Noodling on the type checker

5 files changed, 117 insertions, 79 deletions

diff --git a/src/HSVIS/AST.hs b/src/HSVIS/AST.hs
index 63594bc..6c0b323 100644
--- a/src/HSVIS/AST.hs
+++ b/src/HSVIS/AST.hs
@@ -98,7 +98,7 @@ data Type s
   | TFun (X Type s) (Type s) (Type s)
   | TCon (X Type s) Name
   | TVar (X Type s) Name
-  | TForall (X Type s) Name (Type s)  -- ^ implicit; also, not parsed
+  -- | TForall (X Type s) Name (Type s)  -- ^ implicit; also, not parsed
 
   -- extension point
   | TExt (X Type s) !(E Type s)
@@ -163,8 +163,8 @@ instance Pretty (E Type s) => Pretty (Type s) where
     prettysPrec 0 a . showString " -> " . prettysPrec (-1) b
   prettysPrec _ (TCon _ n) = prettysPrec 11 n
   prettysPrec _ (TVar _ n) = prettysPrec 11 n
-  prettysPrec d (TForall _ n t) = showParen (d > -1) $
-    showString "forall " . prettysPrec 11 n . showString ". " . prettysPrec (-1) t
+  -- prettysPrec d (TForall _ n t) = showParen (d > -1) $
+  --   showString "forall " . prettysPrec 11 n . showString ". " . prettysPrec (-1) t
   prettysPrec d (TExt _ e) = prettysPrec d e
 
 instance (Pretty (X Type s), Pretty (E Type s)) => Pretty (DataDef s) where
@@ -254,7 +254,7 @@ instance HasExt Type where
   extOf (TFun x _ _) = x
   extOf (TCon x _) = x
   extOf (TVar x _) = x
-  extOf (TForall x _ _) = x
+  -- extOf (TForall x _ _) = x
   extOf (TExt x _) = x
 
   -- extMap p ps1 ps2 f g (TApp x a b) = TApp (f x) (extMap p ps1 ps2 f g a) (map (extMap p ps1 ps2 f g) b)
diff --git a/src/HSVIS/Parser.hs b/src/HSVIS/Parser.hs
index 0d35184..2110ca1 100644
--- a/src/HSVIS/Parser.hs
+++ b/src/HSVIS/Parser.hs
@@ -20,8 +20,8 @@ module HSVIS.Parser (
   StageParsed,
   parse,
   -- * Staged AST synonyms
-  PProgram, PDataDef, PFunDef, PFunEq, PType, PPattern, PRHS, PExpr,
-  E(NoTypeSig),
+  PProgram, PDataDef, PFunDef, PTypeSig, PFunEq, PKind, PType, PPattern, PRHS, PExpr,
+  E(NoTypeSig, TForallP),
 ) where
 
 -- import Control.Applicative
@@ -54,20 +54,22 @@ type instance X DataField StageParsed = ()
 type instance X FunDef  StageParsed = Range
 type instance X TypeSig StageParsed = ()
 type instance X FunEq   StageParsed = Range
+type instance X Kind    StageParsed = ()
 type instance X Type    StageParsed = Range
 type instance X Pattern StageParsed = Range
 type instance X RHS     StageParsed = Range
 type instance X Expr    StageParsed = Range
 
 data instance E TypeSig StageParsed = NoTypeSig deriving (Show)
-data instance E Type    StageParsed deriving (Show)
+data instance E Type    StageParsed = TForallP Name (Maybe PKind) PType deriving (Show)
 data instance E Kind    StageParsed deriving (Show)
 
 type PProgram = Program StageParsed
 type PDataDef = DataDef StageParsed
 type PFunDef  = FunDef  StageParsed
--- type PTypeSig = TypeSig StageParsed
+type PTypeSig = TypeSig StageParsed
 type PFunEq   = FunEq   StageParsed
+type PKind    = Kind    StageParsed
 type PType    = Type    StageParsed
 type PPattern = Pattern StageParsed
 type PRHS     = RHS     StageParsed
diff --git a/src/HSVIS/Pretty.hs b/src/HSVIS/Pretty.hs
index 0f98da3..cc8cb2a 100644
--- a/src/HSVIS/Pretty.hs
+++ b/src/HSVIS/Pretty.hs
@@ -12,5 +12,8 @@ instance Pretty Void where
 prettyPrec :: Pretty a => Int -> a -> String
 prettyPrec d x = prettysPrec d x ""
 
+prettys :: Pretty a => a -> ShowS
+prettys = prettysPrec 0
+
 pretty :: Pretty a => a -> String
 pretty x = prettyPrec minBound x
diff --git a/src/HSVIS/Typecheck.hs b/src/HSVIS/Typecheck.hs
index 1e46a99..8b642eb 100644
--- a/src/HSVIS/Typecheck.hs
+++ b/src/HSVIS/Typecheck.hs
@@ -1,13 +1,14 @@
+{-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DerivingStrategies #-}
 {-# LANGUAGE EmptyDataDeriving #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE GADTs #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE ScopedTypeVariables #-}
-{-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TupleSections #-}
-{-# LANGUAGE GADTs #-}
+{-# LANGUAGE TypeFamilies #-}
 module HSVIS.Typecheck (
   StageTyped,
   typecheck,
@@ -52,7 +53,7 @@ type instance X Pattern StageTC = (Range, CType)
 type instance X RHS     StageTC = CType
 type instance X Expr    StageTC = (Range, CType)
 
-data instance E Type StageTC = TUniVar Int deriving (Show, Eq, Ord)
+data instance E Type StageTC = TUniVar Int | TForallC (Name, CKind) CType deriving (Show, Eq, Ord)
 data instance E Kind StageTC = KUniVar Int deriving (Show, Eq, Ord)
 data instance E TypeSig StageTC deriving (Show)
 
@@ -60,6 +61,7 @@ type CProgram = Program StageTC
 type CDataDef = DataDef StageTC
 type CDataField = DataField StageTC
 type CFunDef  = FunDef  StageTC
+type CTypeSig = TypeSig StageTC
 type CFunEq   = FunEq   StageTC
 type CKind    = Kind    StageTC
 type CType    = Type    StageTC
@@ -80,7 +82,7 @@ type instance X Pattern StageTyped = TType
 type instance X RHS     StageTyped = TType
 type instance X Expr    StageTyped = TType
 
-data instance E Type StageTyped deriving (Show)
+data instance E Type StageTyped = TForall (Name, TKind) TType deriving (Show)
 data instance E Kind StageTyped deriving (Show)
 data instance E TypeSig StageTyped deriving (Show)
 
@@ -97,6 +99,8 @@ type TExpr    = Expr    StageTyped
 
 instance Pretty (E Type StageTC) where
   prettysPrec _ (TUniVar n) = showString ("?t" ++ show n)
+  prettysPrec d (TForallC (n, k) t) = showParen (d > 0) $
+    showString "forall (" . prettys n . showString " :: " . prettys k . showString "). " . prettys t
 
 instance Pretty (E Kind StageTC) where
   prettysPrec _ (KUniVar n) = showString ("?k" ++ show n)
@@ -140,17 +144,18 @@ newtype TCM a = TCM {
 
 instance Functor TCM where
   fmap f (TCM g) = TCM $ \ctx i env ->
-    let (ds, cs, i', env', x) = g ctx i env
-    in (ds, cs, i', env', f x)
+    let !(ds, cs, i', env', !x) = g ctx i env
+        !res = f x
+    in (ds, cs, i', env', res)
 
 instance Applicative TCM where
-  pure x = TCM $ \_ i env -> (mempty, mempty, i, env, x)
+  pure x = TCM $ \_ i env -> x `seq` (mempty, mempty, i, env, x)
   (<*>) = ap
 
 instance Monad TCM where
   TCM f >>= g = TCM $ \ctx i1 env1 ->
-    let (ds2, cs2, i2, env2, x) = f ctx i1 env1
-        (ds3, cs3, i3, env3, y) = runTCM (g x) ctx i2 env2
+    let !(ds2, cs2, i2, env2, !x) = f ctx i1 env1
+        !(ds3, cs3, i3, env3, !y) = runTCM (g x) ctx i2 env2
     in (ds2 <> ds3, cs2 <> cs3, i3, env3, y)
 
 class Monad m => MonadRaise m where
@@ -232,14 +237,18 @@ getKind' :: Range -> Name -> TCM CKind
 getKind' rng name = getKind name >>= \case
   Nothing -> do
     raise SError rng $ "Type not in scope: " ++ pretty name
-    genKUniVar
+    k <- genKUniVar  -- insert it now so that all occurrences of this out-of-scope name get the same kind
+    modifyTEnv (Map.insert name k)
+    return k
   Just k -> return k
 
 getType' :: Range -> Name -> TCM CType
 getType' rng name = getType name >>= \case
   Nothing -> do
     raise SError rng $ "Variable not in scope: " ++ pretty name
-    genUniVar (KType ())
+    t <- genUniVar (KType ())  -- insert it now so that all occurrences of this out-of-scope name get the same type
+    modifyVEnv (Map.insert name t)
+    return t
   Just k -> return k
 
 tcTop :: PProgram -> TCM TProgram
@@ -260,8 +269,9 @@ tcProgram (Program ddefs1 fdefs1) = do
   let ddefs3 = map (substDdef kinduvars mempty) ddefs2
   modifyTEnv (Map.map (substKind kinduvars))
 
-  -- generate inverse constructors for all data types
-  forM_ ddefs3 $ \ddef ->
+  -- generate constructor values and inverse constructors for all data types
+  forM_ ddefs3 $ \ddef -> do
+    modifyVEnv (Map.fromList (generateConstructors ddef) <>)
     modifyICEnv (Map.fromList (generateInvCons ddef) <>)
 
   traceM (unlines (map pretty ddefs3))
@@ -309,6 +319,13 @@ generateInvCons (DataDef k tname params cons) =
       resty = TApp (KType ()) (TCon k tname) (map (uncurry TVar) params)
   in [(cname, InvCon tyvars resty (map dataFieldType fields)) | (cname, fields) <- cons]
 
+generateConstructors :: CDataDef -> [(Name, CType)]
+generateConstructors (DataDef k tname params cons) =
+  let resty = TApp (KType ()) (TCon k tname) (map (uncurry TVar) params)
+  in [let funty = foldr (TFun (KType ())) resty (map dataFieldType fields)
+      in (cname, foldr (\(k1, n1) -> TExt (KType ()) . TForallC (n1, k1)) funty params)
+     | (cname, fields) <- cons]
+
 promoteDownK :: Maybe CKind -> TCM CKind
 promoteDownK Nothing = genKUniVar
 promoteDownK (Just k) = return k
@@ -338,7 +355,7 @@ kcType' :: forall ext ret. Monoid ext => KCTypeMode ext ret -> Maybe CKind -> PT
 kcType' mode mdown = \case
   TApp rng t ts -> do
     (ext1, t') <- kcType' mode Nothing t
-    (ext2, ts') <- sequence <$> mapM (kcType' mode Nothing) ts
+    (ext2, ts') <- sequence <$> mapM (kcType' mode Nothing) ts  -- TODO: give more useful down kinds
     retk <- promoteDownK mdown
     let expected = foldr (KFun ()) retk (map extOf ts')
     emit $ CEqK (extOf t') expected rng
@@ -346,22 +363,17 @@ kcType' mode mdown = \case
 
   TTup rng ts -> do
     (ext, ts') <- sequence <$> mapM (kcType' mode (Just (KType ()))) ts
-    forM_ (zip (map extOf ts) ts') $ \(trng, ct) ->
-      emit $ CEqK (extOf ct) (KType ()) trng
     downEqK rng mdown (KType ())
     return (ext, TTup (KType ()) ts')
 
   TList rng t -> do
     (ext, t') <- kcType' mode (Just (KType ())) t
-    emit $ CEqK (extOf t') (KType ()) (extOf t)
     downEqK rng mdown (KType ())
     return (ext, TList (KType ()) t')
 
   TFun rng t1 t2 -> do
     (ext1, t1') <- kcType' mode (Just (KType ())) t1
     (ext2, t2') <- kcType' mode (Just (KType ())) t2
-    emit $ CEqK (extOf t1') (KType ()) (extOf t1)
-    emit $ CEqK (extOf t2') (KType ()) (extOf t2)
     downEqK rng mdown (KType ())
     return (ext1 <> ext2, TFun (KType ()) t1' t2')
 
@@ -371,61 +383,66 @@ kcType' mode mdown = \case
     return (mempty, TCon k n)
 
   TVar rng n -> do
-    k <- getKind' rng n
-    downEqK rng mdown k
-    return (case mode of KCTMNormal -> ()
-                         KCTMOpen -> MMap.singleton n (pure k)
-           ,TVar k n)
-
-  TForall rng n t -> do  -- implicit forall
-    k1 <- genKUniVar
+    mk <- getKind n
+    case mk of
+      Nothing -> do
+        k <- promoteDownK mdown
+        return (case mode of KCTMNormal -> ()
+                             KCTMOpen -> MMap.singleton n (pure k)
+               ,TVar k n)
+      Just k -> do
+        downEqK rng mdown k
+        -- TODO: need to instantiate top-level foralls in k here
+        return (mempty, TVar k n)
+
+  TExt rng (TForallP n mk1 t) -> do  -- implicit forall
+    k1 <- maybe genKUniVar (return . checkKind) mk1
     k2 <- genKUniVar
     downEqK rng mdown k2
     (ext, t') <- scopeTEnv $ do
       modifyTEnv (Map.insert n k1)
       kcType' mode (Just k2) t
-    return (ext, TForall k2 n t')  -- not 'k1 -> k2' because the forall is implicit
+    return (ext, TExt k2 (TForallC (n, k1) t'))  -- 'k2', not 'k1 -> k2', because the forall is implicit
 
 tcFunDefBlock :: [PFunDef] -> TCM [CFunDef]
 tcFunDefBlock fdefs = do
-  -- generate preliminary unification variables for the functions' types
-  bound <- mapM (\(FunDef _ n _ _) -> (n,) <$> genUniVar (KType ())) fdefs
-  defs' <- forM fdefs $ \def@(FunDef _ name _ _) ->
-    scopeVEnv $ do
-      modifyVEnv (Map.fromList [(n, t) | (n, t) <- bound, n /= name] <>)
-      tcFunDef def
-
-  -- take the actual found types for typechecking the body (and link them
-  -- to the variables generated above)
-  let bound2 = map (\(FunDef _ n (TypeSig _ ty) _) -> (n, ty)) defs'
-  forM_ (zip3 fdefs bound bound2) $ \(fdef, (_, tvar), (_, ty)) ->
-    emit $ CEq ty tvar (extOf fdef)  -- which is expected/observed? which range? /shrug/
+  -- collect types for each of the bound functions, or unification variables if there's no type signature
+  bound <- mapM (\(FunDef _ n ts _) -> (n,) <$> tcTypeSig ts) fdefs
+  defs' <- scopeVEnv $ do
+    modifyVEnv (Map.fromList [(n, t) | (n, TypeSig _ t) <- bound] <>)
+    forM (zip fdefs bound) $ \(def, (_, sig)) ->
+      tcFunDef sig def
+
+  -- -- take the actual found types for typechecking the body (and link them
+  -- -- to the variables generated above)
+  -- let bound2 = map (\(FunDef _ n (TypeSig _ ty) _) -> (n, ty)) defs'
+  -- forM_ (zip3 fdefs bound bound2) $ \(fdef, (_, tvar), (_, ty)) ->
+  --   emit $ CEq ty tvar (extOf fdef)  -- which is expected/observed? which range? /shrug/
 
   return defs'
 
-tcFunDef :: PFunDef -> TCM CFunDef
-tcFunDef (FunDef rng name msig eqs) = do
+-- | Just the identity, because there's nothing to be checked in our simple kinds
+checkKind :: PKind -> CKind
+checkKind (KType ()) = KType ()
+checkKind (KFun () k1 k2) = KFun () (checkKind k1) (checkKind k2)
+
+tcTypeSig :: PTypeSig -> TCM CTypeSig
+tcTypeSig (TypeSig () sig) = do
+  (typ, freetvars) <- kcType KCTMOpen (Just (KType ())) sig
+  return $ TypeSig (Just (extOf sig)) $ foldr (\(n, k) -> TExt (KType ()) . TForallC (n, k)) typ (Map.assocs freetvars)
+tcTypeSig (TypeSigExt () NoTypeSig) = TypeSig Nothing <$> genUniVar (KType ())
+
+-- | Typechecks the function definition, but assumes its signature has already
+-- been checked, and is passed separately. Thus the PTypeSig in the PFunDef is
+-- ignored.
+tcFunDef :: CTypeSig -> PFunDef -> TCM CFunDef
+tcFunDef typesig@(TypeSig _ funtyp) (FunDef rng name _ eqs) = do
   when (not $ allEq (fmap (length . funeqPats) eqs)) $
     raise SError rng "Function equations have differing numbers of arguments"
 
-  (typ, msigrng) <- case msig of
-    TypeSig _ sig -> do
-      (typ, freetvars) <- kcType KCTMOpen (Just (KType ())) sig
-      TODO -- We need to check that these free type variables do not escape.
-           -- Perhaps with levels on unification variables? Associate a level
-           -- to a generated uvar, and increment the global level counter when
-           -- passing below a forall.
-           -- But how do we deal with functions without a type signature
-           -- anyway? We should be able to infer a polymorphic type for them.
-      return (foldr (\(n, k) -> TForall k n) typ (Map.assocs freetvars)
-             ,Just (extOf sig))
-    TypeSigExt _ NoTypeSig -> (,Nothing) <$> genUniVar (KType ())
-
-  eqs' <- scopeVEnv $ do
-    modifyVEnv (Map.insert name typ)  -- allow function to be recursive
-    mapM (tcFunEq typ) eqs
-
-  return (FunDef rng name (TypeSig msigrng typ) eqs')
+  eqs' <- scopeVEnv $ mapM (tcFunEq funtyp) eqs
+
+  return (FunDef rng name typesig eqs')
 
 tcFunEq :: CType -> PFunEq -> TCM CFunEq
 tcFunEq down (FunEq rng name pats rhs) = do
@@ -503,13 +520,15 @@ tcExpr down = \case
 
   EVar rng n -> do
     ty <- getType' rng n
-    emit $ CEq down ty rng
-    return $ EVar (rng, ty) n
+    ty' <- instantiateTForallsUni ty
+    emit $ CEq down ty' rng
+    return $ EVar (rng, ty') n
 
   ECon rng n -> do
     ty <- getType' rng n
-    emit $ CEq down ty rng
-    return $ EVar (rng, ty) n
+    ty' <- instantiateTForallsUni ty
+    emit $ CEq down ty' rng
+    return $ EVar (rng, ty') n
 
   EList rng es -> do
     eltty <- genUniVar (KType ())
@@ -574,6 +593,14 @@ tcExpr down = \case
 
   EError rng -> return $ EError (rng, down)
 
+instantiateTForallsUni :: CType -> TCM CType
+instantiateTForallsUni = go mempty
+  where
+    go sub (TExt _ (TForallC (n, k1) t)) = do
+      var <- genUniVar k1
+      go (Map.insert n var sub) t
+    go sub t = return $ substType mempty mempty sub t
+
 unfoldFunTy :: Range -> Int -> CType -> TCM ([CType], CType)
 unfoldFunTy _ n t | n <= 0 = return ([], t)
 unfoldFunTy rng n (TFun _ t1 t2) = do
@@ -680,7 +707,8 @@ solveTypeVars cs = do
       (TFun _ t1 t2, TFun _ t1' t2') -> reduce t1 t1' rng <> reduce t2 t2' rng
       (TCon _ n1, TCon _ n2) | n1 == n2 -> mempty
       (TVar _ n1, TVar _ n2) | n1 == n2 -> mempty
-      (TForall _ n1 t1, TForall k n2 t2) ->
+      -- TODO: this doesn't check that the types are kind-correct. Did we already check that?
+      (TExt _ (TForallC (n1, k) t1), TExt _ (TForallC (n2, _) t2)) ->
         reduce t1 (substType mempty mempty (Map.singleton n2 (TVar k n1)) t2) rng
 
       -- otherwise, this is a kind mismatch
@@ -693,8 +721,8 @@ solveTypeVars cs = do
     typeSize (TFun _ t1 t2) = typeSize t1 + typeSize t2
     typeSize (TCon _ _) = 1
     typeSize (TVar _ _) = 1
-    typeSize (TForall _ _ t) = 1 + typeSize t
     typeSize (TExt _ TUniVar{}) = 2
+    typeSize (TExt _ (TForallC _ t)) = 1 + typeSize t
 
 partitionConstrs :: Foldable t => t Constr -> (Bag (CType, CType, Range), Bag (CKind, CKind, Range))
 partitionConstrs = foldMap $ \case CEq t1 t2 r -> (pure (t1, t2, r), mempty)
@@ -773,8 +801,8 @@ substType mk mt mtv = go
     go (TFun k t1 t2) = TFun (goKind k) (go t1) (go t2)
     go (TCon k n) = TCon (goKind k) n
     go (TVar k n) = fromMaybe (TVar (goKind k) n) (Map.lookup n mtv)
-    go (TForall k n t) = TForall (goKind k) n (go t)
     go (TExt k (TUniVar v)) = fromMaybe (TExt (goKind k) (TUniVar v)) (Map.lookup v mt)
+    go (TExt k (TForallC (n, k1) t)) = TExt (goKind k) (TForallC (n, goKind k1) (go t))
 
     goKind = substKind mk
 
@@ -847,16 +875,17 @@ finaliseExpr = \case
 finaliseType :: MonadRaise m => Range -> CType -> m TType
 finaliseType rng toptype = go toptype
   where
+    go :: MonadRaise m => Type StageTC -> m TType
     go (TApp k t ts) = TApp <$> finaliseKind k <*> go t <*> traverse go ts
     go (TTup k ts) = TTup <$> finaliseKind k <*> traverse go ts
     go (TList k t) = TList <$> finaliseKind k <*> go t
     go (TFun k t1 t2) = TFun <$> finaliseKind k <*> go t1 <*> go t2
     go (TCon k n) = TCon <$> finaliseKind k <*> pure n
     go (TVar k n) = TVar <$> finaliseKind k <*> pure n
-    go (TForall k n t) = TForall <$> finaliseKind k <*> pure n <*> go t
     go t@(TExt k TUniVar{}) = do
       raise SError rng $ "Ambiguous type unification variable " ++ pretty t ++ " in type: " ++ pretty toptype
       TVar <$> finaliseKind k <*> pure (Name "$_error")
+    go (TExt k (TForallC (n, k1) t)) = TExt <$> finaliseKind k <*> (TForall <$> ((,) <$> pure n <*> finaliseKind k1) <*> go t)
 
 finaliseKind :: MonadRaise m => CKind -> m TKind
 finaliseKind = \case
@@ -874,8 +903,8 @@ typeUniVars = \case
   TFun _ t1 t2 -> typeUniVars t1 <> typeUniVars t2
   TCon _ _ -> mempty
   TVar _ _ -> mempty
-  TForall _ _ t -> typeUniVars t
   TExt _ (TUniVar v) -> Set.singleton v
+  TExt _ (TForallC _ t) -> typeUniVars t
 
 kindUniVars :: CKind -> Set Int
 kindUniVars = \case
diff --git a/src/HSVIS/Typecheck/Solve.hs b/src/HSVIS/Typecheck/Solve.hs
index 7250e79..250d7e7 100644
--- a/src/HSVIS/Typecheck/Solve.hs
+++ b/src/HSVIS/Typecheck/Solve.hs
@@ -83,7 +83,8 @@ solveConstraints reduce frees subst detect size = \tupcs ->
       errs'' = fmap constrUnequal errs <> errs'
   in trace ("[solver] Solving:" ++ concat ["\n- " ++ pretty a ++ "  ==  " ++ pretty b ++ "  {" ++ pretty r ++ "}" | Constr a b r <- cs]) $
      trace ("[solver] Result: (with " ++ show (length errs'') ++ " errors)" ++
-              concat ["\n- " ++ show v ++ " = " ++ pretty t | (v, t) <- Map.assocs asg'])
+              concat ["\n- " ++ show v ++ " = " ++ pretty t | (v, t) <- Map.assocs asg'] ++
+              "\n... errors: " ++ intercalate ", " (map summariseUE (toList errs'')))
      (asg', errs'')
   where
     reduce' :: Constr t t -> (Bag (Constr v t), Bag (Constr t t))
@@ -136,10 +137,13 @@ solveConstraints reduce frees subst detect size = \tupcs ->
     applyLog ((v, t) : l) m = applyLog l $ Map.insert v (subst m t) m
     applyLog [] m = m
 
-    -- If there are multiple sources for the same cosntraint, only one of them is kept.
+    -- If there are multiple sources for the same constraint, only one of them is kept.
     dedupRCs :: Ord t => [RConstr t] -> [RConstr t]
     dedupRCs = map head . groupBy ((==) `on` rconType) . sortBy (comparing rconType)
 
+    summariseUE (UEUnequal t1 t2 rng) = "({" ++ pretty rng ++ "} " ++ pretty t1 ++ " /= " ++ pretty t2 ++ ")"
+    summariseUE (UERecursive v t2 rng) = "({" ++ pretty rng ++ "} [" ++ show v ++ "] rec in " ++ pretty t2 ++ ")"
+
 minimumByMay :: Foldable t' => (a -> a -> Ordering) -> t' a -> Maybe a
 minimumByMay cmp = foldl' min' Nothing
   where min' mx y = Just $! case mx of