Fix plugin regression

polysemy-plugin/src/Polysemy/Plugin/Fundep.hs

@@ -1,5 +1,7 @@
 {-# LANGUAGE CPP                        #-}
+{-# LANGUAGE DerivingStrategies         #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE RecordWildCards            #-}
 {-# LANGUAGE ViewPatterns               #-}
 
 ------------------------------------------------------------------------------
@@ -52,13 +54,13 @@ import           Polysemy.Plugin.Fundep.Utils
 
 #if __GLASGOW_HASKELL__ >= 900
 import           GHC.Builtin.Types.Prim (alphaTys)
-import           GHC.Plugins (idType, tyConClass_maybe)
+import           GHC.Plugins (idType, tyConClass_maybe, ppr, Outputable, sep, text, (<+>), parens)
 import           GHC.Tc.Types.Evidence
 import           GHC.Tc.Plugin (TcPluginM, tcPluginIO)
 import           GHC.Tc.Types
 import           GHC.Tc.Types.Constraint
 import           GHC.Tc.Utils.Env (tcGetInstEnvs)
-import           GHC.Tc.Utils.TcType (tcSplitPhiTy, tcSplitTyConApp)
+import           GHC.Tc.Utils.TcType (tcSplitPhiTy, tcSplitTyConApp, tcGetTyVar_maybe, tcSplitAppTy_maybe)
 import           GHC.Tc.Solver.Monad hiding (tcLookupClass)
 import           GHC.Core.Class (classTyCon)
 import           GHC.Core.InstEnv (lookupInstEnv, is_dfun)
@@ -71,14 +73,14 @@ import           Constraint
 #endif
 
 import           Class (classTyCon)
-import           GhcPlugins (idType, tyConClass_maybe)
+import           GhcPlugins (idType, tyConClass_maybe, ppr, Outputable, sep, text, (<+>), parens)
 import           Inst (tcGetInstEnvs)
 import           InstEnv (lookupInstEnv, is_dfun)
 import           MonadUtils (allM, anyM)
 import           TcEvidence
 import           TcPluginM (tcPluginIO)
 import           TcRnTypes
-import           TcType (tcSplitPhiTy, tcSplitTyConApp)
+import           TcType (tcSplitPhiTy, tcSplitTyConApp, tcGetTyVar_maybe, tcSplitAppTy_maybe)
 import           TcSMonad hiding (tcLookupClass)
 import           Type
 import           TysPrim (alphaTys)
@@ -99,6 +101,7 @@ fundepPlugin = TcPlugin
 ------------------------------------------------------------------------------
 -- | Like 'PredType', but has an 'Ord' instance.
 newtype PredType' = PredType' { getPredType :: PredType }
+  deriving newtype Outputable
 
 instance Eq PredType' where
   (==) = ((== EQ) .) . compare
@@ -117,6 +120,13 @@ data FindConstraint = FindConstraint
   , fcRow        :: Type  -- ^ @r@
   }
 
+instance Outputable FindConstraint where
+  ppr FindConstraint{..} = parens $ sep
+    [ text "effect name = " <+> ppr fcEffectName
+    , text "effect = " <+> ppr fcEffect
+    , text "row = " <+> ppr fcRow
+    ]
+
 
 ------------------------------------------------------------------------------
 -- | Given a list of constraints, filter out the 'FindConstraint's.
@@ -213,6 +223,18 @@ mkWantedForce fc given = do
     wanted = fcEffect fc
 
 
+------------------------------------------------------------------------------
+-- | It's very important that we don't try to unify entire effects when we're
+-- in interpreter mode. It's OK to unify @T x ~ T y@, but never @e ~ T y@. This
+-- function takes then "given" of an interpreter, and produces a singleton
+-- skolem set iff the outermost effect to be unified is a tyvar.
+skolemsForInterpreter :: Type -> Set TyVar
+skolemsForInterpreter ty =
+  case tcSplitAppTy_maybe ty of
+    Just (tcGetTyVar_maybe -> Just skolem, _) -> S.singleton skolem
+    _ -> maybe mempty S.singleton $ tcGetTyVar_maybe ty
+
+
 ------------------------------------------------------------------------------
 -- | Generate a wanted unification for the effect described by the
 -- 'FindConstraint' and the given effect --- if they can be unified in this
@@ -222,7 +244,7 @@ mkWanted
     -> SolveContext
     -> Type  -- ^ The given effect.
     -> TcPluginM (Maybe (Unification, Ct))
-mkWanted fc solve_ctx given =
+mkWanted fc solve_ctx given = do
   whenA (not (mustUnify solve_ctx) || isJust (unify solve_ctx wanted given)) $
     mkWantedForce fc given
   where
@@ -359,8 +381,10 @@ solveFundep (ref, stuff) given _ wanted = do
         case splitAppTys r of
           (_, [_, eff', _]) ->
             mkWanted fc
-                     (InterpreterUse $ exactlyOneWantedForR wanted_finds r)
-                     eff'
+              (InterpreterUse
+                (exactlyOneWantedForR wanted_finds r)
+                (skolemsForInterpreter eff'))
+              eff'
           _ -> pure Nothing
 
   -- We only want to emit a unification wanted once, otherwise a type error can

polysemy-plugin/src/Polysemy/Plugin/Fundep/Unification.hs

@@ -17,9 +17,11 @@ import           TcRnTypes
 #if __GLASGOW_HASKELL__ >= 900
 import           GHC.Core.Type
 import           GHC.Core.Unify
+import           GHC.Plugins (Outputable, ppr, parens, text, (<+>))
 #else
 import           Type
 import           Unify
+import           GhcPlugins (Outputable, ppr, parens, text, (<+>))
 #endif
 
 
@@ -32,9 +34,13 @@ data SolveContext
     -- | In the context of running an interpreter. The 'Bool' corresponds to
     -- whether we are only trying to solve a single 'Member' constraint right
     -- now. If so, we *must* produce a unification wanted.
-  | InterpreterUse Bool
+  | InterpreterUse Bool (Set TyVar)
   deriving (Eq, Ord)
 
+instance Outputable SolveContext where
+  ppr (FunctionDef s) = parens $ text "FunctionDef" <+> ppr s
+  ppr (InterpreterUse s ty) = parens $ text "InterpreterUse" <+> ppr s <+> ppr ty
+
 
 ------------------------------------------------------------------------------
 -- | Depending on the context in which we're solving a constraint, we may or
@@ -43,7 +49,7 @@ data SolveContext
 -- r s@, we should unify @s ~ Int@.
 mustUnify :: SolveContext -> Bool
 mustUnify (FunctionDef _) = True
-mustUnify (InterpreterUse b) = b
+mustUnify (InterpreterUse b _) = b
 
 
 ------------------------------------------------------------------------------
@@ -63,8 +69,8 @@ unify solve_ctx = tryUnifyUnivarsButNotSkolems skolems
     skolems :: Set TyVar
     skolems =
       case solve_ctx of
-        InterpreterUse _ -> mempty
-        FunctionDef s    -> s
+        InterpreterUse _ s -> s
+        FunctionDef s      -> s
 
 
 tryUnifyUnivarsButNotSkolems :: Set TyVar -> Type -> Type -> Maybe TCvSubst