chapter 9 in rust

rust/rox/src/environment.rs

@@ -1,4 +1,4 @@
-use std::collections::HashMap;
+use std::{cell::RefCell, collections::HashMap, rc::Rc};
 
 use thiserror::Error;
 
@@ -15,14 +15,15 @@ pub enum EnvironmentError {
 #[derive(Default, Debug, Clone)]
 pub struct Environment {
     values: HashMap<String, Value>,
-    enclosing: Option<Box<Environment>>,
+    enclosing: Option<Rc<RefCell<Environment>>>,
 }
 
 impl Environment {
-    pub fn with_enclosing(enclosing: Environment) -> Self {
+    /// Initialize a new environment with an enclosing one.
+    pub fn with_enclosing(enclosing: Rc<RefCell<Environment>>) -> Self {
         Self {
             values: HashMap::default(),
-            enclosing: Some(Box::new(enclosing)),
+            enclosing: Some(enclosing),
         }
     }
 
@@ -40,7 +41,7 @@ impl Environment {
             self.values.insert(token.lexeme.clone(), value);
             Ok(())
         } else if let Some(enclosing) = &mut self.enclosing {
-            enclosing.assign(token, value)
+            enclosing.borrow_mut().assign(token, value)
         } else {
             Err(EnvironmentError::UndefinedVariable(
                 token.line,
@@ -55,7 +56,7 @@ impl Environment {
         if let Some(v) = self.values.get(token.lexeme.as_str()) {
             Ok(v.clone())
         } else if let Some(enclosing) = &self.enclosing {
-            enclosing.get(token)
+            enclosing.borrow().get(token)
         } else {
             Err(EnvironmentError::UndefinedVariable(
                 token.line,

rust/rox/src/expression.rs

@@ -18,6 +18,11 @@ pub enum Expression {
     Literal {
         value: token::Literal,
     },
+    Logical {
+        left: Box<Expression>,
+        operator: Token,
+        right: Box<Expression>,
+    },
     Unary {
         operator: Token,
         right: Box<Expression>,

rust/rox/src/interpreter.rs

@@ -1,3 +1,5 @@
+use std::{cell::RefCell, rc::Rc};
+
 use thiserror::Error;
 use tracing::error;
 
@@ -28,14 +30,14 @@ pub enum InterpreterError {
 /// Interpreter for the Lox language.
 #[derive(Default, Debug)]
 pub struct Interpreter {
-    environment: Environment,
+    environment: Rc<RefCell<Environment>>,
 }
 
 impl Interpreter {
     /// Try to evaluate an expression and return its result.
     pub fn run(&mut self, statements: Vec<Statement>) -> Result<(), InterpreterError> {
         for stmt in statements {
-            match self.execute(stmt) {
+            match self.execute(&stmt) {
                 Ok(_) => {}
                 Err(e) => error!("{e}"),
             };
@@ -45,7 +47,7 @@ impl Interpreter {
     }
 
     ///Execute a statement.
-    fn execute(&mut self, statement: Statement) -> Result<(), InterpreterError> {
+    fn execute(&mut self, statement: &Statement) -> Result<(), InterpreterError> {
         match statement {
             Statement::Block(statements) => {
                 let sub_env = Environment::with_enclosing(self.environment.clone());
@@ -55,7 +57,18 @@ impl Interpreter {
             Statement::Expression(expression) => {
                 self.evaluate(expression)?;
             }
-            Statement::Var { name, initializer } => self.var_statement(name, *initializer)?,
+            Statement::Var { name, initializer } => {
+                self.var_statement(name, initializer.as_ref().as_ref())?
+            }
+            Statement::If {
+                condition,
+                then_branch,
+                else_branch,
+            } => {
+                let else_branch = else_branch.as_ref().map(|x| *x.clone());
+                self.if_statement(condition, then_branch, else_branch.as_ref())?
+            }
+            Statement::While { condition, body } => self.while_statement(condition, body)?,
         };
 
         Ok(())
@@ -65,11 +78,11 @@ impl Interpreter {
     /// enclosing one).
     fn block(
         &mut self,
-        statements: Vec<Statement>,
+        statements: &Vec<Statement>,
         environment: Environment,
     ) -> Result<(), InterpreterError> {
-        let prev_env = &self.environment.clone();
-        self.environment = environment;
+        let prev_env = self.environment.clone();
+        self.environment = Rc::new(RefCell::new(environment));
 
         for stmt in statements {
             if let Err(e) = self.execute(stmt) {
@@ -83,26 +96,47 @@ impl Interpreter {
     }
 
     /// Evaluate an expression and return its value.
-    fn evaluate(&mut self, expression: Expression) -> Result<Value, InterpreterError> {
+    fn evaluate(&mut self, expression: &Expression) -> Result<Value, InterpreterError> {
         match expression {
-            Expression::Literal { value } => self.literal(value),
-            Expression::Grouping { expression } => self.grouping(*expression),
+            Expression::Literal { value } => self.literal(value.clone()),
+            Expression::Grouping { expression } => self.grouping(expression),
             Expression::Unary {
                 operator: op,
                 right,
-            } => self.unary(op, *right),
+            } => self.unary(op, right),
             Expression::Binary {
                 left,
                 operator,
                 right,
-            } => self.binary(*left, operator, *right),
-            Expression::Variable { name } => self.var_expression(&name),
-            Expression::Assign { name, value } => self.assign(&name, *value),
+            } => self.binary(left, operator, right),
+            Expression::Variable { name } => self.var_expression(name),
+            Expression::Assign { name, value } => self.assign(name, value),
+            Expression::Logical {
+                left,
+                operator,
+                right,
+            } => self.logical_expression(left, operator, right),
+        }
+    }
+
+    /// If the condition evaluates to truthy, execute the then branch, otherwise the else branch.
+    fn if_statement(
+        &mut self,
+        condition: &Expression,
+        then_branch: &Statement,
+        else_branch: Option<&Statement>,
+    ) -> Result<(), InterpreterError> {
+        if self.evaluate(condition)?.is_truthy() {
+            self.execute(then_branch)
+        } else if let Some(else_branch) = else_branch {
+            self.execute(else_branch)
+        } else {
+            Ok(())
         }
     }
 
     /// Evaluate an expression and print its value to stdout.
-    fn print_statement(&mut self, expression: Expression) -> Result<(), InterpreterError> {
+    fn print_statement(&mut self, expression: &Expression) -> Result<(), InterpreterError> {
         let value = self.evaluate(expression)?;
         println!("{value}");
 
@@ -112,8 +146,8 @@ impl Interpreter {
     /// Initialize a variable with an initializer expression or nil.
     fn var_statement(
         &mut self,
-        name: Token,
-        initializer: Option<Expression>,
+        name: &Token,
+        initializer: Option<&Expression>,
     ) -> Result<(), InterpreterError> {
         let value = if let Some(initializer) = initializer {
             self.evaluate(initializer)
@@ -121,16 +155,32 @@ impl Interpreter {
             Ok(Value::Nil)
         }?;
 
-        self.environment.define(name.lexeme, value);
+        self.environment
+            .borrow_mut()
+            .define(name.lexeme.clone(), value);
+
+        Ok(())
+    }
+
+    /// Execute the body as long as the condition evaluates to true.
+    fn while_statement(
+        &mut self,
+        condition: &Expression,
+        body: &Statement,
+    ) -> Result<(), InterpreterError> {
+        while self.evaluate(condition)?.is_truthy() {
+            self.execute(body)?;
+        }
 
         Ok(())
     }
 
     /// Assign the value of an expression to a variable.
-    fn assign(&mut self, name: &Token, value: Expression) -> Result<Value, InterpreterError> {
+    fn assign(&mut self, name: &Token, value: &Expression) -> Result<Value, InterpreterError> {
         let value = self.evaluate(value)?;
 
         self.environment
+            .borrow_mut()
             .assign(name, value.clone())
             .map_err(InterpreterError::UndefinedVariable)?;
         Ok(value)
@@ -141,13 +191,36 @@ impl Interpreter {
         Ok(literal.into())
     }
 
+    /// Evaluate left and if the operator is Or and left is truthy or the operator is not Or and
+    /// not leftis truthy short circuit and return left. Otherwise evaluate and return right.
+    fn logical_expression(
+        &mut self,
+        left: &Expression,
+        operator: &Token,
+        right: &Expression,
+    ) -> Result<Value, InterpreterError> {
+        let left = self.evaluate(left)?;
+
+        let truthy = if operator.token_type == TokenType::Or {
+            left.is_truthy()
+        } else {
+            !left.is_truthy()
+        };
+
+        if truthy {
+            Ok(left)
+        } else {
+            self.evaluate(right)
+        }
+    }
+
     /// Evaluate the inner expression.
-    fn grouping(&mut self, inner: Expression) -> Result<Value, InterpreterError> {
+    fn grouping(&mut self, inner: &Expression) -> Result<Value, InterpreterError> {
         self.evaluate(inner)
     }
 
     /// Evaluate the expression on the right and use its result when evaluating the unary operator.
-    fn unary(&mut self, op: Token, right: Expression) -> Result<Value, InterpreterError> {
+    fn unary(&mut self, op: &Token, right: &Expression) -> Result<Value, InterpreterError> {
         let right = self.evaluate(right)?;
 
         match op.token_type {
@@ -159,13 +232,17 @@ impl Interpreter {
                 }
             }
             TokenType::Bang => Ok(Value::Boolean(!right.is_truthy())),
-            _ => Err(InterpreterError::UnaryOperatorUnknown(op.line, op.lexeme)),
+            _ => Err(InterpreterError::UnaryOperatorUnknown(
+                op.line,
+                op.lexeme.clone(),
+            )),
         }
     }
 
     /// Get the value of a variable.
     fn var_expression(&mut self, name: &Token) -> Result<Value, InterpreterError> {
         self.environment
+            .borrow()
             .get(name)
             .map_err(InterpreterError::UndefinedVariable)
     }
@@ -196,9 +273,9 @@ impl Interpreter {
     /// specified operator.
     fn binary(
         &mut self,
-        left: Expression,
-        op: Token,
-        right: Expression,
+        left: &Expression,
+        op: &Token,
+        right: &Expression,
     ) -> Result<Value, InterpreterError> {
         let left = self.evaluate(left)?;
         let right = self.evaluate(right)?;
@@ -227,7 +304,10 @@ impl Interpreter {
             }
             TokenType::BangEqual => Ok(Value::Boolean(left != right)),
             TokenType::EqualEqual => Ok(Value::Boolean(left == right)),
-            _ => Err(InterpreterError::BinaryOperatorUnknown(op.line, op.lexeme)),
+            _ => Err(InterpreterError::BinaryOperatorUnknown(
+                op.line,
+                op.lexeme.clone(),
+            )),
         }
     }
 }

rust/rox/src/parser.rs

@@ -5,7 +5,7 @@ use crate::{
     expression::Expression,
     statement::Statement,
     token::{
-        self, Token,
+        self, Literal, Token,
         TokenType::{self, *},
     },
 };
@@ -32,6 +32,18 @@ pub enum ParserError {
     InvalidAssignmentTarget(usize),
     #[error("[line {0}] expected '}}' after block.")]
     RightBraceAfterBlockExpected(usize),
+    #[error("[line {0}] expected '(' after if.")]
+    LeftParenAfterIfExpected(usize),
+    #[error("[line {0}] expected ')' after condition.")]
+    RightParenAfterConditionExpected(usize),
+    #[error("[line {0}] expected '(' after while.")]
+    LeftParenAfterWhileExpected(usize),
+    #[error("[line {0}] expected '(' after for.")]
+    LeftParenAfterForExpected(usize),
+    #[error("[line {0}] expected ';' after loop condition.")]
+    SemicolonAfterLoopConditionExpected(usize),
+    #[error("[line {0}] expected ')' after for clauses.")]
+    RightParenAfterForClausesExpected(usize),
 }
 
 /// Parse the Lox language.
@@ -157,8 +169,14 @@ impl Parser {
     }
 
     fn statement(&mut self) -> Result<Statement, ParserError> {
-        if self.matches(&[Print]) {
+        if self.matches(&[For]) {
+            self.for_statement()
+        } else if self.matches(&[If]) {
+            self.if_statement()
+        } else if self.matches(&[Print]) {
             self.print_statement()
+        } else if self.matches(&[While]) {
+            self.while_statement()
         } else if self.matches(&[LeftBrace]) {
             Ok(Statement::Block(self.block()?))
         } else {
@@ -166,6 +184,79 @@ impl Parser {
         }
     }
 
+    /// Build up a while statement from a for statement.
+    fn for_statement(&mut self) -> Result<Statement, ParserError> {
+        let line = self.current_token.line;
+        self.consume(&LeftParen)
+            .ok_or(ParserError::LeftParenAfterForExpected(line))?;
+
+        let initializer = if self.matches(&[Semicolon]) {
+            None
+        } else if self.matches(&[Var]) {
+            Some(self.var_declaration()?)
+        } else {
+            Some(self.expression_statement()?)
+        };
+
+        let condition = if !self.matches(&[Semicolon]) {
+            self.expression()?
+        } else {
+            Expression::Literal {
+                value: Literal::Boolean(true),
+            }
+        };
+        self.consume(&Semicolon)
+            .ok_or(ParserError::SemicolonAfterLoopConditionExpected(line))?;
+
+        let increment = if !self.check(&RightParen) {
+            Some(self.expression()?)
+        } else {
+            None
+        };
+        self.consume(&RightParen)
+            .ok_or(ParserError::RightParenAfterForClausesExpected(line))?;
+
+        let body = self.statement()?;
+        let body = match increment {
+            Some(inc) => Statement::Block(vec![body, Statement::Expression(inc)]),
+            None => body,
+        };
+        let body = Statement::While {
+            condition,
+            body: Box::new(body),
+        };
+        let body = match initializer {
+            Some(initializer) => Statement::Block(vec![initializer, body]),
+            None => body,
+        };
+
+        Ok(body)
+    }
+
+    fn if_statement(&mut self) -> Result<Statement, ParserError> {
+        let line = self.current_token.line;
+        self.consume(&LeftParen)
+            .ok_or(ParserError::LeftParenAfterIfExpected(line))?;
+
+        let condition = self.expression()?;
+
+        self.consume(&RightParen)
+            .ok_or(ParserError::RightParenAfterConditionExpected(line))?;
+
+        let then_branch = self.statement()?;
+        let else_branch = if self.matches(&[Else]) {
+            Some(Box::new(self.statement()?))
+        } else {
+            None
+        };
+
+        Ok(Statement::If {
+            condition,
+            then_branch: Box::new(then_branch),
+            else_branch,
+        })
+    }
+
     fn print_statement(&mut self) -> Result<Statement, ParserError> {
         let value = self.expression()?;
         let line = self.current_token.line;
@@ -197,6 +288,24 @@ impl Parser {
         })
     }
 
+    fn while_statement(&mut self) -> Result<Statement, ParserError> {
+        let line = self.current_token.line;
+        self.consume(&LeftParen)
+            .ok_or(ParserError::LeftParenAfterWhileExpected(line))?;
+
+        let condition = self.expression()?;
+
+        self.consume(&RightParen)
+            .ok_or(ParserError::RightParenAfterConditionExpected(line))?;
+
+        let body = self.statement()?;
+
+        Ok(Statement::While {
+            condition,
+            body: Box::new(body),
+        })
+    }
+
     fn expression_statement(&mut self) -> Result<Statement, ParserError> {
         let expr = self.expression()?;
         let line = self.current_token.line;
@@ -221,7 +330,7 @@ impl Parser {
     }
 
     fn assignment(&mut self) -> Result<Expression, ParserError> {
-        let expr = self.equality()?;
+        let expr = self.or()?;
 
         if self.matches(&[Equal]) {
             let equals = self.previous()?.clone();
@@ -240,6 +349,38 @@ impl Parser {
         }
     }
 
+    fn logical_operator<F>(
+        &mut self,
+        operator: TokenType,
+        parse_fn: F,
+    ) -> Result<Expression, ParserError>
+    where
+        F: Fn(&mut Self) -> Result<Expression, ParserError>,
+    {
+        let mut expr = parse_fn(self)?;
+
+        while self.matches(&[operator]) {
+            let operator = self.previous()?.clone();
+            let right = parse_fn(self)?;
+
+            expr = Expression::Logical {
+                left: Box::new(expr),
+                operator,
+                right: Box::new(right),
+            };
+        }
+
+        Ok(expr)
+    }
+
+    fn or(&mut self) -> Result<Expression, ParserError> {
+        self.logical_operator(Or, Self::and)
+    }
+
+    fn and(&mut self) -> Result<Expression, ParserError> {
+        self.logical_operator(And, Self::equality)
+    }
+
     /// equality -> comparison ( ( "!=" | "==" ) comparison )* ;
     fn equality(&mut self) -> Result<Expression, ParserError> {
         self.binary_expr(Self::comparison, &[BangEqual, EqualEqual])

rust/rox/src/statement.rs

@@ -10,4 +10,13 @@ pub enum Statement {
         name: Token,
         initializer: Box<Option<Expression>>,
     },
+    If {
+        condition: Expression,
+        then_branch: Box<Statement>,
+        else_branch: Option<Box<Statement>>,
+    },
+    While {
+        condition: Expression,
+        body: Box<Statement>,
+    },
 }