src/Example.hs

@@ -101,6 +101,12 @@ exampleNameResolutionAssignment = Block [
 exampleCharIntOperation :: Expression
 exampleCharIntOperation = BinaryOperation Addition (CharacterLiteral 'a') (IntegerLiteral 1)
 
+exampleNullDeclaration :: Statement
+exampleNullDeclaration = LocalVariableDeclaration (VariableDeclaration "Person" "bob" (Just NullLiteral))
+
+exampleNullDeclarationFail :: Statement
+exampleNullDeclarationFail = LocalVariableDeclaration (VariableDeclaration "int" "a" (Just NullLiteral))
+
 testClasses :: [Class]
 testClasses = [
     Class "Person" [
@@ -226,3 +232,17 @@ runTypeCheck = do
         printResult "Result Char Int Operation:" evaluatedCharIntOperation
         ) handleError
 
+    catch (do
+        print "====================================================================================="
+        evaluatedNullDeclaration <- evaluate (typeCheckStatement exampleNullDeclaration [] sampleClasses)
+        printSuccess "Type checking of null declaration completed successfully"
+        printResult "Result Null Declaration:" evaluatedNullDeclaration
+        ) handleError
+
+    catch (do
+        print "====================================================================================="
+        evaluatedNullDeclarationFail <- evaluate (typeCheckStatement exampleNullDeclarationFail [] sampleClasses)
+        printSuccess "Type checking of null declaration failed"
+        printResult "Result Null Declaration:" evaluatedNullDeclarationFail
+        ) handleError
+

src/Typecheck.hs

@@ -226,8 +226,12 @@ typeCheckStatement (LocalVariableDeclaration (VariableDeclaration dataType ident
         let checkedExpr = fmap (\expr -> typeCheckExpression expr symtab classes) maybeExpr
             exprType = fmap getTypeFromExpr checkedExpr
         in case exprType of
-            Just t | t /= dataType -> error $ "Type mismatch in declaration of '" ++ identifier ++ "': expected " ++ dataType ++ ", found " ++ t
+            Just t
-            _ -> TypedStatement dataType (LocalVariableDeclaration (VariableDeclaration dataType identifier checkedExpr))
+                | t == "null" && isObjectType dataType -> 
+                    TypedStatement dataType (LocalVariableDeclaration (VariableDeclaration dataType identifier checkedExpr))
+                | t /= dataType -> error $ "Type mismatch in declaration of '" ++ identifier ++ "': expected " ++ dataType ++ ", found " ++ t
+                | otherwise -> TypedStatement dataType (LocalVariableDeclaration (VariableDeclaration dataType identifier checkedExpr))
+            Nothing -> TypedStatement dataType (LocalVariableDeclaration (VariableDeclaration dataType identifier checkedExpr))
 
 typeCheckStatement (While cond stmt) symtab classes =
   let cond' = typeCheckExpression cond symtab classes
@@ -278,6 +282,9 @@ typeCheckStatement (StatementExpressionStatement stmtExpr) symtab classes =
 
 -- ********************************** Type Checking: Helpers **********************************
 
+isObjectType :: DataType -> Bool
+isObjectType dt = dt /= "int" && dt /= "boolean" && dt /= "char"
+
 getTypeFromExpr :: Expression -> DataType
 getTypeFromExpr (TypedExpression t _) = t
 getTypeFromExpr _ = error "Untyped expression found where typed was expected"