target (TargetModule (mkModuleName file))
+           then pure (TargetModule (mkModuleName file))
            else do
         dflags <- getDynFlags
         liftIO $ throwGhcExceptionIO
@@ -1121,16 +1273,12 @@ guessTarget str mUnitId Nothing
                  text "target" <+> quotes (text file) <+>
                  text "is not a module name or a source file"))
      where
-         (file,obj_allowed)
-                | '*':rest <- str = (rest, False)
-                | otherwise       = (str,  True)
+        file
+          | '*':rest <- str = rest
+          | otherwise       = str
 
-         hs_file  = file <.> "hs"
-         lhs_file = file <.> "lhs"
-
-         target tid = do
-           tuid <- unitIdOrHomeUnit mUnitId
-           pure $ Target tid obj_allowed tuid Nothing
+        hs_file  = file <.> "hs"
+        lhs_file = file <.> "lhs"
 
 -- | Unwrap 'UnitId' or retrieve the 'UnitId'
 -- of the current 'HomeUnit'.
@@ -1251,11 +1399,11 @@ type TypecheckedSource = LHsBinds GhcTc
 --
 -- This function ignores boot modules and requires that there is only one
 -- non-boot module with the given name.
-getModSummary :: GhcMonad m => ModuleName -> m ModSummary
+getModSummary :: GhcMonad m => Module -> m ModSummary
 getModSummary mod = do
    mg <- liftM hsc_mod_graph getSession
    let mods_by_name = [ ms | ms <- mgModSummaries mg
-                      , ms_mod_name ms == mod
+                      , ms_mod ms == mod
                       , isBootSummary ms == NotBoot ]
    case mods_by_name of
      [] -> do dflags <- getDynFlags
@@ -1286,7 +1434,9 @@ typecheckModule pmod = do
  liftIO $ do
    let ms          = modSummary pmod
    let lcl_dflags  = ms_hspp_opts ms -- take into account pragmas (OPTIONS_GHC, etc.)
-   let lcl_hsc_env = hscSetFlags lcl_dflags hsc_env
+   let lcl_hsc_env =
+          hscSetFlags lcl_dflags $
+          hscSetActiveUnitId (toUnitId $ moduleUnit $ ms_mod ms) hsc_env
    let lcl_logger  = hsc_logger lcl_hsc_env
    (tc_gbl_env, rn_info) <- hscTypecheckRename lcl_hsc_env ms $
                         HsParsedModule { hpm_module = parsedSource pmod,
@@ -1431,14 +1581,24 @@ getModuleGraph = liftM hsc_mod_graph getSession
 -- | Return @True@ \<==> module is loaded.
 isLoaded :: GhcMonad m => ModuleName -> m Bool
 isLoaded m = withSession $ \hsc_env -> liftIO $ do
-  hmi <- lookupHpt (hsc_HPT hsc_env) m
-  return $! isJust hmi
+  hmis <- HUG.lookupAllHug (hsc_HUG hsc_env) m
+  return $! not (null hmis)
 
+-- | Check whether a 'ModuleName' is found in the 'HomePackageTable'
+-- for the given 'UnitId'.
 isLoadedModule :: GhcMonad m => UnitId -> ModuleName -> m Bool
 isLoadedModule uid m = withSession $ \hsc_env -> liftIO $ do
   hmi <- HUG.lookupHug (hsc_HUG hsc_env) uid m
   return $! isJust hmi
 
+-- | Check whether 'Module' is part of the 'HomeUnitGraph'.
+--
+-- Similar to 'isLoadedModule', but for 'Module's.
+isLoadedHomeModule :: GhcMonad m => Module -> m Bool
+isLoadedHomeModule m = withSession $ \hsc_env -> liftIO $ do
+  hmi <- HUG.lookupHugByModule m (hsc_HUG hsc_env)
+  return $! isJust hmi
+
 -- | Return the bindings for the current interactive session.
 getBindings :: GhcMonad m => m [TyThing]
 getBindings = withSession $ \hsc_env ->
@@ -1470,7 +1630,7 @@ data ModuleInfo = ModuleInfo {
 -- | Request information about a loaded 'Module'
 getModuleInfo :: GhcMonad m => Module -> m (Maybe ModuleInfo)  -- XXX: Maybe X
 getModuleInfo mdl = withSession $ \hsc_env -> do
-  if moduleUnitId mdl `S.member` hsc_all_home_unit_ids hsc_env
+  if HUG.memberHugUnit (moduleUnit mdl) (hsc_HUG hsc_env)
         then liftIO $ getHomeModuleInfo hsc_env mdl
         else liftIO $ getPackageModuleInfo hsc_env mdl
 
@@ -1826,6 +1986,50 @@ lookupLoadedHomeModule uid mod_name = withSession $ \hsc_env -> liftIO $ do
     Just mod_info      -> return (Just (mi_module (hm_iface mod_info)))
     _not_a_home_module -> return Nothing
 
+-- | Lookup the given 'ModuleName' in the 'HomeUnitGraph'.
+--
+-- Returns 'Nothing' if no 'Module' has the given 'ModuleName'.
+-- Otherwise, returns all 'Module's that have the given 'ModuleName'.
+--
+-- A 'ModuleName' is generally not enough to uniquely identify a 'Module', since
+-- there can be multiple units exposing the same 'ModuleName' in the case of
+-- multiple home units.
+-- Thus, this function may return more than one possible 'Module'.
+-- We leave it up to the caller to decide how to handle the ambiguity.
+-- For example, GHCi may prompt the user to clarify which 'Module' is the correct one.
+--
+lookupLoadedHomeModuleByModuleName :: GhcMonad m => ModuleName -> m (Maybe [Module])
+lookupLoadedHomeModuleByModuleName mod_name = withSession $ \hsc_env -> liftIO $ do
+  trace_if (hsc_logger hsc_env) (text "lookupLoadedHomeModuleByModuleName" <+> ppr mod_name)
+  HUG.lookupAllHug (hsc_HUG hsc_env) mod_name >>= \case
+    []        -> return Nothing
+    mod_infos -> return (Just (mi_module . hm_iface <$> mod_infos))
+
+-- | Given a 'ModuleName' and 'PkgQual', lookup all 'Module's that may fit the criteria.
+--
+-- Identically to 'lookupLoadedHomeModuleByModuleName', there may be more than one
+-- 'Module' in the 'HomeUnitGraph' that has the given 'ModuleName'.
+--
+-- The result is guaranteed to be non-empty, if no 'Module' can be found,
+-- this function throws an error.
+lookupAllQualifiedModuleNames :: GhcMonad m => PkgQual -> ModuleName -> m [Module]
+lookupAllQualifiedModuleNames NoPkgQual mod_name = withSession $ \hsc_env -> do
+  home <- lookupLoadedHomeModuleByModuleName mod_name
+  case home of
+    Just m  -> return m
+    Nothing -> liftIO $ do
+      let fc     = hsc_FC hsc_env
+      let units  = hsc_units hsc_env
+      let dflags = hsc_dflags hsc_env
+      let fopts  = initFinderOpts dflags
+      res <- findExposedPackageModule fc fopts units mod_name NoPkgQual
+      case res of
+        Found _ m -> return [m]
+        err       -> throwOneError $ noModError hsc_env noSrcSpan mod_name err
+lookupAllQualifiedModuleNames pkgqual mod_name = do
+  m <- findQualifiedModule pkgqual mod_name
+  pure [m]
+
 -- | Check that a module is safe to import (according to Safe Haskell).
 --
 -- We return True to indicate the import is safe and False otherwise


=====================================
compiler/GHC/Driver/Downsweep.hs
=====================================
@@ -281,7 +281,7 @@ downsweepInteractiveImports hsc_env ic = unsafeInterleaveIO $ do
     -- A simple edge to a module from the same home unit
     mkEdge (IIModule n) =
       let unitId = homeUnitId $ hsc_home_unit hsc_env
-      in (unitId, NormalLevel, NoPkgQual, GWIB (noLoc n) NotBoot)
+      in (unitId, NormalLevel, NoPkgQual, GWIB (noLoc $ moduleName n) NotBoot)
     -- A complete import statement
     mkEdge (IIDecl i) =
       let lvl = convImportLevel (ideclLevelSpec i)


=====================================
compiler/GHC/Driver/Session.hs
=====================================
@@ -162,6 +162,7 @@ module GHC.Driver.Session (
         updOptLevel,
         setTmpDir,
         setUnitId,
+        setHomeUnitId,
 
         TurnOnFlag,
         turnOn,
@@ -3114,6 +3115,9 @@ parseUnitArg =
 setUnitId :: String -> DynFlags -> DynFlags
 setUnitId p d = d { homeUnitId_ = stringToUnitId p }
 
+setHomeUnitId :: UnitId -> DynFlags -> DynFlags
+setHomeUnitId p d = d { homeUnitId_ = p }
+
 setWorkingDirectory :: String -> DynFlags -> DynFlags
 setWorkingDirectory p d = d { workingDirectory =  Just p }
 


=====================================
compiler/GHC/Iface/Load.hs
=====================================
@@ -918,12 +918,10 @@ findAndReadIface hsc_env doc_str mod wanted_mod hi_boot_file = do
           case mb_found of
               InstalledFound loc -> do
                   -- See Note [Home module load error]
-                  case mhome_unit of
-                    Just home_unit
-                      | isHomeInstalledModule home_unit mod
-                      , not (isOneShot (ghcMode dflags))
-                      -> return (Failed (HomeModError mod loc))
-                    _ -> do
+                  if HUG.memberHugUnitId (moduleUnit mod) (hsc_HUG hsc_env)
+                      && not (isOneShot (ghcMode dflags))
+                    then return (Failed (HomeModError mod loc))
+                    else do
                         r <- read_file logger name_cache unit_state dflags wanted_mod (ml_hi_file loc)
                         case r of
                           Failed err


=====================================
compiler/GHC/Rename/Unbound.hs
=====================================
@@ -364,7 +364,7 @@ importSuggestions looking_for ic currMod imports rdr_name
   pick_interactive :: InteractiveImport -> Bool
   pick_interactive (IIDecl d)   | mod_name == Just (unLoc (ideclName d)) = True
                                 | mod_name == fmap unLoc (ideclAs d) = True
-  pick_interactive (IIModule m) | mod_name == Just m = True
+  pick_interactive (IIModule m) | mod_name == Just (moduleName m) = True
   pick_interactive _ = False
 
   -- We want to keep only one for each original module; preferably one with an


=====================================
compiler/GHC/Runtime/Context.hs
=====================================
@@ -115,6 +115,51 @@ The details are a bit tricky though:
   modules.
 
 
+Note [Relation between the 'InteractiveContext' and 'interactiveGhciUnitId']
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The 'InteractiveContext' is used to store 'DynFlags', 'Plugins' and similar
+information about the so-called interactive "home unit". We are using
+quotes here, since, originally, GHC wasn't aware of more than one 'HomeUnitEnv's.
+So the 'InteractiveContext' was a hack/solution to have 'DynFlags' and 'Plugins'
+independent of the 'DynFlags' and 'Plugins' stored in 'HscEnv'.
+Nowadays, GHC has support for multiple home units via the 'HomeUnitGraph', thus,
+this part of the 'InteractiveContext' is strictly speaking redundant, as we
+can simply manage one 'HomeUnitEnv' for the 'DynFlags' and 'Plugins' that are
+currently stored in the 'InteractiveContext'.
+
+As a matter of fact, that's exactly what we do nowadays.
+That means, we can also lift other restrictions in the future, for example
+allowing @:seti@ commands to modify the package-flags, since we now have a
+separate 'UnitState' for the interactive session.
+However, we did not rip out 'ic_dflags' and 'ic_plugins', yet, as it makes
+it easier to access them for functions that want to use the interactive 'DynFlags',
+such as 'runInteractiveHsc' and 'mkInteractiveHscEnv', without having to look that
+information up in the 'HomeUnitGraph'.
+It is reasonable to change this in the future, and remove 'ic_dflags' and 'ic_plugins'.
+
+We keep 'ic_dflags' and 'ic_plugins' around, but we also store a 'HomeUnitEnv'
+for the 'DynFlags' and 'Plugins' of the interactive session.
+
+It is important to keep the 'DynFlags' in these two places consistent.
+
+In other words, whenever you update the 'DynFlags' of the 'interactiveGhciUnitId'
+in the 'HscEnv', then you also need to update the 'DynFlags' of the
+'InteractiveContext'.
+The easiest way to update them is via 'setInteractiveDynFlags'.
+However, careful, footgun! It is very easy to call 'setInteractiveDynFlags'
+and forget to call 'normaliseInteractiveDynFlags' on the 'DynFlags' in the
+'HscEnv'! This is important, because you may, accidentally, have enabled
+Language Extensions that are not supported in the interactive ghc session,
+which we do not want.
+
+To summarise, the 'ic_dflags' and 'ic_plugins' are currently used to
+conveniently cache them for easy access.
+The 'ic_dflags' must be identical to the 'DynFlags' stored in the 'HscEnv'
+for the 'HomeUnitEnv' identified by 'interactiveGhciUnitId'.
+
+See Note [Multiple Home Units aware GHCi] for the design and rationale for
+the current 'interactiveGhciUnitId'.
+
 Note [Interactively-bound Ids in GHCi]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 The Ids bound by previous Stmts in GHCi are currently
@@ -296,7 +341,7 @@ data InteractiveImport
       -- ^ Bring the exports of a particular module
       -- (filtered by an import decl) into scope
 
-  | IIModule ModuleName
+  | IIModule Module
       -- ^ Bring into scope the entire top-level envt of
       -- of this module, including the things imported
       -- into it.


=====================================
compiler/GHC/Runtime/Eval.hs
=====================================
@@ -119,7 +119,6 @@ import GHC.Unit
 import GHC.Unit.Module.Graph
 import GHC.Unit.Module.ModIface
 import GHC.Unit.Home.ModInfo
-import GHC.Unit.Home.PackageTable
 
 import GHC.Tc.Module ( runTcInteractive, tcRnTypeSkolemising, loadUnqualIfaces )
 import GHC.Tc.Solver (simplifyWantedsTcM)
@@ -823,16 +822,17 @@ findGlobalRdrEnv hsc_env imports
     idecls :: [LImportDecl GhcPs]
     idecls = [noLocA d | IIDecl d <- imports]
 
-    imods :: [ModuleName]
+    imods :: [Module]
     imods = [m | IIModule m <- imports]
 
-    mkEnv mod = mkTopLevEnv hsc_env mod >>= \case
-      Left err -> pure $ Left (mod, err)
-      Right env -> pure $ Right env
+    mkEnv mod = do
+      mkTopLevEnv hsc_env mod >>= \case
+        Left err -> pure $ Left (moduleName mod, err)
+        Right env -> pure $ Right env
 
-mkTopLevEnv :: HscEnv -> ModuleName -> IO (Either String GlobalRdrEnv)
+mkTopLevEnv :: HscEnv -> Module -> IO (Either String GlobalRdrEnv)
 mkTopLevEnv hsc_env modl
-  = lookupHpt hpt modl >>= \case
+  = HUG.lookupHugByModule modl hug >>= \case
       Nothing -> pure $ Left "not a home module"
       Just details ->
          case mi_top_env (hm_iface details) of
@@ -857,7 +857,7 @@ mkTopLevEnv hsc_env modl
                   let exports_env = mkGlobalRdrEnv $ gresFromAvails hsc_env Nothing (getDetOrdAvails exports)
                   pure $ Right $ plusGlobalRdrEnv imports_env exports_env
   where
-    hpt = hsc_HPT hsc_env
+    hug = hsc_HUG hsc_env
 
 -- | Get the interactive evaluation context, consisting of a pair of the
 -- set of modules from which we take the full top-level scope, and the set
@@ -870,11 +870,9 @@ getContext = withSession $ \HscEnv{ hsc_IC=ic } ->
 -- its full top-level scope available.
 moduleIsInterpreted :: GhcMonad m => Module -> m Bool
 moduleIsInterpreted modl = withSession $ \h ->
- if notHomeModule (hsc_home_unit h) modl
-        then return False
-        else liftIO (HUG.lookupHugByModule modl (hsc_HUG h)) >>= \case
-              Just hmi       -> return (isJust $ homeModInfoByteCode hmi)
-              _not_a_home_module -> return False
+  liftIO (HUG.lookupHugByModule modl (hsc_HUG h)) >>= \case
+    Just hmi           -> return (isJust $ homeModInfoByteCode hmi)
+    _not_a_home_module -> return False
 
 -- | Looks up an identifier in the current interactive context (for :info)
 -- Filter the instances by the ones whose tycons (or classes resp)


=====================================
compiler/GHC/StgToByteCode.hs
=====================================
@@ -78,7 +78,7 @@ import Control.Monad
 import Data.Char
 
 import GHC.Unit.Module
-import GHC.Unit.Home.PackageTable (lookupHpt)
+import qualified GHC.Unit.Home.Graph as HUG
 
 import Data.Array
 import Data.Coerce (coerce)
@@ -458,8 +458,7 @@ schemeER_wrk d p rhs = schemeE d 0 p rhs
 -- If that is 'Nothing', consider breakpoints to be disabled and skip the
 -- instruction.
 --
--- If the breakpoint is inlined from another module, look it up in the home
--- package table.
+-- If the breakpoint is inlined from another module, look it up in the HUG (home unit graph).
 -- If the module doesn't exist there, or its module pointer is null (which means
 -- that the 'ModBreaks' value is uninitialized), skip the instruction.
 break_info ::
@@ -472,7 +471,7 @@ break_info hsc_env mod current_mod current_mod_breaks
   | mod == current_mod
   = pure $ check_mod_ptr =<< current_mod_breaks
   | otherwise
-  = ioToBc (lookupHpt (hsc_HPT hsc_env) (moduleName mod)) >>= \case
+  = ioToBc (HUG.lookupHugByModule mod (hsc_HUG hsc_env)) >>= \case
       Just hp -> pure $ check_mod_ptr (getModBreaks hp)
       Nothing -> pure Nothing
   where


=====================================
compiler/GHC/StgToJS/Linker/Linker.hs
=====================================
@@ -461,7 +461,7 @@ computeLinkDependencies cfg unit_env link_spec finder_opts finder_cache ar_cache
 
   -- all the units we want to link together, without their dependencies
   let root_units = filter (/= ue_currentUnit unit_env)
-                   $ filter (/= interactiveUnitId)
+                   $ filter (/= interactiveUnitId) -- TODO @fendor: what does this do?
                    $ nub
                    $ rts_wired_units ++ reverse obj_units ++ reverse units
 


=====================================
compiler/GHC/Tc/Module.hs
=====================================
@@ -150,7 +150,6 @@ import GHC.Types.Basic hiding( SuccessFlag(..) )
 import GHC.Types.Annotations
 import GHC.Types.SrcLoc
 import GHC.Types.SourceFile
-import GHC.Types.PkgQual
 import qualified GHC.LanguageExtensions as LangExt
 
 import GHC.Unit.Env as UnitEnv
@@ -2119,15 +2118,18 @@ runTcInteractive hsc_env thing_inside
                                                  , let local_gres = filter isLocalGRE gres
                                                  , not (null local_gres) ]) ]
 
-       ; let getOrphans m mb_pkg = fmap (\iface -> mi_module iface
-                                          : dep_orphs (mi_deps iface))
-                                 (loadSrcInterface (text "runTcInteractive") m
-                                                   NotBoot mb_pkg)
+       ; let getOrphansForModuleName m mb_pkg = do
+              iface <- loadSrcInterface (text "runTcInteractive") m NotBoot mb_pkg
+              pure $ mi_module iface : dep_orphs (mi_deps iface)
+
+             getOrphansForModule m = do
+              iface <- loadModuleInterface (text "runTcInteractive") m
+              pure $ mi_module iface : dep_orphs (mi_deps iface)
 
        ; !orphs <- fmap (force . concat) . forM (ic_imports icxt) $ \i ->
             case i of                   -- force above: see #15111
-                IIModule n -> getOrphans n NoPkgQual
-                IIDecl i   -> getOrphans (unLoc (ideclName i))
+                IIModule n -> getOrphansForModule n
+                IIDecl i   -> getOrphansForModuleName (unLoc (ideclName i))
                                          (renameRawPkgQual (hsc_unit_env hsc_env) (unLoc $ ideclName i) (ideclPkgQual i))
 
 


=====================================
compiler/GHC/Types/Name/Ppr.hs
=====================================
@@ -13,6 +13,7 @@ import GHC.Data.FastString
 
 import GHC.Unit
 import GHC.Unit.Env
+import qualified GHC.Unit.Home.Graph as HUG
 
 import GHC.Types.Name
 import GHC.Types.Name.Reader
@@ -72,12 +73,11 @@ mkNamePprCtx :: Outputable info => PromotionTickContext -> UnitEnv -> GlobalRdrE
 mkNamePprCtx ptc unit_env env
  = QueryQualify
       (mkQualName env)
-      (mkQualModule unit_state home_unit)
+      (mkQualModule unit_state unit_env)
       (mkQualPackage unit_state)
       (mkPromTick ptc env)
   where
   unit_state = ue_homeUnitState unit_env
-  home_unit  = ue_homeUnit unit_env
 
 mkQualName :: Outputable info => GlobalRdrEnvX info -> QueryQualifyName
 mkQualName env = qual_name where
@@ -215,10 +215,12 @@ Side note (int-index):
 -- | Creates a function for formatting modules based on two heuristics:
 -- (1) if the module is the current module, don't qualify, and (2) if there
 -- is only one exposed package which exports this module, don't qualify.
-mkQualModule :: UnitState -> Maybe HomeUnit -> QueryQualifyModule
-mkQualModule unit_state mhome_unit mod
-     | Just home_unit <- mhome_unit
-     , isHomeModule home_unit mod = False
+mkQualModule :: UnitState -> UnitEnv -> QueryQualifyModule
+mkQualModule unit_state unitEnv mod
+       -- Check whether the unit of the module is in the HomeUnitGraph.
+       -- If it is, then we consider this 'mod' to be "local" and don't
+       -- want to qualify it.
+     | HUG.memberHugUnit (moduleUnit mod) (ue_home_unit_graph unitEnv) = False
 
      | [(_, pkgconfig)] <- lookup,
        mkUnit pkgconfig == moduleUnit mod


=====================================
compiler/GHC/Unit/Env.hs
=====================================
@@ -241,7 +241,7 @@ isUnitEnvInstalledModule ue m = maybe False (`isHomeInstalledModule` m) hu
 -- -------------------------------------------------------
 
 ue_findHomeUnitEnv :: HasDebugCallStack => UnitId -> UnitEnv -> HomeUnitEnv
-ue_findHomeUnitEnv uid e = case HUG.lookupHugUnit uid (ue_home_unit_graph e) of
+ue_findHomeUnitEnv uid e = case HUG.lookupHugUnitId uid (ue_home_unit_graph e) of
   Nothing -> pprPanic "Unit unknown to the internal unit environment"
               $  text "unit (" <> ppr uid <> text ")"
               $$ ppr (HUG.allUnits (ue_home_unit_graph e))
@@ -311,7 +311,7 @@ ue_unitHomeUnit uid = expectJust . ue_unitHomeUnit_maybe uid
 
 ue_unitHomeUnit_maybe :: UnitId -> UnitEnv -> Maybe HomeUnit
 ue_unitHomeUnit_maybe uid ue_env =
-  HUG.homeUnitEnv_home_unit =<< HUG.lookupHugUnit uid (ue_home_unit_graph ue_env)
+  HUG.homeUnitEnv_home_unit =<< HUG.lookupHugUnitId uid (ue_home_unit_graph ue_env)
 
 -- -------------------------------------------------------
 -- Query and modify the currently active unit
@@ -319,7 +319,7 @@ ue_unitHomeUnit_maybe uid ue_env =
 
 ue_currentHomeUnitEnv :: HasDebugCallStack => UnitEnv -> HomeUnitEnv
 ue_currentHomeUnitEnv e =
-  case HUG.lookupHugUnit (ue_currentUnit e) (ue_home_unit_graph e) of
+  case HUG.lookupHugUnitId (ue_currentUnit e) (ue_home_unit_graph e) of
     Just unitEnv -> unitEnv
     Nothing -> pprPanic "packageNotFound" $
       (ppr $ ue_currentUnit e) $$ ppr (HUG.allUnits (ue_home_unit_graph e))
@@ -389,7 +389,7 @@ ue_transitiveHomeDeps uid e =
 -- FIXME: Shouldn't this be a proper assertion only used in debug mode?
 assertUnitEnvInvariant :: HasDebugCallStack => UnitEnv -> UnitEnv
 assertUnitEnvInvariant u =
-  case HUG.lookupHugUnit (ue_current_unit u) (ue_home_unit_graph u) of
+  case HUG.lookupHugUnitId (ue_current_unit u) (ue_home_unit_graph u) of
     Just _ -> u
     Nothing ->
       pprPanic "invariant" (ppr (ue_current_unit u) $$ ppr (HUG.allUnits (ue_home_unit_graph u)))


=====================================
compiler/GHC/Unit/Home/Graph.hs
=====================================
@@ -34,7 +34,10 @@ module GHC.Unit.Home.Graph
   , lookupHug
   , lookupHugByModule
   , lookupHugUnit
-
+  , lookupHugUnitId
+  , lookupAllHug
+  , memberHugUnit
+  , memberHugUnitId
   -- ** Reachability
   , transitiveHomeDeps
 
@@ -62,6 +65,8 @@ module GHC.Unit.Home.Graph
   , unitEnv_insert
   , unitEnv_new
   , unitEnv_lookup
+  , unitEnv_traverseWithKey
+  , unitEnv_assocs
   ) where
 
 import GHC.Prelude
@@ -73,6 +78,7 @@ import GHC.Unit.Home.PackageTable
 import GHC.Unit.Module
 import GHC.Unit.Module.ModIface
 import GHC.Unit.State
+import GHC.Utils.Monad (mapMaybeM)
 import GHC.Utils.Outputable
 import GHC.Utils.Panic
 
@@ -222,7 +228,7 @@ updateUnitFlags uid f = unitEnv_adjust update uid
 -- | Compute the transitive closure of a unit in the 'HomeUnitGraph'.
 -- If the argument unit is not present in the graph returns Nothing.
 transitiveHomeDeps :: UnitId -> HomeUnitGraph -> Maybe [UnitId]
-transitiveHomeDeps uid hug = case lookupHugUnit uid hug of
+transitiveHomeDeps uid hug = case lookupHugUnitId uid hug of
   Nothing -> Nothing
   Just hue -> Just $
     Set.toList (loop (Set.singleton uid) (homeUnitDepends (homeUnitEnv_units hue)))
@@ -234,7 +240,7 @@ transitiveHomeDeps uid hug = case lookupHugUnit uid hug of
           let hue = homeUnitDepends
                     . homeUnitEnv_units
                     . expectJust
-                    $ lookupHugUnit uid hug
+                    $ lookupHugUnitId uid hug
           in loop (Set.insert uid acc) (hue ++ uids)
 
 --------------------------------------------------------------------------------
@@ -246,21 +252,47 @@ transitiveHomeDeps uid hug = case lookupHugUnit uid hug of
 lookupHug :: HomeUnitGraph -> UnitId -> ModuleName -> IO (Maybe HomeModInfo)
 lookupHug hug uid mod = do
   case unitEnv_lookup_maybe uid hug of
-    -- Really, here we want "lookup HPT" rather than unitEnvLookup
     Nothing -> pure Nothing
     Just hue -> lookupHpt (homeUnitEnv_hpt hue) mod
 
 -- | Lookup the 'HomeModInfo' of a 'Module' in the 'HomeUnitGraph' (via the 'HomePackageTable' of the corresponding unit)
 lookupHugByModule :: Module -> HomeUnitGraph -> IO (Maybe HomeModInfo)
-lookupHugByModule mod hug
-  | otherwise = do
-      case unitEnv_lookup_maybe (toUnitId $ moduleUnit mod) hug of
-        Nothing -> pure Nothing
-        Just env -> lookupHptByModule (homeUnitEnv_hpt env) mod
+lookupHugByModule mod hug =
+  case lookupHugUnit (moduleUnit mod) hug of
+    Nothing -> pure Nothing
+    Just env -> lookupHptByModule (homeUnitEnv_hpt env) mod
+
+-- | Lookup all 'HomeModInfo' that have the same 'ModuleName' as the given 'ModuleName'.
+-- 'ModuleName's are not unique in the case of multiple home units, so there can be
+-- more than one possible 'HomeModInfo'.
+--
+-- You should always prefer 'lookupHug' and 'lookupHugByModule' when possible.
+lookupAllHug :: HomeUnitGraph -> ModuleName -> IO [HomeModInfo]
+lookupAllHug hug mod = mapMaybeM (\uid -> lookupHug hug uid mod) (Set.toList $ unitEnv_keys hug)
 
 -- | Lookup a 'HomeUnitEnv' by 'UnitId' in a 'HomeUnitGraph'
-lookupHugUnit :: UnitId -> HomeUnitGraph -> Maybe HomeUnitEnv
-lookupHugUnit = unitEnv_lookup_maybe
+lookupHugUnitId :: UnitId -> HomeUnitGraph -> Maybe HomeUnitEnv
+lookupHugUnitId = unitEnv_lookup_maybe
+
+-- | Check whether the 'UnitId' is present in the 'HomeUnitGraph'
+memberHugUnitId :: UnitId -> HomeUnitGraph -> Bool
+memberHugUnitId u = isJust . lookupHugUnitId u
+
+-- | Lookup up the 'HomeUnitEnv' by the 'Unit' in the 'HomeUnitGraph'.
+-- If the 'Unit' can be turned into a 'UnitId', we behave identical to 'lookupHugUnitId'.
+--
+-- A 'HoleUnit' is never part of the 'HomeUnitGraph', only instantiated 'Unit's
+lookupHugUnit :: Unit -> HomeUnitGraph -> Maybe HomeUnitEnv
+lookupHugUnit unit hug =
+  if isHoleUnit unit
+    then Nothing
+    else lookupHugUnitId (toUnitId unit) hug
+
+-- | Check whether the 'Unit' is present in the 'HomeUnitGraph'
+--
+-- A 'HoleUnit' is never part of the 'HomeUnitGraph', only instantiated 'Unit's
+memberHugUnit :: Unit -> HomeUnitGraph -> Bool
+memberHugUnit u = isJust . lookupHugUnit u
 
 --------------------------------------------------------------------------------
 -- * Internal representation map
@@ -313,6 +345,13 @@ unitEnv_foldWithKey f z (UnitEnvGraph g)= Map.foldlWithKey' f z g
 unitEnv_lookup :: UnitEnvGraphKey -> UnitEnvGraph v -> v
 unitEnv_lookup u env = expectJust $ unitEnv_lookup_maybe u env
 
+unitEnv_traverseWithKey :: Applicative f => (UnitEnvGraphKey -> a -> f b) -> UnitEnvGraph a -> f (UnitEnvGraph b)
+unitEnv_traverseWithKey f unitEnv =
+  UnitEnvGraph <$> Map.traverseWithKey f (unitEnv_graph unitEnv)
+
+unitEnv_assocs :: UnitEnvGraph a -> [(UnitEnvGraphKey, a)]
+unitEnv_assocs (UnitEnvGraph x) = Map.assocs x
+
 --------------------------------------------------------------------------------
 -- * Utilities
 --------------------------------------------------------------------------------


=====================================
compiler/GHC/Unit/Types.hs
=====================================
@@ -63,12 +63,16 @@ module GHC.Unit.Types
    , mainUnitId
    , thisGhcUnitId
    , interactiveUnitId
+   , interactiveGhciUnitId
+   , interactiveSessionUnitId
 
    , ghcInternalUnit
    , rtsUnit
    , mainUnit
    , thisGhcUnit
    , interactiveUnit
+   , interactiveGhciUnit
+   , interactiveSessionUnit
 
    , isInteractiveModule
    , wiredInUnitIds
@@ -588,20 +592,24 @@ Make sure you change 'GHC.Unit.State.findWiredInUnits' if you add an entry here.
 -}
 
 ghcInternalUnitId, rtsUnitId,
-  mainUnitId, thisGhcUnitId, interactiveUnitId :: UnitId
+  mainUnitId, thisGhcUnitId, interactiveUnitId, interactiveGhciUnitId, interactiveSessionUnitId :: UnitId
 
 ghcInternalUnit, rtsUnit,
-  mainUnit, thisGhcUnit, interactiveUnit :: Unit
+  mainUnit, thisGhcUnit, interactiveUnit, interactiveGhciUnit, interactiveSessionUnit :: Unit
 
 ghcInternalUnitId = UnitId (fsLit "ghc-internal")
 rtsUnitId         = UnitId (fsLit "rts")
 thisGhcUnitId     = UnitId (fsLit cProjectUnitId) -- See Note [GHC's Unit Id]
 interactiveUnitId = UnitId (fsLit "interactive")
+interactiveGhciUnitId = UnitId (fsLit "interactive-ghci")
+interactiveSessionUnitId = UnitId (fsLit "interactive-session")
 
 ghcInternalUnit   = RealUnit (Definite ghcInternalUnitId)
 rtsUnit           = RealUnit (Definite rtsUnitId)
 thisGhcUnit       = RealUnit (Definite thisGhcUnitId)
 interactiveUnit   = RealUnit (Definite interactiveUnitId)
+interactiveGhciUnit = RealUnit (Definite interactiveGhciUnitId)
+interactiveSessionUnit = RealUnit (Definite interactiveSessionUnitId)
 
 -- | This is the package Id for the current program.  It is the default
 -- package Id if you don't specify a package name.  We don't add this prefix


=====================================
ghc/GHCi/UI.hs
=====================================
@@ -113,6 +113,7 @@ import GHC.Utils.Misc
 import qualified GHC.LanguageExtensions as LangExt
 import qualified GHC.Data.Strict as Strict
 import GHC.Types.Error
+import qualified GHC.Unit.Home.Graph as HUG
 
 -- Haskell Libraries
 import System.Console.Haskeline as Haskeline
@@ -129,6 +130,7 @@ import Data.Array
 import qualified Data.ByteString.Char8 as BS
 import Data.Char
 import Data.Function
+import qualified Data.Foldable as Foldable
 import Data.IORef ( IORef, modifyIORef, newIORef, readIORef, writeIORef )
 import Data.List ( find, intercalate, intersperse,
                    isPrefixOf, isSuffixOf, nub, partition, sort, sortBy, (\\) )
@@ -204,31 +206,31 @@ ghciCommands = map mkCmd [
   -- Hugs users are accustomed to :e, so make sure it doesn't overlap
   ("?",         keepGoing help,                 noCompletion),
   ("add",       keepGoingPaths addModule,       completeFilename),
-  ("abandon",   keepGoing abandonCmd,           noCompletion),
-  ("break",     keepGoing breakCmd,             completeBreakpoint),
-  ("back",      keepGoing backCmd,              noCompletion),
+  ("abandon",   keepGoing  abandonCmd,          noCompletion),
+  ("break",     keepGoing  breakCmd,            completeBreakpoint),
+  ("back",      keepGoing  backCmd,             noCompletion),
   ("browse",    keepGoing' (browseCmd False),   completeModule),
   ("browse!",   keepGoing' (browseCmd True),    completeModule),
-  ("cd",        keepGoingMulti' changeDirectory,     completeFilename),
-  ("continue",  keepGoing continueCmd,          noCompletion),
+  ("cd",        keepGoing' changeDirectory,     completeFilename),
+  ("continue",  keepGoing' continueCmd,         noCompletion),
   ("cmd",       keepGoing cmdCmd,               completeExpression),
   ("def",       keepGoing (defineMacro False),  completeExpression),
   ("def!",      keepGoing (defineMacro True),   completeExpression),
   ("delete",    keepGoing deleteCmd,            noCompletion),
   ("disable",   keepGoing disableCmd,           noCompletion),
   ("doc",       keepGoing' docCmd,              completeIdentifier),
-  ("edit",      keepGoingMulti' editFile,            completeFilename),
+  ("edit",      keepGoing' editFile,            completeFilename),
   ("enable",    keepGoing enableCmd,            noCompletion),
   ("force",     keepGoing forceCmd,             completeExpression),
   ("forward",   keepGoing forwardCmd,           noCompletion),
-  ("help",      keepGoingMulti help,                 noCompletion),
-  ("history",   keepGoingMulti historyCmd,           noCompletion),
-  ("info",      keepGoingMulti' (info False),        completeIdentifier),
-  ("info!",     keepGoingMulti' (info True),         completeIdentifier),
+  ("help",      keepGoing help,                 noCompletion),
+  ("history",   keepGoing historyCmd,           noCompletion),
+  ("info",      keepGoing' (info False),        completeIdentifier),
+  ("info!",     keepGoing' (info True),         completeIdentifier),
   ("issafe",    keepGoing' isSafeCmd,           completeModule),
   ("ignore",    keepGoing ignoreCmd,            noCompletion),
-  ("kind",      keepGoingMulti' (kindOfType False),  completeIdentifier),
-  ("kind!",     keepGoingMulti' (kindOfType True),   completeIdentifier),
+  ("kind",      keepGoing' (kindOfType False),  completeIdentifier),
+  ("kind!",     keepGoing' (kindOfType True),   completeIdentifier),
   ("load",      keepGoingPaths loadModule_,     completeHomeModuleOrFile),
   ("load!",     keepGoingPaths loadModuleDefer, completeHomeModuleOrFile),
   ("list",      keepGoing' listCmd,             noCompletion),
@@ -236,19 +238,19 @@ ghciCommands = map mkCmd [
   ("main",      keepGoing runMain,              completeFilename),
   ("print",     keepGoing printCmd,             completeExpression),
   ("quit",      quit,                           noCompletion),
-  ("reload",    keepGoingMulti' reloadModule,   noCompletion),
-  ("reload!",   keepGoingMulti' reloadModuleDefer,   noCompletion),
-  ("run",       keepGoing runRun,               completeFilename),
+  ("reload",    keepGoing' reloadModule,        noCompletion),
+  ("reload!",   keepGoing' reloadModuleDefer,   noCompletion),
+  ("run",       keepGoing' runRun,              completeFilename),
   ("script",    keepGoing' scriptCmd,           completeFilename),
-  ("set",       keepGoingMulti setCmd,          completeSetOptions),
-  ("seti",      keepGoingMulti setiCmd,         completeSeti),
-  ("show",      keepGoingMulti' showCmd,        completeShowOptions),
-  ("showi",     keepGoing showiCmd,             completeShowiOptions),
+  ("set",       keepGoing setCmd,               completeSetOptions),
+  ("seti",      keepGoing setiCmd,              completeSeti),
+  ("show",      keepGoing' showCmd,             completeShowOptions),
+  ("showi",     keepGoing  showiCmd,            completeShowiOptions),
   ("sprint",    keepGoing sprintCmd,            completeExpression),
   ("step",      keepGoing stepCmd,              completeIdentifier),
   ("steplocal", keepGoing stepLocalCmd,         completeIdentifier),
   ("stepmodule",keepGoing stepModuleCmd,        completeIdentifier),
-  ("type",      keepGoingMulti' typeOfExpr,          completeExpression),
+  ("type",      keepGoing' typeOfExpr,          completeExpression),
   ("trace",     keepGoing traceCmd,             completeExpression),
   ("unadd",     keepGoingPaths unAddModule,     completeFilename),
   ("undef",     keepGoing undefineMacro,        completeMacro),
@@ -316,24 +318,11 @@ showSDocForUserQualify doc = do
 keepGoing :: (String -> GHCi ()) -> (String -> InputT GHCi CmdExecOutcome)
 keepGoing a str = keepGoing' (lift . a) str
 
-keepGoingMulti :: (String -> GHCi ()) -> (String -> InputT GHCi CmdExecOutcome)
-keepGoingMulti a str = keepGoingMulti' (lift . a) str
-
 keepGoing' :: GhciMonad m => (a -> m ()) -> a -> m CmdExecOutcome
 keepGoing' a str = do
-  in_multi <- inMultiMode
-  if in_multi
-    then reportError GhciCommandNotSupportedInMultiMode
-    else a str
+  a str
   return CmdSuccess
 
--- For commands which are actually support in multi-mode, initially just :reload
-keepGoingMulti' :: GhciMonad m => (String -> m ()) -> String -> m CmdExecOutcome
-keepGoingMulti' a str = a str >> return CmdSuccess
-
-inMultiMode :: GhciMonad m => m Bool
-inMultiMode = multiMode <$> getGHCiState
-
 keepGoingPaths :: ([FilePath] -> InputT GHCi ()) -> (String -> InputT GHCi CmdExecOutcome)
 keepGoingPaths a str
  = do case toArgsNoLoc str of
@@ -489,9 +478,6 @@ default_args = []
 interactiveUI :: GhciSettings -> [(FilePath, Maybe UnitId, Maybe Phase)] -> Maybe [String]
               -> Ghc ()
 interactiveUI config srcs maybe_exprs = do
-   -- This is a HACK to make sure dynflags are not overwritten when setting
-   -- options. When GHCi is made properly multi component it should be removed.
-   modifySession (\env -> hscSetActiveUnitId (hscActiveUnitId env) env)
    -- HACK! If we happen to get into an infinite loop (eg the user
    -- types 'let x=x in x' at the prompt), then the thread will block
    -- on a blackhole, and become unreachable during GC.  The GC will
@@ -507,21 +493,7 @@ interactiveUI config srcs maybe_exprs = do
     -- Initialise buffering for the *interpreted* I/O system
    (nobuffering, flush) <- runInternal initInterpBuffering
 
-   -- The initial set of DynFlags used for interactive evaluation is the same
-   -- as the global DynFlags, plus -XExtendedDefaultRules and
-   -- -XNoMonomorphismRestriction.
-   -- See Note [Changing language extensions for interactive evaluation] #10857
-   dflags <- getDynFlags
-   let dflags' = (xopt_set_unlessExplSpec
-                      LangExt.ExtendedDefaultRules xopt_set)
-               . (xopt_set_unlessExplSpec
-                      LangExt.MonomorphismRestriction xopt_unset)
-               $ dflags
-   GHC.setInteractiveDynFlags dflags'
-   _ <- GHC.setProgramDynFlags
-               -- Set Opt_KeepGoing so that :reload loads as much as
-               -- possible
-               (gopt_set dflags Opt_KeepGoing)
+   installInteractiveHomeUnit
 
    -- Update the LogAction. Ensure we don't override the user's log action lest
    -- we break -ddump-json (#14078)
@@ -553,9 +525,6 @@ interactiveUI config srcs maybe_exprs = do
          case simpleImportDecl preludeModuleName of
            -- Set to True because Prelude is implicitly imported.
            impDecl@ImportDecl{ideclExt=ext} -> impDecl{ideclExt = ext{ideclImplicit=True}}
-   hsc_env <- GHC.getSession
-   let !in_multi = length (hsc_all_home_unit_ids hsc_env) > 1
-        -- We force this to make sure we don't retain the hsc_env when reloading
    empty_cache <- liftIO newIfaceCache
    startGHCi (runGHCi srcs maybe_exprs)
         GHCiState{ progname           = default_progname,
@@ -566,7 +535,6 @@ interactiveUI config srcs maybe_exprs = do
                    stop               = default_stop,
                    editor             = default_editor,
                    options            = [],
-                   multiMode          = in_multi,
                    localConfig        = SourceLocalConfig,
                    -- We initialize line number as 0, not 1, because we use
                    -- current line number while reporting errors which is
@@ -595,6 +563,243 @@ interactiveUI config srcs maybe_exprs = do
 
    return ()
 
+{-
+Note [Multiple Home Units aware GHCi]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+GHCi supported multiple home units up to a certain degree for quite a while now.
+The supported feature set was limited, due to a design impasse:
+One of the home units must be "active", e.g., there must be one 'HomeUnit'
+whose 'UnitId' is "active" which is returned when calling
+
+@'hscActiveUnitId' \<$\> 'getSession'@
+
+This makes sense in a GHC session, since you are always compiling a particular
+Module, but it makes less intuitive sense in an interactive session.
+Given an expression to evaluate, we can't easily tell in which "context" the expression
+should be parsed, typechecked and evaluated.
+That's why initially, most of GHCi features, except for `:reload`ing were disabled
+if the GHCi session had more than one 'HomeUnitEnv'.
+
+We lift this restriction, enabling all features of GHCi for the multiple home unit case.
+To do this, we fundamentally change the 'HomeUnitEnv' graph to be multiple home unit first.
+Instead of differentiating the case were we have a single home unit and multiple,
+we now always set up a multiple home unit session that scales seamlessly to an arbitrary
+amount of home units.
+
+We introduce two new 'HomeUnitEnv's that are always added to the 'HomeUnitGraph'.
+They are:
+
+The "interactive-ghci", called the 'interactiveGhciUnit', contains the same
+'DynFlags' that are used by the 'InteractiveContext' for interactive evaluation
+of expressions.
+This 'HomeUnitEnv' is only used on the prompt of GHCi, so we may refer to it as
+"interactive-prompt" unit.
+See Note [Relation between the 'InteractiveContext' and 'interactiveGhciUnitId']
+for discussing its role.
+
+And the 'interactive-session', called 'interactiveSessionUnit' or
+'interactiveSessionUnitId', which is used for loading Scripts into
+GHCi that are not 'Target's of any home unit, via `:load` or `:add`.
+
+Both of these "interactive" home units depend on all other 'HomeUnitEnv's that
+are passed as arguments on the cli.
+Additionally, the "interactive-ghci" unit depends on 'interactive-session'.
+
+We always evaluate expressions in the context of the
+"interactive-ghci" session.
+Since "interactive-ghci" depends on all home units, we can import any 'Module'
+from the other home units with ease.
+
+As we have a clear 'HomeUnitGraph' hierarchy, we can set 'interactiveGhciUnitId'
+as the active home unit for the full duration of the GHCi session.
+In GHCi, we always set 'interactiveGhciUnitId' to be the currently active home unit.
+
+=== Single Home Unit Case Diagram
+
+ Example:       ghci -this-unit-id main ...
+ Equivalent to: ghci -unit @unitA
+
+ ┌───────────────────┐        ┌─────────────────────┐
+ │ Interactive Prompt│        │ Interactive Session │
+ │                   │───────►│                     │
+ │  interactive-ghci │        │ interactive-session │
+ └────────┬──────────┘        └──────────┬──────────┘
+          │                              │
+          └───────────────┬──────────────┘
+                          │
+                          │
+                     ┌────▼───┐
+                     │ Unit A │
+                     │ main   │
+                     └────────┘
+
+
+=== Multi Home Unit Case Diagram
+
+ Example:       ghci -unit @unitA -unit @unitB -unit @unitC
+
+ ┌───────────────────┐        ┌─────────────────────┐
+ │ Interactive Prompt│        │ Interactive Session │
+ │                   │───────►│                     │
+ │  interactive-ghci │        │ interactive-session │
+ └────────┬──────────┘        └──────────┬──────────┘
+          │                              │
+          └───────────────┬──────────────┘
+                          │
+            ┌─────────────┼─────────────┐
+       ┌────▼───┐    ┌────▼───┐    ┌────▼───┐
+       │ Unit A │    │ Unit B │    │ Unit C │
+       │ a-0.0  │    │ b-0.0  │    │ c-0.0  │
+       └────────┘    └────────┘    └────────┘
+
+As we can see, this design can be scaled to an arbitrary number of Home Units.
+
+=== 'interactiveGhciUnit' Home Unit
+
+The 'interactiveGhciUnit' home unit is used for storing the 'DynFlags' of
+the interactive context.
+There is considerable overlap with the 'InteractiveContext,
+see Note [Relation between the 'InteractiveContext' and 'interactiveGhciUnitId']
+for details.
+
+The 'DynFlags' of the 'interactiveGhciUnit' can be modified by using `:seti`
+commands in the GHCi session.
+
+=== 'interactiveSessionUnit' Home Unit
+
+The 'interactiveSessionUnit' home unit is used as a kitchen sink for Modules that
+are not part of a home unit already.
+When the user types ":load", it is not trivial to figure to which home unit the module
+should be added to.
+Especially, when there is more than home unit. Thus, we always ":load"ed modules
+to this home unit.
+
+The 'DynFlags' of the 'interactiveSessionUnit' can be modified via the ':set'
+commands in the GHCi session.
+-}
+
+-- | Set up the multiple home unit session.
+-- Installs a 'HomeUnitEnv' for the ghci-prompt and one for the ghci-session in the
+-- current 'HscEnv'.
+--
+-- Installs the two home units 'interactiveGhciUnit' and 'interactiveSessionUnit', which
+-- both depend on any other 'HomeUnitEnv' that is already present in the current
+-- 'HomeUnitGraph'.
+--
+-- In other words, in each GHCi session, there are always at least three 'HomeUnitEnv's:
+--
+-- * 'interactiveGhciUnit'
+-- * 'interactiveSessionUnit'
+-- * 'mainUnit' (by default)
+--
+-- The 'interactiveGhciUnit' is the currently active unit, i.e. @hscActiveUnit hsc_env == 'interactiveGhciUnitId'@,
+-- and it stays as the active unit for the entire duration of GHCi.
+-- Within GHCi, you can rely on this property.
+--
+-- For motivation and design, see Note [Multiple Home Units aware GHCi]
+installInteractiveHomeUnit :: GHC.GhcMonad m => m ()
+installInteractiveHomeUnit = do
+  logger <- getLogger
+  hsc_env <- GHC.getSession
+  -- The initial set of DynFlags used for interactive evaluation is the same
+  -- as the global DynFlags, plus:
+  -- * -XExtendedDefaultRules and
+  -- * -XNoMonomorphismRestriction.
+  -- See Note [Changing language extensions for interactive evaluation] #10857
+  dflags <- getDynFlags
+  let
+    dflags0' =
+      (xopt_set_unlessExplSpec LangExt.ExtendedDefaultRules xopt_set) .
+      (xopt_set_unlessExplSpec LangExt.MonomorphismRestriction xopt_unset) $
+      dflags
+    -- Disable warnings about unused packages
+    -- It doesn't matter for the interactive session.
+    -- See Note [No unused package warnings for the interactive session]
+    dflags0 = wopt_unset dflags0' Opt_WarnUnusedPackages
+
+    -- Trivial '-package-id <uid>' flag
+    homeUnitPkgFlag uid =
+      ExposePackage
+        (unitIdString uid)
+        (UnitIdArg $ RealUnit (Definite uid))
+        (ModRenaming False [])
+
+    sessionUnitExposedFlag =
+      homeUnitPkgFlag interactiveSessionUnitId
+
+  -- Explicitly depend on all home units and 'sessionUnitExposedFlag'.
+  -- Normalise the 'dflagsPrompt', as they will be used for 'ic_dflags'
+  -- of the 'InteractiveContext'.
+  dflagsPrompt <- GHC.normaliseInteractiveDynFlags logger $
+    setHomeUnitId interactiveGhciUnitId $ dflags0
+      { packageFlags =
+        [ sessionUnitExposedFlag ] ++
+        [ homeUnitPkgFlag uid
+        | homeUnitEnv <- Foldable.toList $ hsc_HUG hsc_env
+        , Just homeUnit <- [homeUnitEnv_home_unit homeUnitEnv]
+        , let uid = homeUnitId homeUnit
+        ] ++
+        (packageFlags dflags0)
+      , importPaths = [] -- TODO @fendor: do we need this?
+      }
+
+  let
+    -- Explicitly depend on all current home units.
+    dflagsSession =
+      setHomeUnitId interactiveSessionUnitId $ dflags
+        { packageFlags =
+          [ homeUnitPkgFlag uid
+          | homeUnitEnv <- Foldable.toList $ hsc_HUG hsc_env
+          , Just homeUnit <- [homeUnitEnv_home_unit homeUnitEnv]
+          , let uid = homeUnitId homeUnit
+          ] ++
+          (packageFlags dflags)
+        , importPaths = [] -- TODO @fendor: do we need this?
+        }
+
+  let
+    cached_unit_dbs =
+        concat
+      . catMaybes
+      . fmap homeUnitEnv_unit_dbs
+      $ Foldable.toList
+      $ hsc_HUG hsc_env
+
+    all_unit_ids =
+      S.insert interactiveGhciUnitId $
+      S.insert interactiveSessionUnitId $
+      hsc_all_home_unit_ids hsc_env
+
+  ghciPromptUnit  <- setupHomeUnitFor logger dflagsPrompt  all_unit_ids cached_unit_dbs
+  ghciSessionUnit <- setupHomeUnitFor logger dflagsSession all_unit_ids cached_unit_dbs
+  let
+    -- Setup up the HUG, install the interactive home units
+    withInteractiveUnits =
+        HUG.unitEnv_insert interactiveGhciUnitId ghciPromptUnit
+        . HUG.unitEnv_insert interactiveSessionUnitId ghciSessionUnit
+
+  -- Finish up the setup, install the new HUG and make the 'interactiveGhciUnitId'
+  -- the active unit.
+  modifySessionM (\env -> do
+    -- Set the new HUG
+    let newEnv0 = hscUpdateHUG       withInteractiveUnits  env
+    -- Make sure the 'interactiveGhciUnitId' is active and 'hsc_dflags'
+    -- are populated correctly.
+    -- The 'interactiveGhciUnitId' will stay as the active unit within GHCi.
+    let newEnv1 = hscSetActiveUnitId interactiveGhciUnitId newEnv0
+    -- Use the 'DynFlags' of the 'interactiveGhciUnitId' for the 'InteractiveContext'.
+    GHC.initialiseInteractiveDynFlags dflagsPrompt newEnv1
+    )
+
+  pure ()
+  where
+    setupHomeUnitFor :: GHC.GhcMonad m => Logger -> DynFlags -> S.Set UnitId -> [UnitDatabase UnitId] -> m HomeUnitEnv
+    setupHomeUnitFor logger dflags all_home_units cached_unit_dbs = do
+      (dbs,unit_state,home_unit,_mconstants) <-
+        liftIO $ initUnits logger dflags (Just cached_unit_dbs) all_home_units
+      hpt <- liftIO emptyHomePackageTable
+      pure (HUG.mkHomeUnitEnv unit_state (Just dbs) dflags hpt (Just home_unit))
+
 reportError :: GhciMonad m => GhciCommandMessage -> m ()
 reportError err = do
   printError err
@@ -933,7 +1138,7 @@ getInfoForPrompt = do
                       | otherwise           = unLoc (ideclName d)
 
         modules_names =
-             ['*':(moduleNameString m) | IIModule m <- rev_imports] ++
+             ['*':(moduleNameString (moduleName m)) | IIModule m <- rev_imports] ++
              [moduleNameString (myIdeclName d) | IIDecl d <- rev_imports]
         line = 1 + line_number st
 
@@ -1971,13 +2176,27 @@ wrapDeferTypeErrors :: GHC.GhcMonad m => m a -> m a
 wrapDeferTypeErrors load =
   MC.bracket
     (do
-      -- Force originalFlags to avoid leaking the associated HscEnv
-      !originalFlags <- getDynFlags
-      void $ GHC.setProgramDynFlags $
-         setGeneralFlag' Opt_DeferTypeErrors originalFlags
-      return originalFlags)
-    (\originalFlags -> void $ GHC.setProgramDynFlags originalFlags)
+      -- Force originalHUG to avoid leaking the associated HscEnv
+      !originalHUG <- hsc_HUG <$> GHC.getSession
+      _ <- GHC.setProgramHUG (fmap deferTypeErrors originalHUG)
+      return originalHUG)
+    (\originalHUG ->
+      -- Restore the old 'DynFlags' for each home unit.
+      -- This makes sure that '-fdefer-type-errors' is unset again, iff it wasn't set before.
+      modifySession (hscUpdateHUG (restoreOriginalDynFlags originalHUG)))
     (\_ -> load)
+  where
+    deferTypeErrors home_unit_env =
+      home_unit_env
+        { homeUnitEnv_dflags =
+            setGeneralFlag' Opt_DeferTypeErrors (homeUnitEnv_dflags home_unit_env)
+        }
+
+    restoreOriginalDynFlags (HUG.UnitEnvGraph old) (HUG.UnitEnvGraph new) = HUG.UnitEnvGraph $
+      M.unionWith (\b a ->
+        a { homeUnitEnv_dflags = homeUnitEnv_dflags b
+          })
+        old new
 
 loadModule :: GhciMonad m => [(FilePath, Maybe UnitId, Maybe Phase)] -> m SuccessFlag
 loadModule fs = do
@@ -1986,7 +2205,7 @@ loadModule fs = do
 
 -- | @:load@ command
 loadModule_ :: GhciMonad m => [FilePath] -> m ()
-loadModule_ fs = void $ loadModule (zip3 fs (repeat Nothing) (repeat Nothing))
+loadModule_ fs = void $ loadModule (zip3 fs (repeat (Just interactiveSessionUnitId)) (repeat Nothing))
 
 loadModuleDefer :: GhciMonad m => [FilePath] -> m ()
 loadModuleDefer = wrapDeferTypeErrors . loadModule_
@@ -2030,7 +2249,8 @@ addModule :: GhciMonad m => [FilePath] -> m ()
 addModule files = do
   revertCAFs -- always revert CAFs on load/add.
   files' <- mapM expandPath files
-  targets <- mapM (\m -> GHC.guessTarget m Nothing Nothing) files'
+  -- New targets are always added to the 'interactiveSessionUnitId' 'HomeUnitEnv'.
+  targets <- mapM (\m -> GHC.guessTarget m (Just interactiveSessionUnitId) Nothing) files'
   targets' <- filterM checkTarget targets
   -- remove old targets with the same id; e.g. for :add *M
   mapM_ GHC.removeTarget [ tid | Target { targetId = tid } <- targets' ]
@@ -2063,7 +2283,8 @@ addModule files = do
 unAddModule :: GhciMonad m => [FilePath] -> m ()
 unAddModule files = do
   files' <- mapM expandPath files
-  targets <- mapM (\m -> GHC.guessTarget m Nothing Nothing) files'
+  -- New targets are always added to the 'interactiveSessionUnitId' 'HomeUnitEnv'.
+  targets <- mapM (\m -> GHC.guessTarget m (Just interactiveSessionUnitId) Nothing) files'
   let removals = [ tid | Target { targetId = tid } <- targets ]
   mapM_ GHC.removeTarget removals
   _ <- doLoadAndCollectInfo (Unadd $ length removals) LoadAllTargets
@@ -2105,7 +2326,7 @@ doLoadAndCollectInfo load_type howmuch = do
       -- MP: :set +c code path only works in single package mode atm, hence
       -- this call to isLoaded is ok. collectInfo needs to be modified further to
       -- work with :set +c so I have punted on that for now.
-      loaded <- filterM GHC.isLoaded (map ms_mod_name mod_summaries)
+      loaded <- filterM GHC.isLoadedHomeModule (map ms_mod mod_summaries)
       v <- mod_infos <$> getGHCiState
       !newInfos <- collectInfo v loaded
       modifyGHCiState (\st -> st { mod_infos = newInfos })
@@ -2187,7 +2408,7 @@ setContextAfterLoad keep_ctxt (Just graph) = do
               -- We import the module with a * iff
               --   - it is interpreted, and
               --   - -XSafe is off (it doesn't allow *-imports)
-        let new_ctx | star_ok   = [mkIIModule (GHC.moduleName m)]
+        let new_ctx | star_ok   = [mkIIModule m]
                     | otherwise = [mkIIDecl   (GHC.moduleName m)]
         setContextKeepingPackageModules keep_ctxt new_ctx
 
@@ -2607,7 +2828,7 @@ guessCurrentModule cmd = do
   imports <- GHC.getContext
   case imports of
     [] -> throwGhcException $ CmdLineError (':' : cmd ++ ": no current module")
-    IIModule m : _ -> GHC.findQualifiedModule NoPkgQual m
+    IIModule m : _ -> pure m
     IIDecl d : _ -> do
       pkgqual <- GHC.renameRawPkgQualM (unLoc $ ideclName d) (ideclPkgQual d)
       GHC.findQualifiedModule pkgqual (unLoc (ideclName d))
@@ -2628,7 +2849,7 @@ browseModule bang modl exports_only = do
           then pure $ GHC.modInfoExports mod_info
           else do
             hsc_env <- GHC.getSession
-            mmod_env <- liftIO $ mkTopLevEnv hsc_env (moduleName modl)
+            mmod_env <- liftIO $ mkTopLevEnv hsc_env modl
             case mmod_env of
               Left err -> throwGhcException (CmdLineError (GHC.moduleNameString (GHC.moduleName modl) ++ " " ++ err))
               Right mod_env -> pure $ map greName . globalRdrEnvElts $ mod_env
@@ -2737,8 +2958,9 @@ addModulesToContext starred unstarred = restoreContextOnFailure $ do
 
 addModulesToContext_ :: GhciMonad m => [ModuleName] -> [ModuleName] -> m ()
 addModulesToContext_ starred unstarred = do
-   mapM_ addII (map mkIIModule starred ++ map mkIIDecl unstarred)
-   setGHCContextFromGHCiState
+  starredModules <- traverse lookupModuleName starred
+  mapM_ addII (map mkIIModule starredModules ++ map mkIIDecl unstarred)
+  setGHCContextFromGHCiState
 
 remModulesFromContext :: GhciMonad m => [ModuleName] -> [ModuleName] -> m ()
 remModulesFromContext  starred unstarred = do
@@ -2804,14 +3026,14 @@ checkAdd ii = do
   dflags <- getDynFlags
   let safe = safeLanguageOn dflags
   case ii of
-    IIModule modname
+    IIModule mod
        | safe -> throwGhcException $ CmdLineError "can't use * imports with Safe Haskell"
-       | otherwise -> wantInterpretedModuleName modname >> return ()
+       | otherwise -> checkInterpretedModule mod >> return ()
 
     IIDecl d -> do
        let modname = unLoc (ideclName d)
        pkgqual <- GHC.renameRawPkgQualM modname (ideclPkgQual d)
-       m <- GHC.lookupQualifiedModule pkgqual modname
+       m <- lookupQualifiedModuleName pkgqual modname
        when safe $ do
            t <- GHC.isModuleTrusted m
            unless t $ throwGhcException $ ProgramError $ ""
@@ -2874,13 +3096,13 @@ getImplicitPreludeImports iidecls = do
 -- -----------------------------------------------------------------------------
 -- Utils on InteractiveImport
 
-mkIIModule :: ModuleName -> InteractiveImport
+mkIIModule :: Module -> InteractiveImport
 mkIIModule = IIModule
 
 mkIIDecl :: ModuleName -> InteractiveImport
 mkIIDecl = IIDecl . simpleImportDecl
 
-iiModules :: [InteractiveImport] -> [ModuleName]
+iiModules :: [InteractiveImport] -> [Module]
 iiModules is = [m | IIModule m <- is]
 
 isIIModule :: InteractiveImport -> Bool
@@ -2888,7 +3110,7 @@ isIIModule (IIModule _) = True
 isIIModule _ = False
 
 iiModuleName :: InteractiveImport -> ModuleName
-iiModuleName (IIModule m) = m
+iiModuleName (IIModule m) = moduleName m
 iiModuleName (IIDecl d)   = unLoc (ideclName d)
 
 preludeModuleName :: ModuleName
@@ -2990,8 +3212,23 @@ showOptions show_all
                    then text "none."
                    else hsep (map (\o -> char '+' <> text (optToStr o)) opts)
            ))
-       liftIO $ showDynFlags show_all dflags
-
+       mapNonInteractiveHomeUnitsM (liftIO . showDynFlags show_all)
+
+mapNonInteractiveHomeUnitsM :: GHC.GhcMonad m => (DynFlags -> m ()) -> m ()
+mapNonInteractiveHomeUnitsM printer = do
+  hug <- hsc_HUG <$> GHC.getSession
+  singleOrMultipleHomeUnits
+      $ map (\(uid, homeUnit) -> (uid, homeUnitEnv_dflags homeUnit))
+      $ filter (\(uid, _) -> uid /= interactiveSessionUnitId
+                          && uid /= interactiveGhciUnitId)
+      $ HUG.unitEnv_assocs hug
+  where
+    singleOrMultipleHomeUnits [] =
+      liftIO $ putStrLn "GHCi: internal error - no home unit configured"
+    singleOrMultipleHomeUnits [(_, dflags)] = printer dflags
+    singleOrMultipleHomeUnits xs = mapM_ (\(uid, dflags) -> do
+      liftIO $ putStrLn (showSDoc dflags (text "Unit ID:" <+> ppr uid))
+      printer dflags) xs
 
 showDynFlags :: Bool -> DynFlags -> IO ()
 showDynFlags show_all dflags = do
@@ -3117,69 +3354,200 @@ setOptions wds =
       -- then, dynamic flags
       when (not (null minus_opts)) $ newDynFlags False minus_opts
 
--- | newDynFlags will *not* read package environment files, therefore we
--- use 'parseDynamicFlagsCmdLine' rather than 'parseDynamicFlags'. This
--- function is called very often and results in repeatedly loading
--- environment files (see #19650)
+-- Note [No unused package warnings for the interactive session]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+--
+-- The interactive session (also called "interactive-prompt" occassionally) should not
+-- report unused packages, as it will essentially always report packages
+-- as unused.
+-- The "interactive-prompt" doesn't contain any 'Module's, so most packages
+-- are unused.
+-- As this would flood the user with warnings they can't do anything about,
+-- we decide to unconditionally turn off the warning 'Opt_WarnUnusedPackages'.
+--
+-- Unused packages in GHCi are still reported via the 'interactive-session' unit.
+-- See Note [Multiple Home Units aware GHCi] for an explanation about the
+-- "interactive-prompt" and 'interactive-session' unit.
+
+-- | 'newDynFlags' adds the given user options to the session.
+--
+-- If 'True' is passed, we add the options only to the interactive 'DynFlags'.
+-- Otherwise, the options are added to each 'HomeUnitEnv' in the current session.
+--
+-- This function will check whether we need to re-initialise the 'UnitState',
+-- for example when the user writes ':set -package containers'.
+--
+-- Any warnings during parsing, or validation of the new 'DynFlags' will be
+-- directly reported to the user.
 newDynFlags :: GhciMonad m => Bool -> [String] -> m ()
 newDynFlags interactive_only minus_opts = do
-      let lopts = map noLoc minus_opts
+  let lopts = map noLoc minus_opts
 
-      logger <- getLogger
-      idflags0 <- GHC.getInteractiveDynFlags
-      (idflags1, leftovers, warns) <- DynFlags.parseDynamicFlagsCmdLine logger idflags0 lopts
+  case interactive_only of
+    True -> addToInteractiveDynFlags lopts
+    False -> addToProgramDynFlags lopts
 
-      liftIO $ printOrThrowDiagnostics logger (initPrintConfig idflags1) (initDiagOpts idflags1) (GhcDriverMessage <$> warns)
+  idflags <- hsc_dflags <$> GHC.getSession
+  installInteractivePrint (interactivePrint idflags) False
+
+-- | Add the given options to the interactive 'DynFlags'.
+-- This function will normalise and validate the 'DynFlags' and report warnings
+-- directly to the user.
+--
+-- Updates both the 'hsc_dflags' of 'HscEnv', and the 'ic_dflags' of the 'InteractiveContext'.
+--
+-- 'addToInteractiveDynFlags' will *not* read package environment files, therefore we
+-- use 'parseDynamicFlagsCmdLine' rather than 'parseDynamicFlags'. This
+-- function is called very often and results in repeatedly loading
+-- environment files (see #19650)
+addToInteractiveDynFlags :: GhciMonad m => [Located String] -> m ()
+addToInteractiveDynFlags lopts = do
+  logger <- getLogger
+  idflags0 <- hsc_dflags <$> GHC.getSession
+  (idflags1, leftovers, warns) <- DynFlags.parseDynamicFlagsCmdLine logger idflags0 lopts
+
+  liftIO $ printOrThrowDiagnostics logger (initPrintConfig idflags1) (initDiagOpts idflags1) (GhcDriverMessage <$> warns)
+  when (not $ null leftovers) (unknownFlagsErr $ map unLoc leftovers)
+
+  when (packageFlagsChanged idflags1 idflags0) $ do
+    liftIO $ hPutStrLn stderr "cannot set package flags with :seti; use :set"
+
+  GHC.setInteractiveDynFlags idflags1
+  idflags_norm <- ic_dflags . hsc_IC <$> GHC.getSession
+  -- TODO @fendor: why not 'setProgramHUG_'?
+  _ <- GHC.setProgramDynFlags idflags_norm
+  pure ()
+
+-- | Add the given options to all 'DynFlags' in the 'HomeUnitGraph'.
+-- This function will validate the 'DynFlags' and report warnings directly to the user.
+--
+-- We additionally normalise the 'DynFlags' for the 'interactiveGhciUnitId' for use
+-- in the 'InteractiveContext'.
+--
+-- 'addToProgramDynFlags' will *not* read package environment files, therefore we
+-- use 'parseDynamicFlagsCmdLine' rather than 'parseDynamicFlags'. This
+-- function is called very often and results in repeatedly loading
+-- environment files (see #19650)
+addToProgramDynFlags :: GhciMonad m => [Located String] -> m ()
+addToProgramDynFlags lopts = do
+  logger <- getLogger
+  initial_hug <- hsc_HUG <$> GHC.getSession
+  -- Update the 'DynFlags' of each 'HomeUnitEnv'.
+  -- Parse the new 'DynFlags', and report potential issues once.
+  -- Arguably, we may want to report issues for each non-builtin 'HomeUnitEnv'
+  -- individually.
+  updates <- HUG.unitEnv_traverseWithKey (\uid homeUnitEnv -> do
+    let oldFlags = HUG.homeUnitEnv_dflags homeUnitEnv
+    (newFlags, leftovers, warns) <- DynFlags.parseDynamicFlagsCmdLine logger oldFlags lopts
+    -- We only want to report inconsistencies and warnings once.
+    -- Thus, we do it only once for the 'interactiveGhciUnitId'
+    when (uid == interactiveGhciUnitId) $ do
+      liftIO $ printOrThrowDiagnostics logger (initPrintConfig newFlags) (initDiagOpts newFlags) (GhcDriverMessage <$> warns)
       when (not $ null leftovers) (unknownFlagsErr $ map unLoc leftovers)
 
-      when (interactive_only && packageFlagsChanged idflags1 idflags0) $ do
-          liftIO $ hPutStrLn stderr "cannot set package flags with :seti; use :set"
-      GHC.setInteractiveDynFlags idflags1
-      installInteractivePrint (interactivePrint idflags1) False
-
-      dflags0 <- getDynFlags
-
-      when (not interactive_only) $ do
-        (dflags1, _, _) <- liftIO $ DynFlags.parseDynamicFlagsCmdLine logger dflags0 lopts
-        must_reload <- GHC.setProgramDynFlags dflags1
-
-        -- if the package flags changed, reset the context and link
-        -- the new packages.
-        hsc_env <- GHC.getSession
-        let dflags2 = hsc_dflags hsc_env
-        let interp  = hscInterp hsc_env
-        when (packageFlagsChanged dflags2 dflags0) $ do
-          when (verbosity dflags2 > 0) $
-            liftIO . putStrLn $
-              "package flags have changed, resetting and loading new packages..."
-          -- Clear caches and eventually defined breakpoints. (#1620)
-          clearCaches
-          when must_reload $ do
-            let units = preloadUnits (hsc_units hsc_env)
-            liftIO $ Loader.loadPackages interp hsc_env units
-          -- package flags changed, we can't re-use any of the old context
-          setContextAfterLoad False Nothing
-          -- and copy the package flags to the interactive DynFlags
-          idflags <- GHC.getInteractiveDynFlags
-          GHC.setInteractiveDynFlags
-              idflags{ packageFlags = packageFlags dflags2 }
-
-        let ld0length   = length $ ldInputs dflags0
-            fmrk0length = length $ cmdlineFrameworks dflags0
-
-            newLdInputs     = drop ld0length (ldInputs dflags2)
-            newCLFrameworks = drop fmrk0length (cmdlineFrameworks dflags2)
-
-            dflags'  = dflags2 { ldInputs = newLdInputs
-                               , cmdlineFrameworks = newCLFrameworks
-                               }
-            hsc_env' = hscSetFlags dflags' hsc_env
-
-        when (not (null newLdInputs && null newCLFrameworks)) $
-          liftIO $ Loader.loadCmdLineLibs (hscInterp hsc_env') hsc_env'
-
-      return ()
+    -- Special Logic!
+    -- Currently, the interactive 'DynFlags' have additional restrictions,
+    -- for example modifying package flags is not supported!
+    -- The interactive 'DynFlags' get normalised to uphold this restriction.
+    -- As a special precaution, we also don't want to report unusued packages warnings
+    -- for the interactive session.
+    -- See Note [No unused package warnings for the interactive session]
+    --
+    -- See Note [Multiple Home Units aware GHCi] for details about how
+    -- the interactive session is structured.
+    newFlags' <-
+      if uid == interactiveGhciUnitId
+        then do
+          -- See Note [No unused package warnings for the interactive session]
+          let icdflags1 = wopt_unset newFlags Opt_WarnUnusedPackages
+          GHC.normaliseInteractiveDynFlags logger icdflags1
+        else
+          pure newFlags
+    pure (homeUnitEnv { homeUnitEnv_dflags = newFlags' })
+    )
+    initial_hug
+  -- Update the HUG! This might force us to reload the 'UnitState' of each 'HomeUnitEnv'
+  -- if package flags were changed.
+  must_reload <- GHC.setProgramHUG_ True updates
+
+  -- Initialise the Interactive DynFlags.
+  -- Sets the 'ic_dflags' and initialises the 'ic_plugins'.
+  -- See Note [Relation between the 'InteractiveContext' and 'interactiveGhciUnitId']
+  icdflags <- hsc_dflags <$> GHC.getSession
+  modifySessionM (GHC.initialiseInteractiveDynFlags icdflags)
+
+  -- if the package flags changed, reset the context and link
+  -- the new packages.
+  hsc_env <- GHC.getSession
+  let dflags2 = hsc_dflags hsc_env
+  when must_reload $ do
+    when (verbosity dflags2 > 0) $
+      liftIO . putStrLn $
+        "package flags have changed, resetting and loading new packages..."
+
+    -- Clear caches and eventually defined breakpoints. (#1620)
+    clearCaches
 
+    reloadPackages hsc_env
+
+  reloadLinkerOptions hsc_env initial_hug
+
+reloadPackages :: GhciMonad m => HscEnv -> m ()
+reloadPackages hsc_env = do
+  let
+      units =
+        concatMap (preloadUnits . HUG.homeUnitEnv_units)
+                  (Foldable.toList $ hsc_HUG hsc_env)
+  liftIO $ Loader.loadPackages (hscInterp hsc_env) hsc_env units
+  -- package flags changed, we can't re-use any of the old context
+  setContextAfterLoad False Nothing
+
+-- | Reload the linker options.
+--
+-- Synopsis: @'reloadLinkerOptions' hsc_env old_hug@
+--
+-- After the HUG is modified, the linker may need to be reloaded.
+-- The linker is reloaded via 'loadCmdLineLibs', if the library inputs
+-- have changed.
+-- To determine whether the library inputs have changed, we need the
+-- old HUG, which is passed as the argument 'old_hug'.
+--
+-- This function will crash, if the 'old_hug' doesn't have exactly
+-- the same keys has the given 'hsc_env'. I.e.
+--
+-- @
+--   HUG.unitEnv_keys old_hug == HUG.unitEnv_keys (hsc_HUG hsc_env)
+-- @
+reloadLinkerOptions :: MonadIO m => HscEnv -> HomeUnitGraph -> m ()
+reloadLinkerOptions hsc_env old_hug = do
+  let
+    new_hug = hsc_HUG hsc_env
+  let
+    (needs_updates, updated_hug) = HUG.unitEnv_traverseWithKey (\key unitEnv ->
+      let
+        old_flags = homeUnitEnv_dflags (HUG.unitEnv_lookup key old_hug)
+        new_flags = homeUnitEnv_dflags unitEnv
+        ld0length   = length $ ldInputs old_flags
+        fmrk0length = length $ cmdlineFrameworks old_flags
+
+        newLdInputs     = drop ld0length (ldInputs new_flags)
+        newCLFrameworks = drop fmrk0length (cmdlineFrameworks new_flags)
+
+        dflags'  = new_flags { ldInputs = newLdInputs
+                            , cmdlineFrameworks = newCLFrameworks
+                            }
+      in
+        (S.Any (not (null newLdInputs && null newCLFrameworks)),
+           unitEnv { homeUnitEnv_dflags = dflags' })
+      ) new_hug
+
+    hsc_env' =
+      hscSetActiveUnitId (hscActiveUnitId hsc_env)
+      $ hscUpdateHUG (const updated_hug)
+      $ hsc_env
+
+  when (S.getAny needs_updates) $
+    liftIO $ Loader.loadCmdLineLibs (hscInterp hsc_env') hsc_env'
 
 unknownFlagsErr :: GhciMonad m => [String] -> m ()
 unknownFlagsErr fs = mapM_ (\f -> reportError (GhciUnknownFlag f (suggestions f))) fs
@@ -3261,7 +3629,6 @@ showCmd ""   = showOptions False
 showCmd "-a" = showOptions True
 showCmd str = do
     st <- getGHCiState
-    dflags <- getDynFlags
     hsc_env <- GHC.getSession
 
     let lookupCmd :: String -> Maybe (m ())
@@ -3299,8 +3666,10 @@ showCmd str = do
     case words str of
       [w] | Just action <- lookupCmd w -> action
 
-      _ -> let helpCmds = [ text name | (True, name, _) <- cmds ]
-           in throwGhcException $ CmdLineError $ showSDoc dflags
+      _ -> do
+        let helpCmds = [ text name | (True, name, _) <- cmds ]
+        dflags <- getDynFlags
+        throwGhcException $ CmdLineError $ showSDoc dflags
               $ hang (text "syntax:") 4
               $ hang (text ":show") 6
               $ brackets (fsep $ punctuate (text " |") helpCmds)
@@ -3321,7 +3690,7 @@ showImports = do
       trans_ctx = transient_ctx st
 
       show_one (IIModule star_m)
-          = ":module +*" ++ moduleNameString star_m
+          = ":module +*" ++ moduleNameString (moduleName star_m)
       show_one (IIDecl imp) = showPpr dflags imp
 
   prel_iidecls <- getImplicitPreludeImports (rem_ctx ++ trans_ctx)
@@ -3427,16 +3796,14 @@ pprStopped res =
   mb_mod_name = moduleName <$> ibi_tick_mod <$> GHC.resumeBreakpointId res
 
 showUnits :: GHC.GhcMonad m => m ()
-showUnits = do
-  dflags <- getDynFlags
+showUnits = mapNonInteractiveHomeUnitsM $ \dflags -> do
   let pkg_flags = packageFlags dflags
   liftIO $ putStrLn $ showSDoc dflags $
     text ("active package flags:"++if null pkg_flags then " none" else "") $$
       nest 2 (vcat (map pprFlag pkg_flags))
 
 showPaths :: GHC.GhcMonad m => m ()
-showPaths = do
-  dflags <- getDynFlags
+showPaths = mapNonInteractiveHomeUnitsM $ \dflags -> do
   liftIO $ do
     cwd <- getCurrentDirectory
     putStrLn $ showSDoc dflags $
@@ -3448,7 +3815,7 @@ showPaths = do
         nest 2 (vcat (map text ipaths))
 
 showLanguages :: GHC.GhcMonad m => m ()
-showLanguages = getDynFlags >>= liftIO . showLanguages' False
+showLanguages = mapNonInteractiveHomeUnitsM $ liftIO . showLanguages' False
 
 showiLanguages :: GHC.GhcMonad m => m ()
 showiLanguages = GHC.getInteractiveDynFlags >>= liftIO . showLanguages' False
@@ -3627,11 +3994,11 @@ completeBreakpoint = wrapCompleter spaces $ \w -> do          -- #3000
         filterM GHC.moduleIsInterpreted hmods
 
     -- Return all possible bids for a given Module
-    bidsByModule :: GhciMonad m => [ModuleName] -> Module -> m [String]
+    bidsByModule :: GhciMonad m => [Module] -> Module -> m [String]
     bidsByModule nonquals mod = do
       (_, decls) <- getModBreak mod
       let bids = nub $ declPath <$> elems decls
-      pure $ case (moduleName mod) `elem` nonquals of
+      pure $ case mod `elem` nonquals of
               True  -> bids
               False -> (combineModIdent (showModule mod)) <$> bids
 
@@ -4036,8 +4403,7 @@ breakSwitch (arg1:rest)
    | all isDigit arg1 = do
         imports <- GHC.getContext
         case iiModules imports of
-           (mn : _) -> do
-              md <- lookupModuleName mn
+           (md : _) -> do
               breakByModuleLine md (read arg1) rest
            [] -> do
               liftIO $ putStrLn "No modules are loaded with debugging support."
@@ -4169,8 +4535,7 @@ list2 [arg] | all isDigit arg = do
     case iiModules imports of
         [] -> liftIO $ putStrLn "No module to list"
         (mn : _) -> do
-          md <- lookupModuleName mn
-          listModuleLine md (read arg)
+          listModuleLine mn (read arg)
 list2 [arg1,arg2] | looksLikeModuleName arg1, all isDigit arg2 = do
         md <- wantInterpretedModule arg1
         listModuleLine md (read arg2)
@@ -4426,7 +4791,20 @@ lookupModule :: GHC.GhcMonad m => String -> m Module
 lookupModule mName = lookupModuleName (GHC.mkModuleName mName)
 
 lookupModuleName :: GHC.GhcMonad m => ModuleName -> m Module
-lookupModuleName mName = GHC.lookupQualifiedModule NoPkgQual mName
+lookupModuleName mName = lookupQualifiedModuleName NoPkgQual mName
+
+lookupQualifiedModuleName :: GHC.GhcMonad m => PkgQual -> ModuleName -> m Module
+lookupQualifiedModuleName qual modl = do
+  GHC.lookupAllQualifiedModuleNames qual modl >>= \case
+    [] -> throwGhcException (CmdLineError ("module '" ++ str ++ "' could not be found."))
+    [m] -> pure m
+    ms -> throwGhcException (CmdLineError ("module name '" ++ str ++ "' is ambiguous:\n" ++ errorMsg ms))
+  where
+    str = moduleNameString modl
+    errorMsg ms = intercalate "\n"
+      [ "- " ++ unitIdString (toUnitId (moduleUnit m)) ++ ":" ++ moduleNameString (moduleName m)
+      | m <- ms
+      ]
 
 isMainUnitModule :: Module -> Bool
 isMainUnitModule m = GHC.moduleUnit m == mainUnit
@@ -4476,15 +4854,19 @@ wantInterpretedModule str = wantInterpretedModuleName (GHC.mkModuleName str)
 
 wantInterpretedModuleName :: GHC.GhcMonad m => ModuleName -> m Module
 wantInterpretedModuleName modname = do
-   modl <- lookupModuleName modname
-   let str = moduleNameString modname
-   home_unit <- hsc_home_unit <$> GHC.getSession
-   unless (isHomeModule home_unit modl) $
-      throwGhcException (CmdLineError ("module '" ++ str ++ "' is from another package;\nthis command requires an interpreted module"))
-   is_interpreted <- GHC.moduleIsInterpreted modl
-   when (not is_interpreted) $
-       throwGhcException (CmdLineError ("module '" ++ str ++ "' is not interpreted; try \':add *" ++ str ++ "' first"))
-   return modl
+  modl <- lookupModuleName modname
+  checkInterpretedModule modl
+
+checkInterpretedModule :: GHC.GhcMonad m => Module -> m Module
+checkInterpretedModule modl = do
+  let str = moduleNameString $ moduleName modl
+  hug <- hsc_HUG <$> GHC.getSession
+  unless (HUG.memberHugUnit (moduleUnit modl) hug) $
+    throwGhcException (CmdLineError ("module '" ++ str ++ "' is from another package;\nthis command requires an interpreted module"))
+  is_interpreted <- GHC.moduleIsInterpreted modl
+  when (not is_interpreted) $
+      throwGhcException (CmdLineError ("module '" ++ str ++ "' is not interpreted; try \':add *" ++ str ++ "' first"))
+  return modl
 
 wantNameFromInterpretedModule :: GHC.GhcMonad m
                               => (Name -> SDoc -> m ())


=====================================
ghc/GHCi/UI/Exception.hs
=====================================
@@ -465,7 +465,7 @@ instance DiagnosticCodeNameSpace GHCi where
 
 type GhciDiagnosticCode :: Symbol -> Nat
 type family GhciDiagnosticCode c = n | n -> c where
-  GhciDiagnosticCode "GhciCommandNotSupportedInMultiMode" = 83514
+  GhciDiagnosticCode "GhciCommandNotSupportedInMultiMode" = Outdated 83514
   GhciDiagnosticCode "GhciInvalidArgumentString"          = 68894
   GhciDiagnosticCode "GhciCommandSyntaxError"             = 72682
   GhciDiagnosticCode "GhciInvalidPromptString"            = 50882


=====================================
ghc/GHCi/UI/Info.hs
=====================================
@@ -46,6 +46,7 @@ import           GHC.Tc.Types
 import           GHC.Types.Name.Reader
 import           GHC.Types.Name.Set
 import           GHC.Utils.Outputable
+import           GHC.Unit.Types
 import           GHC.Types.SrcLoc
 import           GHC.Types.Var
 import qualified GHC.Data.Strict as Strict
@@ -113,7 +114,7 @@ srcSpanFilePath = unpackFS . srcSpanFile
 -- | Try to find the location of the given identifier at the given
 -- position in the module.
 findLoc :: GhcMonad m
-        => Map ModuleName ModInfo
+        => Map Module ModInfo
         -> RealSrcSpan
         -> String
         -> ExceptT GhciModuleError m (ModInfo,Name,SrcSpan)
@@ -133,7 +134,7 @@ findLoc infos span0 string = do
 
 -- | Find any uses of the given identifier in the codebase.
 findNameUses :: (GhcMonad m)
-             => Map ModuleName ModInfo
+             => Map Module ModInfo
              -> RealSrcSpan
              -> String
              -> ExceptT GhciModuleError m [SrcSpan]
@@ -160,7 +161,7 @@ stripSurrounding xs = filter (not . isRedundant) xs
 -- | Try to resolve the name located at the given position, or
 -- otherwise resolve based on the current module's scope.
 findName :: GhcMonad m
-         => Map ModuleName ModInfo
+         => Map Module ModInfo
          -> RealSrcSpan
          -> ModInfo
          -> String
@@ -186,11 +187,11 @@ findName infos span0 mi string =
 
 -- | Try to resolve the name from another (loaded) module's exports.
 resolveNameFromModule :: GhcMonad m
-                      => Map ModuleName ModInfo
+                      => Map Module ModInfo
                       -> Name
                       -> ExceptT GhciModuleError m Name
 resolveNameFromModule infos name = do
-     info <- maybe (throwE $ GhciNoModuleForName name) pure (nameModule_maybe name >>= \modL -> M.lookup (moduleName modL) infos)
+     info <- maybe (throwE $ GhciNoModuleForName name) pure (nameModule_maybe name >>= \modL -> M.lookup modL infos)
      let all_names = modInfo_rdrs info
      maybe (throwE GhciNoMatchingModuleExport) pure $
          find (matchName name) all_names
@@ -206,7 +207,7 @@ resolveName spans' si = listToMaybe $ mapMaybe spaninfoVar $
 
 -- | Try to find the type of the given span.
 findType :: GhcMonad m
-         => Map ModuleName ModInfo
+         => Map Module ModInfo
          -> RealSrcSpan
          -> String
          -> ExceptT GhciModuleError m (ModInfo, Type)
@@ -228,14 +229,14 @@ findType infos span0 string = do
 
 -- | Guess a module name from a file path.
 guessModule :: GhcMonad m
-            => Map ModuleName ModInfo -> FilePath -> MaybeT m ModuleName
+            => Map Module ModInfo -> FilePath -> MaybeT m Module
 guessModule infos fp = do
-    target <- lift $ guessTarget fp Nothing Nothing
-    case targetId target of
-        TargetModule mn  -> return mn
+    target <- lift $ guessTargetId fp
+    case target of
+        TargetModule mn  -> MaybeT $ pure $ findModByModuleName mn
         TargetFile fp' _ -> guessModule' fp'
   where
-    guessModule' :: GhcMonad m => FilePath -> MaybeT m ModuleName
+    guessModule' :: GhcMonad m => FilePath -> MaybeT m Module
     guessModule' fp' = case findModByFp fp' of
         Just mn -> return mn
         Nothing -> do
@@ -243,19 +244,21 @@ guessModule infos fp = do
 
             target' <- lift $ guessTarget fp'' Nothing Nothing
             case targetId target' of
-                TargetModule mn -> return mn
+                TargetModule mn -> return $ mkModule (RealUnit $ Definite $ targetUnitId target') mn
                 _               -> MaybeT . pure $ findModByFp fp''
 
-    findModByFp :: FilePath -> Maybe ModuleName
+    findModByFp :: FilePath -> Maybe Module
     findModByFp fp' = fst <$> find ((Just fp' ==) . mifp) (M.toList infos)
       where
-        mifp :: (ModuleName, ModInfo) -> Maybe FilePath
+        mifp :: (Module, ModInfo) -> Maybe FilePath
         mifp = ml_hs_file . ms_location . modinfoSummary . snd
 
+    findModByModuleName :: ModuleName -> Maybe Module
+    findModByModuleName mn = find ((== mn) . moduleName) (M.keys infos)
 
 -- | Collect type info data for the loaded modules.
-collectInfo :: (GhcMonad m) => Map ModuleName ModInfo -> [ModuleName]
-               -> m (Map ModuleName ModInfo)
+collectInfo :: (GhcMonad m) => Map Module ModInfo -> [Module]
+               -> m (Map Module ModInfo)
 collectInfo ms loaded = do
     df <- getDynFlags
     unit_state <- hsc_units <$> getSession
@@ -299,17 +302,17 @@ srcFilePath modSum = fromMaybe obj_fp src_fp
         ms_loc = ms_location modSum
 
 -- | Get info about the module: summary, types, etc.
-getModInfo :: (GhcMonad m) => ModuleName -> m ModInfo
-getModInfo name = do
-    m <- getModSummary name
-    p <- parseModule m
+getModInfo :: (GhcMonad m) => Module -> m ModInfo
+getModInfo m = do
+    mod_summary <- getModSummary m
+    p <- parseModule mod_summary
     typechecked <- typecheckModule p
     let allTypes = processAllTypeCheckedModule typechecked
     let !rdr_env = tcg_rdr_env (fst $ tm_internals_ typechecked)
-    ts <- liftIO $ getModificationTime $ srcFilePath m
+    ts <- liftIO $ getModificationTime $ srcFilePath mod_summary
     return $
       ModInfo
-        { modinfoSummary    = m
+        { modinfoSummary    = mod_summary
         , modinfoSpans      = allTypes
         , modinfoRdrEnv     = forceGlobalRdrEnv rdr_env
         , modinfoLastUpdate = ts


=====================================
ghc/GHCi/UI/Monad.hs
=====================================
@@ -91,7 +91,6 @@ data GHCiState = GHCiState
         prompt_cont    :: PromptFunction,
         editor         :: String,
         stop           :: String,
-        multiMode      :: Bool,
         localConfig    :: LocalConfigBehaviour,
         options        :: [GHCiOption],
         line_number    :: !Int,         -- ^ input line
@@ -155,7 +154,7 @@ data GHCiState = GHCiState
         long_help  :: String,
         lastErrorLocations :: IORef [(FastString, Int)],
 
-        mod_infos  :: !(Map ModuleName ModInfo),
+        mod_infos  :: !(Map Module ModInfo),
 
         flushStdHandles :: ForeignHValue,
             -- ^ @hFlush stdout; hFlush stderr@ in the interpreter


=====================================
ghc/Main.hs
=====================================
@@ -302,7 +302,8 @@ ghciUI units srcs maybe_expr = do
         [] -> return []
         _  -> do
           s <- initMake srcs
-          return $ map (uncurry (,Nothing,)) s
+          dflags <- getDynFlags
+          return $ map (uncurry (,Just $ homeUnitId_ dflags,)) s
   interactiveUI defaultGhciSettings hs_srcs maybe_expr
 #endif
 


=====================================
testsuite/tests/linters/notes.stdout
=====================================
@@ -6,39 +6,37 @@ ref    compiler/GHC/Core/Opt/Simplify/Iteration.hs:2556:55:     Note [Plan (AFTE
 ref    compiler/GHC/Core/Opt/Simplify/Iteration.hs:2985:13:     Note [Case binder next]
 ref    compiler/GHC/Core/Opt/Simplify/Iteration.hs:4345:8:     Note [Lambda-bound unfoldings]
 ref    compiler/GHC/Core/Opt/Simplify/Utils.hs:1387:37:     Note [Gentle mode]
-ref    compiler/GHC/Core/Opt/Specialise.hs:1761:29:     Note [Arity decrease]
+ref    compiler/GHC/Core/Opt/Specialise.hs:1758:29:     Note [Arity decrease]
 ref    compiler/GHC/Core/TyCo/Rep.hs:1783:31:     Note [What prevents a constraint from floating]
-ref    compiler/GHC/Driver/DynFlags.hs:1218:52:     Note [Eta-reduction in -O0]
-ref    compiler/GHC/Driver/Main.hs:1901:34:     Note [simpleTidyPgm - mkBootModDetailsTc]
+ref    compiler/GHC/Driver/DynFlags.hs:1217:52:     Note [Eta-reduction in -O0]
+ref    compiler/GHC/Driver/Main.hs:1886:34:     Note [simpleTidyPgm - mkBootModDetailsTc]
 ref    compiler/GHC/Hs/Expr.hs:189:63:     Note [Pending Splices]
-ref    compiler/GHC/Hs/Expr.hs:2194:87:     Note [Lifecycle of a splice]
-ref    compiler/GHC/Hs/Expr.hs:2230:7:     Note [Pending Splices]
-ref    compiler/GHC/Hs/Extension.hs:148:5:     Note [Strict argument type constraints]
+ref    compiler/GHC/Hs/Expr.hs:2208:87:     Note [Lifecycle of a splice]
+ref    compiler/GHC/Hs/Expr.hs:2244:7:     Note [Pending Splices]
+ref    compiler/GHC/Hs/Extension.hs:151:5:     Note [Strict argument type constraints]
 ref    compiler/GHC/Hs/Pat.hs:151:74:     Note [Lifecycle of a splice]
 ref    compiler/GHC/HsToCore/Pmc/Solver.hs:860:20:     Note [COMPLETE sets on data families]
 ref    compiler/GHC/HsToCore/Quote.hs:1533:7:     Note [How brackets and nested splices are handled]
 ref    compiler/GHC/Stg/Unarise.hs:457:32:     Note [Renaming during unarisation]
 ref    compiler/GHC/Tc/Gen/HsType.hs:563:56:     Note [Skolem escape prevention]
-ref    compiler/GHC/Tc/Gen/HsType.hs:2717:7:     Note [Matching a kind signature with a declaration]
+ref    compiler/GHC/Tc/Gen/HsType.hs:2718:7:     Note [Matching a kind signature with a declaration]
 ref    compiler/GHC/Tc/Gen/Pat.hs:284:20:     Note [Typing patterns in pattern bindings]
-ref    compiler/GHC/Tc/Gen/Pat.hs:1378:7:     Note [Matching polytyped patterns]
+ref    compiler/GHC/Tc/Gen/Pat.hs:1380:7:     Note [Matching polytyped patterns]
 ref    compiler/GHC/Tc/Gen/Sig.hs:91:10:     Note [Overview of type signatures]
-ref    compiler/GHC/Tc/Gen/Splice.hs:368:16:     Note [How brackets and nested splices are handled]
-ref    compiler/GHC/Tc/Gen/Splice.hs:543:35:     Note [PendingRnSplice]
-ref    compiler/GHC/Tc/Gen/Splice.hs:670:7:     Note [How brackets and nested splices are handled]
+ref    compiler/GHC/Tc/Gen/Splice.hs:367:16:     Note [How brackets and nested splices are handled]
+ref    compiler/GHC/Tc/Gen/Splice.hs:542:35:     Note [PendingRnSplice]
+ref    compiler/GHC/Tc/Gen/Splice.hs:669:7:     Note [How brackets and nested splices are handled]
 ref    compiler/GHC/Tc/Gen/Splice.hs:909:11:     Note [How brackets and nested splices are handled]
 ref    compiler/GHC/Tc/Instance/Family.hs:458:35:     Note [Constrained family instances]
-ref    compiler/GHC/Tc/Solver/Rewrite.hs:1015:7:     Note [Stability of rewriting]
-ref    compiler/GHC/Tc/TyCl.hs:1322:6:     Note [Unification variables need fresh Names]
+ref    compiler/GHC/Tc/Solver/Rewrite.hs:1020:7:     Note [Stability of rewriting]
+ref    compiler/GHC/Tc/TyCl.hs:1662:6:     Note [Unification variables need fresh Names]
 ref    compiler/GHC/Tc/Types/Constraint.hs:209:9:     Note [NonCanonical Semantics]
 ref    compiler/GHC/Types/Demand.hs:304:25:     Note [Preserving Boxity of results is rarely a win]
-ref    compiler/GHC/Unit/Module/Deps.hs:86:13:     Note [Structure of dep_boot_mods]
+ref    compiler/GHC/Unit/Module/Deps.hs:97:13:     Note [Structure of dep_boot_mods]
 ref    compiler/GHC/Utils/Monad.hs:415:34:     Note [multiShotIO]
 ref    compiler/Language/Haskell/Syntax/Binds.hs:206:31:     Note [fun_id in Match]
 ref    configure.ac:205:10:     Note [Linking ghc-bin against threaded stage0 RTS]
 ref    docs/core-spec/core-spec.mng:177:6:     Note [TyBinders]
-ref    ghc/GHCi/UI.hs:3292:17:     Note [Multiple Home Units aware GHCi]
-ref    ghc/GHCi/UI.hs:3292:17:     Note [Relation between the InteractiveContext and 'interactiveGhciUnitId']
 ref    hadrian/src/Expression.hs:145:30:     Note [Linking ghc-bin against threaded stage0 RTS]
 ref    linters/lint-notes/Notes.hs:32:29:     Note [" <> T.unpack x <> "]
 ref    linters/lint-notes/Notes.hs:69:22:     Note [...]



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/05f613722fd027733eff60dde2576bf...

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/05f613722fd027733eff60dde2576bf...
You're receiving this email because of your account on gitlab.haskell.org.

    

Hannes Siebenhandl (＠fendor)

tags

participants (1)