handle expressions + conditionals + return statemetnts + function calls

src/ast/mod.rs

@@ -1,3 +1,5 @@
+use std::rc::Rc;
+
 use crate::token::Token;
 
 pub trait NodeTrait {
@@ -162,7 +164,7 @@ impl StatementTrait for ExpressionStatement {
     fn statement_node(&self) {}
 }
 
-#[derive(Debug, Default)]
+#[derive(Debug, Default, Clone)]
 pub struct Identifier {
     pub token: Token,
     pub value: String,
@@ -329,7 +331,7 @@ impl NodeTrait for BlockStatement {
 pub struct FunctionLiteral {
     pub token: Token,
     pub parameters: Vec<Identifier>,
-    pub body: BlockStatement,
+    pub body: Rc<BlockStatement>,
 }
 
 impl NodeTrait for FunctionLiteral {

src/evaluator/mod.rs

@@ -1,75 +1,366 @@
-use crate::ast::Program;
-use crate::ast::{self};
-use crate::object::{self, Null, Object};
+use std::cell::RefCell;
+use std::rc::Rc;
+
+use crate::ast::{self, NodeTrait};
+use crate::object::{self, Environment, Null, Object, ObjectType};
 
 const NULL: Null = Null;
 const BOOL_TRUE: object::Boolean = object::Boolean { value: true };
 const BOOL_FALSE: object::Boolean = object::Boolean { value: false };
 
-fn eval(node: Box<dyn ast::NodeTrait>) -> Option<Box<dyn object::Object>> {
-    let node = node.as_ref();
+pub fn eval_program(program: ast::Program) -> Option<Box<dyn Object>> {
+    let env = Rc::new(RefCell::new(Environment::new()));
+    let mut result: Option<Box<dyn Object>> = None;
+
+    for stmt in program.statements {
+        result = eval(stmt.as_ref(), Rc::clone(&env));
+
+        if let Some(ref obj) = result {
+            if obj.object_type() == ObjectType::RETURN_VALUE {
+                return Some(unwrap_return_value(obj.clone_box()));
+            }
+            if obj.object_type() == ObjectType::ERROR {
+                return result;
+            }
+        }
+    }
+
+    result
+}
+
+fn eval(node: &dyn NodeTrait, env: Rc<RefCell<Environment>>) -> Option<Box<dyn Object>> {
     if let Some(stmt) = node.as_any().downcast_ref::<ast::ExpressionStatement>() {
-        return eval_expression(&stmt.expression);
+        return eval_expression(&stmt.expression, env);
     }
     if let Some(stmt) = node.as_any().downcast_ref::<ast::LetStatement>() {
-        return None;
+        let value = eval_expression(&stmt.value, Rc::clone(&env))?;
+        if is_error(value.as_ref()) {
+            return Some(value);
+        }
+        env.borrow_mut().set(&stmt.name.value, value);
+        return Some(Box::new(NULL));
     }
     if let Some(stmt) = node.as_any().downcast_ref::<ast::ReturnStatement>() {
-        return eval_expression(&stmt.return_value);
+        let value = eval_expression(&stmt.return_value, Rc::clone(&env))?;
+        if is_error(value.as_ref()) {
+            return Some(value);
+        }
+        return Some(Box::new(object::ReturnValue { value }));
     }
-    if let Some(expr) = node.as_any().downcast_ref::<ast::Expression>() {
-        return eval_expression(expr);
-    }
-    if let Some(expr) = node.as_any().downcast_ref::<ast::PrefixExpression>() {
-        let right = &eval(expr.right);
-        return eval_prefix_expression(expr.operator, right);
+    if let Some(block) = node.as_any().downcast_ref::<ast::BlockStatement>() {
+        return eval_block_statement(block, env);
     }
+
     None
 }
 
-pub fn eval_program(program: Program) -> Option<Box<dyn object::Object>> {
-    let mut res: Option<Box<dyn object::Object>> = None;
-    for stmt in program.statements {
-        res = eval(stmt)
+fn eval_block_statement(
+    block: &ast::BlockStatement,
+    env: Rc<RefCell<Environment>>,
+) -> Option<Box<dyn Object>> {
+    let mut result: Option<Box<dyn Object>> = None;
+
+    for stmt in &block.statements {
+        result = eval(stmt.as_ref(), Rc::clone(&env));
+
+        if let Some(ref obj) = result {
+            let obj_type = obj.object_type();
+            if obj_type == ObjectType::RETURN_VALUE || obj_type == ObjectType::ERROR {
+                return result;
+            }
+        }
     }
-    res
+
+    result
 }
 
-fn eval_expression(expression: &ast::Expression) -> Option<Box<dyn object::Object>> {
+fn eval_expression(
+    expression: &ast::Expression,
+    env: Rc<RefCell<Environment>>,
+) -> Option<Box<dyn Object>> {
     match expression {
+        ast::Expression::Identifier(ident) => eval_identifier(ident, env),
         ast::Expression::IntegerLiteral(int) => {
             Some(Box::new(object::Integer { value: int.value }))
         }
         ast::Expression::Boolean(b) => Some(Box::new(native_bool_to_bool_object(b.value))),
-        _ => None,
+        ast::Expression::PrefixExpression(expr) => {
+            let right = eval_expression(expr.right.as_ref(), Rc::clone(&env))?;
+            if is_error(right.as_ref()) {
+                return Some(right);
+            }
+            eval_prefix_expression(&expr.operator, right)
+        }
+        ast::Expression::InfixExpression(expr) => {
+            let left = eval_expression(expr.left.as_ref(), Rc::clone(&env))?;
+            if is_error(left.as_ref()) {
+                return Some(left);
+            }
+            let right = eval_expression(expr.right.as_ref(), Rc::clone(&env))?;
+            if is_error(right.as_ref()) {
+                return Some(right);
+            }
+            eval_infix_expression(&expr.operator, left, right)
+        }
+        ast::Expression::IfExpression(expr) => eval_if_expression(expr, env),
+        ast::Expression::FunctionLiteral(func) => Some(Box::new(object::Function {
+            parameters: func.parameters.clone(),
+            body: Rc::clone(&func.body),
+            env: Rc::clone(&env),
+        })),
+        ast::Expression::CallExpression(call) => {
+            let function = eval_expression(call.function.as_ref(), Rc::clone(&env))?;
+            if is_error(function.as_ref()) {
+                return Some(function);
+            }
+            let args = eval_expressions(&call.arguments, Rc::clone(&env))?;
+            if args.len() == 1 && is_error(args[0].as_ref()) {
+                return Some(args.into_iter().next().unwrap());
+            }
+            apply_function(function, args)
+        }
     }
 }
 
+fn eval_identifier(
+    ident: &ast::Identifier,
+    env: Rc<RefCell<Environment>>,
+) -> Option<Box<dyn Object>> {
+    if let Some(value) = env.borrow().get(&ident.value) {
+        return Some(value);
+    }
+    Some(new_error(format!("identifier not found: {}", ident.value)))
+}
+
+fn eval_expressions(
+    expressions: &[ast::Expression],
+    env: Rc<RefCell<Environment>>,
+) -> Option<Vec<Box<dyn Object>>> {
+    let mut result = Vec::new();
+
+    for expr in expressions {
+        let evaluated = eval_expression(expr, Rc::clone(&env))?;
+        if is_error(evaluated.as_ref()) {
+            return Some(vec![evaluated]);
+        }
+        result.push(evaluated);
+    }
+
+    Some(result)
+}
+
+fn apply_function(
+    function: Box<dyn Object>,
+    args: Vec<Box<dyn Object>>,
+) -> Option<Box<dyn Object>> {
+    if let Some(func) = function.as_any().downcast_ref::<object::Function>() {
+        let extended_env = extend_function_env(func, args);
+        let evaluated = eval_block_statement(&func.body, Rc::clone(&extended_env));
+
+        if let Some(obj) = evaluated {
+            if obj.object_type() == ObjectType::RETURN_VALUE {
+                return Some(unwrap_return_value(obj));
+            }
+            return Some(obj);
+        }
+        return Some(Box::new(NULL));
+    }
+
+    Some(new_error(format!(
+        "not a function: {:?}",
+        function.object_type()
+    )))
+}
+
+fn extend_function_env(
+    func: &object::Function,
+    args: Vec<Box<dyn Object>>,
+) -> Rc<RefCell<Environment>> {
+    let mut env = Environment::from_outer(Rc::clone(&func.env));
+
+    for (i, param) in func.parameters.iter().enumerate() {
+        if let Some(arg) = args.get(i) {
+            env.set(&param.value, arg.clone_box());
+        }
+    }
+
+    Rc::new(RefCell::new(env))
+}
+
 fn eval_prefix_expression(
-    operator: String,
-    right: &Option<Box<dyn object::Object>>,
-) -> Option<Box<dyn object::Object>> {
-    match operator.as_str() {
+    operator: &str,
+    right: Box<dyn Object>,
+) -> Option<Box<dyn Object>> {
+    match operator {
         "!" => eval_bang_operator_expression(right),
-        _ => Some(Box::new(NULL)),
+        "-" => eval_minus_prefix_operator_expression(right),
+        _ => Some(new_error(format!("unknown operator: {}{:?}", operator, right.object_type()))),
     }
 }
 
-fn eval_bang_operator_expression(
-    right: &Option<Box<dyn object::Object>>,
-) -> Option<Box<dyn object::Object>> {
-    let bool_true = Box::new(BOOL_TRUE);
-    let bool_false = Box::new(BOOL_FALSE);
-    let null = Box::new(NULL);
-
-    match right..unwrap() {
-        bool_true => Some(bool_false),
-        bool_false => Some(bool_true),
-        null => Some(bool_true),
-        _ => Some(bool_false),
+fn eval_bang_operator_expression(right: Box<dyn Object>) -> Option<Box<dyn Object>> {
+    match right.object_type() {
+        ObjectType::BOOLEAN => {
+            if let Some(b) = right.as_any().downcast_ref::<object::Boolean>() {
+                Some(Box::new(native_bool_to_bool_object(!b.value)))
+            } else {
+                Some(Box::new(NULL))
+            }
+        }
+        ObjectType::NULL => Some(Box::new(BOOL_TRUE)),
+        _ => Some(Box::new(BOOL_FALSE)),
     }
 }
 
+fn eval_minus_prefix_operator_expression(right: Box<dyn Object>) -> Option<Box<dyn Object>> {
+    if let Some(int) = right.as_any().downcast_ref::<object::Integer>() {
+        return Some(Box::new(object::Integer { value: -int.value }));
+    }
+    Some(new_error(format!(
+        "unknown operator: -{:?}",
+        right.object_type()
+    )))
+}
+
+fn eval_infix_expression(
+    operator: &str,
+    left: Box<dyn Object>,
+    right: Box<dyn Object>,
+) -> Option<Box<dyn Object>> {
+    if left.object_type() == ObjectType::INTEGER && right.object_type() == ObjectType::INTEGER {
+        return eval_integer_infix_expression(operator, left, right);
+    }
+
+    if left.object_type() == ObjectType::BOOLEAN && right.object_type() == ObjectType::BOOLEAN {
+        return eval_boolean_infix_expression(operator, left, right);
+    }
+
+    if left.object_type() != right.object_type() {
+        return Some(new_error(format!(
+            "type mismatch: {:?} {} {:?}",
+            left.object_type(),
+            operator,
+            right.object_type()
+        )));
+    }
+
+    Some(new_error(format!(
+        "unknown operator: {:?} {} {:?}",
+        left.object_type(),
+        operator,
+        right.object_type()
+    )))
+}
+
+fn eval_integer_infix_expression(
+    operator: &str,
+    left: Box<dyn Object>,
+    right: Box<dyn Object>,
+) -> Option<Box<dyn Object>> {
+    let left_val = left.as_any().downcast_ref::<object::Integer>().unwrap().value;
+    let right_val = right
+        .as_any()
+        .downcast_ref::<object::Integer>()
+        .unwrap()
+        .value;
+
+    match operator {
+        "+" => Some(Box::new(object::Integer {
+            value: left_val + right_val,
+        })),
+        "-" => Some(Box::new(object::Integer {
+            value: left_val - right_val,
+        })),
+        "*" => Some(Box::new(object::Integer {
+            value: left_val * right_val,
+        })),
+        "/" => Some(Box::new(object::Integer {
+            value: left_val / right_val,
+        })),
+        "<" => Some(Box::new(native_bool_to_bool_object(left_val < right_val))),
+        ">" => Some(Box::new(native_bool_to_bool_object(left_val > right_val))),
+        "==" => Some(Box::new(native_bool_to_bool_object(left_val == right_val))),
+        "!=" => Some(Box::new(native_bool_to_bool_object(left_val != right_val))),
+        _ => Some(new_error(format!(
+            "unknown operator: {:?} {} {:?}",
+            left.object_type(),
+            operator,
+            right.object_type()
+        ))),
+    }
+}
+
+fn eval_boolean_infix_expression(
+    operator: &str,
+    left: Box<dyn Object>,
+    right: Box<dyn Object>,
+) -> Option<Box<dyn Object>> {
+    let left_val = left.as_any().downcast_ref::<object::Boolean>().unwrap().value;
+    let right_val = right
+        .as_any()
+        .downcast_ref::<object::Boolean>()
+        .unwrap()
+        .value;
+
+    match operator {
+        "==" => Some(Box::new(native_bool_to_bool_object(left_val == right_val))),
+        "!=" => Some(Box::new(native_bool_to_bool_object(left_val != right_val))),
+        _ => Some(new_error(format!(
+            "unknown operator: {:?} {} {:?}",
+            left.object_type(),
+            operator,
+            right.object_type()
+        ))),
+    }
+}
+
+fn eval_if_expression(
+    expr: &ast::IfExpression,
+    env: Rc<RefCell<Environment>>,
+) -> Option<Box<dyn Object>> {
+    let condition = eval_expression(expr.condition.as_ref(), Rc::clone(&env))?;
+    if is_error(condition.as_ref()) {
+        return Some(condition);
+    }
+
+    if is_truthy(condition.as_ref()) {
+        eval_block_statement(&expr.consequence, env)
+    } else if let Some(ref alternative) = expr.alternative {
+        eval_block_statement(alternative, env)
+    } else {
+        Some(Box::new(NULL))
+    }
+}
+
+fn is_truthy(obj: &dyn Object) -> bool {
+    match obj.object_type() {
+        ObjectType::NULL => false,
+        ObjectType::BOOLEAN => {
+            let b = obj.as_any().downcast_ref::<object::Boolean>().unwrap();
+            b.value
+        }
+        _ => true,
+    }
+}
+
+fn unwrap_return_value(obj: Box<dyn Object>) -> Box<dyn Object> {
+    if let Some(ret) = obj.as_any().downcast_ref::<object::ReturnValue>() {
+        return ret.value.clone_box();
+    }
+    obj
+}
+
+fn is_error(obj: &dyn Object) -> bool {
+    obj.object_type() == ObjectType::ERROR
+}
+
+fn new_error(message: String) -> Box<dyn Object> {
+    Box::new(object::Error { message })
+}
+
 fn native_bool_to_bool_object(input: bool) -> object::Boolean {
     if input { BOOL_TRUE } else { BOOL_FALSE }
 }
+
+#[cfg(test)]
+mod tests;

src/evaluator/tests.rs

@@ -0,0 +1,225 @@
+use super::*;
+use crate::lexer;
+use crate::parser::new_parser;
+
+fn test_eval(input: &str) -> Option<Box<dyn Object>> {
+    let lexer = lexer::new(input.to_string());
+    let mut parser = new_parser(lexer);
+    let program = parser.parse_program();
+    eval_program(program)
+}
+
+fn test_integer_object(obj: &dyn Object, expected: i64) {
+    assert_eq!(obj.object_type(), ObjectType::INTEGER);
+    let int = obj.as_any().downcast_ref::<object::Integer>().unwrap();
+    assert_eq!(int.value, expected);
+}
+
+fn test_boolean_object(obj: &dyn Object, expected: bool) {
+    assert_eq!(obj.object_type(), ObjectType::BOOLEAN);
+    let b = obj.as_any().downcast_ref::<object::Boolean>().unwrap();
+    assert_eq!(b.value, expected);
+}
+
+fn test_null_object(obj: &dyn Object) {
+    assert_eq!(obj.object_type(), ObjectType::NULL);
+}
+
+#[test]
+fn test_eval_integer_expression() {
+    let tests = vec![("5", 5), ("10", 10), ("-5", -5), ("-10", -10)];
+
+    for (input, expected) in tests {
+        let evaluated = test_eval(input).unwrap();
+        test_integer_object(evaluated.as_ref(), expected);
+    }
+}
+
+#[test]
+fn test_eval_boolean_expression() {
+    let tests = vec![
+        ("true", true),
+        ("false", false),
+        ("1 < 2", true),
+        ("1 > 2", false),
+        ("1 < 1", false),
+        ("1 > 1", false),
+        ("1 == 1", true),
+        ("1 != 1", false),
+        ("1 == 2", false),
+        ("1 != 2", true),
+        ("true == true", true),
+        ("false == false", true),
+        ("true == false", false),
+        ("true != false", true),
+        ("false != true", true),
+    ];
+
+    for (input, expected) in tests {
+        let evaluated = test_eval(input).unwrap();
+        test_boolean_object(evaluated.as_ref(), expected);
+    }
+}
+
+#[test]
+fn test_bang_operator() {
+    let tests = vec![
+        ("!true", false),
+        ("!false", true),
+        ("!5", false),
+        ("!!true", true),
+        ("!!false", false),
+        ("!!5", true),
+    ];
+
+    for (input, expected) in tests {
+        let evaluated = test_eval(input).unwrap();
+        test_boolean_object(evaluated.as_ref(), expected);
+    }
+}
+
+#[test]
+fn test_eval_integer_arithmetic() {
+    let tests = vec![
+        ("5 + 5", 10),
+        ("5 - 5", 0),
+        ("5 * 5", 25),
+        ("5 / 5", 1),
+        ("5 + 5 + 5 + 5 - 10", 10),
+        ("2 * 2 * 2 * 2 * 2", 32),
+        ("-50 + 100 + -50", 0),
+        ("5 * 2 + 10", 20),
+        ("5 + 2 * 10", 25),
+        ("20 + 2 * -10", 0),
+        ("50 / 2 * 2 + 10", 60),
+        ("2 * (5 + 10)", 30),
+        ("3 * 3 * 3 + 10", 37),
+        ("3 * (3 * 3) + 10", 37),
+        ("(5 + 10 * 2 + 15 / 3) * 2 + -10", 50),
+    ];
+
+    for (input, expected) in tests {
+        let evaluated = test_eval(input).unwrap();
+        test_integer_object(evaluated.as_ref(), expected);
+    }
+}
+
+#[test]
+fn test_if_else_expressions() {
+    let tests = vec![
+        ("if (true) { 10 }", Some(10)),
+        ("if (false) { 10 }", None),
+        ("if (1) { 10 }", Some(10)),
+        ("if (1 < 2) { 10 }", Some(10)),
+        ("if (1 > 2) { 10 }", None),
+        ("if (1 > 2) { 10 } else { 20 }", Some(20)),
+        ("if (1 < 2) { 10 } else { 20 }", Some(10)),
+    ];
+
+    for (input, expected) in tests {
+        let evaluated = test_eval(input).unwrap();
+        match expected {
+            Some(value) => test_integer_object(evaluated.as_ref(), value),
+            None => test_null_object(evaluated.as_ref()),
+        }
+    }
+}
+
+#[test]
+fn test_return_statements() {
+    let tests = vec![
+        ("return 10;", 10),
+        ("return 10; 9;", 10),
+        ("return 2 * 5; 9;", 10),
+        ("9; return 2 * 5; 9;", 10),
+        (
+            "if (10 > 1) { if (10 > 1) { return 10; } return 1; }",
+            10,
+        ),
+    ];
+
+    for (input, expected) in tests {
+        let evaluated = test_eval(input).unwrap();
+        test_integer_object(evaluated.as_ref(), expected);
+    }
+}
+
+#[test]
+fn test_let_statements() {
+    let tests = vec![
+        ("let a = 5; a;", 5),
+        ("let a = 5 * 5; a;", 25),
+        ("let a = 5; let b = a; b;", 5),
+        ("let a = 5; let b = a; let c = a + b + 5; c;", 15),
+    ];
+
+    for (input, expected) in tests {
+        let evaluated = test_eval(input).unwrap();
+        test_integer_object(evaluated.as_ref(), expected);
+    }
+}
+
+#[test]
+fn test_function_object() {
+    let input = "fn(x) { x + 2; };";
+    let evaluated = test_eval(input).unwrap();
+
+    assert_eq!(evaluated.object_type(), ObjectType::FUNCTION);
+    let func = evaluated.as_any().downcast_ref::<object::Function>().unwrap();
+    assert_eq!(func.parameters.len(), 1);
+    assert_eq!(func.parameters[0].value, "x");
+    assert_eq!(func.body.string(), "(x + 2)");
+}
+
+#[test]
+fn test_function_application() {
+    let tests = vec![
+        ("let identity = fn(x) { x; }; identity(5);", 5),
+        ("let identity = fn(x) { return x; }; identity(5);", 5),
+        ("let double = fn(x) { x * 2; }; double(5);", 10),
+        ("let add = fn(x, y) { x + y; }; add(5, 5);", 10),
+        ("let add = fn(x, y) { x + y; }; add(5 + 5, add(5, 5));", 20),
+        ("fn(x) { x; }(5)", 5),
+    ];
+
+    for (input, expected) in tests {
+        let evaluated = test_eval(input).unwrap();
+        test_integer_object(evaluated.as_ref(), expected);
+    }
+}
+
+#[test]
+fn test_enclosing_environments() {
+    let input = "
+        let first = 10;
+        let second = 10;
+        let third = 10;
+        let ourFunction = fn(first) {
+            let second = 20;
+            first + second + third;
+        };
+        ourFunction(20) + first + second;
+    ";
+    let evaluated = test_eval(input).unwrap();
+    test_integer_object(evaluated.as_ref(), 70);
+}
+
+#[test]
+fn test_error_handling() {
+    let tests = vec![
+        ("5 + true;", "type mismatch: INTEGER + BOOLEAN"),
+        ("5 + true; 5;", "type mismatch: INTEGER + BOOLEAN"),
+        ("-true", "unknown operator: -BOOLEAN"),
+        ("true + false;", "unknown operator: BOOLEAN + BOOLEAN"),
+        ("5; true + false; 5", "unknown operator: BOOLEAN + BOOLEAN"),
+        ("if (10 > 1) { true + false; }", "unknown operator: BOOLEAN + BOOLEAN"),
+        ("foobar", "identifier not found: foobar"),
+    ];
+
+    for (input, expected) in tests {
+        let evaluated = test_eval(input).unwrap();
+        assert_eq!(evaluated.object_type(), ObjectType::ERROR);
+        let err = evaluated.as_any().downcast_ref::<object::Error>().unwrap();
+        assert!(err.message.contains(expected), "expected '{}' to contain '{}'", err.message, expected);
+    }
+}

src/object/mod.rs

@@ -1,12 +1,25 @@
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::rc::Rc;
+
+use crate::ast::{self, NodeTrait};
+
+#[derive(Debug, PartialEq, Eq)]
+#[allow(non_camel_case_types)]
 pub enum ObjectType {
     INTEGER,
     BOOLEAN,
     NULL,
+    RETURN_VALUE,
+    ERROR,
+    FUNCTION,
 }
 
 pub trait Object {
     fn object_type(&self) -> ObjectType;
     fn inspect(&self) -> String;
+    fn clone_box(&self) -> Box<dyn Object>;
+    fn as_any(&self) -> &dyn std::any::Any;
 }
 
 pub struct Integer {
@@ -21,6 +34,14 @@ impl Object for Integer {
     fn inspect(&self) -> String {
         self.value.to_string()
     }
+
+    fn clone_box(&self) -> Box<dyn Object> {
+        Box::new(Integer { value: self.value })
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
 
 pub struct Boolean {
@@ -35,6 +56,14 @@ impl Object for Boolean {
     fn inspect(&self) -> String {
         self.value.to_string()
     }
+
+    fn clone_box(&self) -> Box<dyn Object> {
+        Box::new(Boolean { value: self.value })
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
 }
 
 pub struct Null;
@@ -47,4 +76,129 @@ impl Object for Null {
     fn inspect(&self) -> String {
         "null".to_string()
     }
+
+    fn clone_box(&self) -> Box<dyn Object> {
+        Box::new(Null)
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
+
+pub struct ReturnValue {
+    pub value: Box<dyn Object>,
+}
+
+impl Object for ReturnValue {
+    fn object_type(&self) -> ObjectType {
+        ObjectType::RETURN_VALUE
+    }
+
+    fn inspect(&self) -> String {
+        self.value.inspect()
+    }
+
+    fn clone_box(&self) -> Box<dyn Object> {
+        Box::new(ReturnValue {
+            value: self.value.clone_box(),
+        })
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
+
+pub struct Error {
+    pub message: String,
+}
+
+impl Object for Error {
+    fn object_type(&self) -> ObjectType {
+        ObjectType::ERROR
+    }
+
+    fn inspect(&self) -> String {
+        format!("ERROR: {}", self.message)
+    }
+
+    fn clone_box(&self) -> Box<dyn Object> {
+        Box::new(Error {
+            message: self.message.clone(),
+        })
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
+
+pub struct Function {
+    pub parameters: Vec<ast::Identifier>,
+    pub body: Rc<ast::BlockStatement>,
+    pub env: Rc<RefCell<Environment>>,
+}
+
+impl Object for Function {
+    fn object_type(&self) -> ObjectType {
+        ObjectType::FUNCTION
+    }
+
+    fn inspect(&self) -> String {
+        let params: Vec<String> = self.parameters.iter().map(|p| p.string()).collect();
+        format!(
+            "fn({}) {{\n{}\n}}",
+            params.join(", "),
+            self.body.string()
+        )
+    }
+
+    fn clone_box(&self) -> Box<dyn Object> {
+        Box::new(Function {
+            parameters: self.parameters.clone(),
+            body: Rc::clone(&self.body),
+            env: Rc::clone(&self.env),
+        })
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
+
+#[derive(Default)]
+pub struct Environment {
+    store: HashMap<String, Box<dyn Object>>,
+    outer: Option<Rc<RefCell<Environment>>>,
+}
+
+impl Environment {
+    pub fn new() -> Self {
+        Self {
+            store: HashMap::new(),
+            outer: None,
+        }
+    }
+
+    pub fn from_outer(outer: Rc<RefCell<Environment>>) -> Self {
+        Self {
+            store: HashMap::new(),
+            outer: Some(outer),
+        }
+    }
+
+    pub fn get(&self, name: &str) -> Option<Box<dyn Object>> {
+        if let Some(value) = self.store.get(name) {
+            return Some(value.clone_box());
+        }
+        if let Some(ref outer) = self.outer {
+            return outer.borrow().get(name);
+        }
+        None
+    }
+
+    pub fn set(&mut self, name: &str, value: Box<dyn Object>) {
+        self.store.insert(name.to_string(), value);
+    }
 }

src/parser/mod.rs

@@ -1,3 +1,5 @@
+use std::rc::Rc;
+
 use crate::{
     ast::{self},
     lexer::{self, LexerTraits},
@@ -378,7 +380,7 @@ impl Parser {
         if !self.expect_peek(TokenType::Lbrace) {
             return None;
         }
-        let body = self.parse_block_statement()?;
+        let body = Rc::new(self.parse_block_statement()?);
         Some(ast::Expression::FunctionLiteral(ast::FunctionLiteral {
             token,
             parameters,