feat: add variable definition and reference

src/ast/expr.py

@@ -33,6 +33,10 @@ class Expr(ABC):
         def visit_literal_expr(self, expr: LiteralExpr) -> T:
             ...
 
+        @abstractmethod
+        def visit_variable_expr(self, expr: VariableExpr) -> T:
+            ...
+
 
 @dataclass(frozen=True)
 class BinaryExpr(Expr):
@@ -67,3 +71,11 @@ class LiteralExpr(Expr):
 
     def accept(self, visitor: Expr.Visitor[T]) -> T:
         return visitor.visit_literal_expr(self)
+
+
+@dataclass(frozen=True)
+class VariableExpr(Expr):
+    name: Token
+
+    def accept(self, visitor: Expr.Visitor[T]) -> T:
+        return visitor.visit_variable_expr(self)

src/ast/stmt.py

@@ -2,9 +2,10 @@ from __future__ import annotations
 
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
-from typing import TypeVar, Generic
+from typing import TypeVar, Generic, Optional
 
 from src.ast.expr import Expr
+from src.token import Token
 
 T = TypeVar("T")
 
@@ -24,6 +25,10 @@ class Stmt(ABC):
         def visit_print_stmt(self, stmt: PrintStmt) -> T:
             ...
 
+        @abstractmethod
+        def visit_let_stmt(self, stmt: LetStmt) -> T:
+            ...
+
 
 @dataclass(frozen=True)
 class ExpressionStmt(Stmt):
@@ -39,3 +44,12 @@ class PrintStmt(Stmt):
 
     def accept(self, visitor: Stmt.Visitor[T]) -> T:
         return visitor.visit_print_stmt(self)
+
+
+@dataclass(frozen=True)
+class LetStmt(Stmt):
+    name: Token
+    initializer: Optional[Expr]
+
+    def accept(self, visitor: Stmt.Visitor[T]) -> T:
+        return visitor.visit_let_stmt(self)

src/interpreter/interpreter.py

@@ -1,14 +1,34 @@
 from typing import Any
 
-from src.ast.expr import LiteralExpr, GroupingExpr, UnaryExpr, BinaryExpr, Expr
+from src.ast.expr import LiteralExpr, GroupingExpr, UnaryExpr, BinaryExpr, Expr, VariableExpr
-from src.ast.stmt import Stmt, PrintStmt, T, ExpressionStmt
+from src.ast.stmt import Stmt, PrintStmt, T, ExpressionStmt, LetStmt
 from src.interpreter.error import PebbleRuntimeError
 from src.pebble import Pebble
 from src.token import TokenType, Token
 
 
 class Interpreter(Expr.Visitor[Any], Stmt.Visitor[None]):
+    class Environment:
+        def __init__(self):
+            self.values: dict[str, Any] = {}
+
+        def define(self, name: str, value: Any):
+            self.values[name] = value
+
+        def get(self, name: Token) -> Any:
+            try:
+                return self.values[name.lexeme]
+            except IndexError:
+                raise PebbleRuntimeError(name, f"Undefined variable '{name.lexeme}'.")
+
+        def clear(self):
+            self.values = {}
+
+    def __init__(self):
+        self.env: Interpreter.Environment = Interpreter.Environment()
+
     def interpret(self, statements: list[Stmt]) -> None:
+        self.env.clear()
         try:
             for stmt in statements:
                 self.execute(stmt)
@@ -80,6 +100,9 @@ class Interpreter(Expr.Visitor[Any], Stmt.Visitor[None]):
     def visit_literal_expr(self, expr: LiteralExpr) -> Any:
         return expr.value
 
+    def visit_variable_expr(self, expr: VariableExpr) -> Any:
+        return self.env.get(expr.name)
+
     def visit_expression_stmt(self, stmt: ExpressionStmt) -> None:
         self.evaluate(stmt.expression)
 
@@ -87,6 +110,12 @@ class Interpreter(Expr.Visitor[Any], Stmt.Visitor[None]):
         value: Any = self.evaluate(stmt.expression)
         print(value)
 
+    def visit_let_stmt(self, stmt: LetStmt) -> None:
+        value: Any = None
+        if stmt.initializer is not None:
+            value = self.evaluate(stmt.initializer)
+        self.env.define(stmt.name.lexeme, value)
+
     @staticmethod
     def is_truthy(value: Any) -> bool:
         if value is None or value is False:

src/parser/parser.py

@@ -1,5 +1,7 @@
-from src.ast.expr import Expr, BinaryExpr, UnaryExpr, LiteralExpr, GroupingExpr
+from typing import Optional
-from src.ast.stmt import Stmt, PrintStmt, ExpressionStmt
+
+from src.ast.expr import Expr, BinaryExpr, UnaryExpr, LiteralExpr, GroupingExpr, VariableExpr
+from src.ast.stmt import Stmt, PrintStmt, ExpressionStmt, LetStmt
 from src.parser.error import ParsingError
 from src.pebble import Pebble
 from src.token import Token, TokenType
@@ -31,7 +33,7 @@ class Parser:
 
         statements: list[Stmt] = []
         while not self.is_at_end():
-            statements.append(self.statement())
+            statements.append(self.declaration())
         return statements
 
     def is_at_end(self) -> bool:
@@ -60,8 +62,9 @@ class Parser:
                 return True
         return False
 
-    def consume(self, token_type: TokenType, error_msg: str):
+    def consume(self, token_type: TokenType, error_msg: str) -> Token:
-        if not self.match(token_type):
+        if self.check(token_type):
+            return self.advance()
         raise self.error(self.peek(), error_msg)
 
     def expect_eol(self, error_msg: str):
@@ -80,6 +83,23 @@ class Parser:
                 return
             self.advance()
 
+    def declaration(self) -> Optional[Stmt]:
+        try:
+            if self.match(TokenType.LET):
+                return self.var_declaration()
+            return self.statement()
+        except ParsingError:
+            self.synchronize()
+            return None
+
+    def var_declaration(self) -> Stmt:
+        name: Token = self.consume(TokenType.IDENTIFIER, "Expected variable name.")
+        initializer: Optional[Expr] = None
+        if self.match(TokenType.EQUAL):
+            initializer = self.expression()
+        self.expect_eol("Expected end of line after variable initialization")
+        return LetStmt(name, initializer)
+
     def statement(self) -> Stmt:
         if self.match(TokenType.PRINT):
             return self.print_stmt()
@@ -150,6 +170,9 @@ class Parser:
         if self.match(TokenType.NUMBER, TokenType.STRING):
             return LiteralExpr(self.previous().value)
 
+        if self.match(TokenType.IDENTIFIER):
+            return VariableExpr(self.previous())
+
         if self.match(TokenType.LEFT_PAREN):
             expr: Expr = self.expression()
             self.consume(TokenType.RIGHT_PAREN, "Unclosed parenthesis")