challenge 6.3 done

Author: BaLiKfromUA

pylox/parser.py

@@ -9,7 +9,7 @@
 # recursive descent, top-down parser
 class Parser:
     def __init__(
-            self, tokens: List[Token], report_error: Optional[Callable] = None
+        self, tokens: List[Token], report_error: Optional[Callable] = None
     ) -> None:
         self.tokens = tokens
         self.current = 0
@@ -104,10 +104,10 @@ def comparison(self) -> ast.Expr:
         expr = self.term()
 
         while self.match(
-                TokenType.GREATER,
-                TokenType.GREATER_EQUAL,
-                TokenType.LESS,
-                TokenType.LESS_EQUAL,
+            TokenType.GREATER,
+            TokenType.GREATER_EQUAL,
+            TokenType.LESS,
+            TokenType.LESS_EQUAL,
         ):
             op = self.previous()
             right = self.term()
@@ -163,4 +163,35 @@ def primary(self) -> ast.Expr:
             self.consume(TokenType.RIGHT_PAREN, "Expect ')' after expression.")
             return ast.Grouping(expr)
 
+        # Error handling
+        if self.match(TokenType.BANG_EQUAL, TokenType.EQUAL_EQUAL):
+            err = self.error(self.previous(), "Missing left-hand operand.")
+            self.equality()
+            raise err
+
+        if self.match(TokenType.BANG_EQUAL, TokenType.EQUAL_EQUAL):
+            err = self.error(self.previous(), "Missing left-hand operand.")
+            self.equality()
+            raise err
+
+        if self.match(
+            TokenType.GREATER,
+            TokenType.GREATER_EQUAL,
+            TokenType.LESS,
+            TokenType.LESS_EQUAL,
+        ):
+            err = self.error(self.previous(), "Missing left-hand operand.")
+            self.comparison()
+            raise err
+
+        if self.match(TokenType.PLUS):
+            err = self.error(self.previous(), "Missing left-hand operand.")
+            self.term()
+            raise err
+
+        if self.match(TokenType.SLASH, TokenType.STAR):
+            err = self.error(self.previous(), "Missing left-hand operand.")
+            self.factor()
+            raise err
+
         raise self.error(self.peek(), "Expect expression")

tests/test_parser.py

@@ -39,3 +39,46 @@ def test_if_parser_handles_empty_right_hand_operand_inside_binary_expression() -
         Parser(tokens).expression()
     # THEN
     assert "Expect expression" in err.value.message
+
+
+def test_if_parser_handles_empty_left_hand_operand_inside_binary_expression() -> (
+    None
+):
+    # GIVEN
+    src = "* 123"
+    # WHEN
+    tokens = Scanner(src).scan_tokens()
+    with pytest.raises(LoxParseError) as err:
+        Parser(tokens).expression()
+    # THEN
+    assert "Missing left-hand operand." in err.value.message
+
+
+def test_if_parser_parse_and_discard_right_hand_operand_in_case_of_empty_left_hand_operant() -> (
+    None
+):
+    # GIVEN
+    src = "* (123"
+
+    # MOCK
+    cnt = 0
+
+    def handler(err: LoxParseError):
+        nonlocal cnt
+
+        if cnt == 0:
+            assert err.message == "Missing left-hand operand."
+        elif cnt == 1:
+            assert err.message == "Expect ')' after expression."
+        else:
+            assert False
+
+        cnt += 1
+
+    # WHEN
+    tokens = Scanner(src).scan_tokens()
+    ast = Parser(tokens, handler).parse()
+
+    # THEN
+    assert ast is None
+    assert cnt == 2  # both errors have been handled