Fix unification and make union types workHEADmaster

1 files changed, 12 insertions, 3 deletions

diff --git a/typecheck/CC/Typecheck/Types.hs b/typecheck/CC/Typecheck/Types.hs
index 3009ca1..dc0740d 100644
--- a/typecheck/CC/Typecheck/Types.hs
+++ b/typecheck/CC/Typecheck/Types.hs
@@ -14,17 +14,22 @@ import CC.Pretty
 import CC.Types
 
 
-data TCError = TypeError SourceRange T.Type T.Type
+data TCError = UnifyError SourceRange T.Type T.Type T.Type T.Type (Maybe UnifyReason)
              | RefError SourceRange Name
              | TypeArityError SourceRange Name Int Int
              | DupTypeError Name
   deriving (Show)
 
+data UnifyReason = URNotInUnion | URAmbiguousWeakening
+  deriving (Show)
+
 instance Pretty TCError where
-    pretty (TypeError sr real expect) =
+    pretty (UnifyError sr real expect unifyt1 unifyt2 mreason) =
         "Type error: Expression at " ++ pretty sr ++
             " has type " ++ pretty real ++
-            ", but should have type " ++ pretty expect
+            ", but should have type " ++ pretty expect ++
+            " (when unifying " ++ pretty unifyt1 ++ " and " ++ pretty unifyt2 ++ ")" ++
+            maybe "" (\r -> " (reason: " ++ pretty r ++ ")") mreason
     pretty (RefError sr name) =
         "Reference error: Variable '" ++ name ++ "' out of scope at " ++ pretty sr
     pretty (TypeArityError sr name wanted got) =
@@ -33,6 +38,10 @@ instance Pretty TCError where
     pretty (DupTypeError name) =
         "Duplicate types: Type '" ++ name ++ "' defined multiple times"
 
+instance Pretty UnifyReason where
+    pretty URNotInUnion = "type not found in union"
+    pretty URAmbiguousWeakening = "type unifies with multiple items in the union"
+
 type TM a = ExceptT TCError (State Int) a
 
 genId :: TM Int