fix(parser): add parsing error handling

main.py

@@ -32,7 +32,10 @@ def main():
     print(printer.print(ast))
 
     parser: Parser = Parser()
-    ast = parser.process(tokens)
+    ast = parser.parse(tokens)
+
+    if ast is None:
+        return
 
     print(printer.print(ast))
 

src/parser/error.py

@@ -0,0 +1,2 @@
+class ParsingError(RuntimeError):
+    pass

src/parser/parser.py

@@ -1,4 +1,8 @@
+from typing import Optional
+
 from src.ast.expr import Expr, BinaryExpr, UnaryExpr, LiteralExpr, GroupingExpr
+from src.parser.error import ParsingError
+from src.pebble import Pebble
 from src.token import Token, TokenType
 
 
@@ -7,21 +11,29 @@ class Parser:
         TokenType.WHITESPACE, TokenType.COMMENT
     }
 
+    STATEMENT_BOUNDARY: set[TokenType] = {
+        TokenType.FOR, TokenType.WHILE, TokenType.IF, TokenType.PRINT
+    }
+
     def __init__(self):
         self.tokens: list[Token] = []
         self.current: int = 0
         self.length: int = 0
 
-    def error(self, token: Token, msg: str):
-        lexeme: str = "end" if token.type == TokenType.EOF else f"'{token.lexeme}'"
-        raise SyntaxError(f"[ERROR] Invalid syntax at {lexeme} ({token.position}): {msg}")
+    @staticmethod
+    def error(token: Token, msg: str):
+        Pebble.token_error(token, msg)
+        return ParsingError()
 
-    def process(self, tokens: list[Token]):
+    def parse(self, tokens: list[Token]) -> Optional[Expr]:
         self.tokens = list(filter(lambda t: t.type not in self.IGNORE, tokens))
         self.current = 0
         self.length = len(self.tokens)
 
-        return self.expression()
+        try:
+            return self.expression()
+        except ParsingError:
+            return None
 
     def is_at_end(self) -> bool:
         return self.current >= self.length
@@ -51,9 +63,17 @@ class Parser:
 
     def consume(self, token_type: TokenType, error_msg: str):
         if not self.match(token_type):
-            self.error(self.peek(), error_msg)
+            raise self.error(self.peek(), error_msg)
 
     # Parsing
+    def synchronize(self):
+        self.advance()
+        while not self.is_at_end():
+            # TODO: if self.previous().type == TokenType.NEWLINE: return
+            if self.peek().type in self.STATEMENT_BOUNDARY:
+                return
+            self.advance()
+
     def expression(self) -> Expr:
         return self.equality()
 
@@ -112,4 +132,4 @@ class Parser:
             self.consume(TokenType.RIGHT_PAREN, "Unclosed parenthesis")
             return GroupingExpr(expr)
 
-        self.error(self.peek(), "Malformed expression")
+        raise self.error(self.peek(), "Expected expression")

src/pebble.py

@@ -0,0 +1,19 @@
+from src.position import Position
+from src.token import Token, TokenType
+
+
+class Pebble:
+    @staticmethod
+    def report(position: Position, where: str, msg: str):
+        print(f"({position}) Error{where}: {msg}")
+
+    @classmethod
+    def error(cls, position: Position, msg: str):
+        cls.report(position, "", msg)
+
+    @classmethod
+    def token_error(cls, token: Token, msg: str):
+        if token.type == TokenType.EOF:
+            cls.report(token.position, " at end", msg)
+        else:
+            cls.report(token.position, f" at '{token.lexeme}'", msg)