use crate::errors::{Error, ErrorKind}; use crate::parser::{self, Block, Expr, Literal, Statement}; use crate::scanner::{self, TokenKind}; use std::collections::HashMap; use std::rc::Rc; use std::sync::RwLock; // Implementation of built-in functions. mod builtins; #[cfg(test)] mod tests; // Tree-walk interpreter // Representation of all callables, including builtins & user-defined // functions. #[derive(Clone, Debug)] pub enum Callable { Builtin(&'static dyn builtins::Builtin), Function(Rc), } impl Callable { fn arity(&self) -> usize { match self { Callable::Builtin(builtin) => builtin.arity(), Callable::Function(func) => func.params.len(), } } fn call(&self, lox: &mut Interpreter, args: Vec) -> Result { match self { Callable::Builtin(builtin) => builtin.call(args), Callable::Function(func) => { let mut fn_env: Environment = Default::default(); for (param, value) in func.params.iter().zip(args.into_iter()) { fn_env.define(param, value)?; } let result = lox.interpret_block(Rc::new(RwLock::new(fn_env)), &func.body); match result { // extract returned values if applicable Err(Error { kind: ErrorKind::FunctionReturn(value), .. }) => Ok(value), // otherwise just return the result itself _ => result, } } } } } // Representation of an in-language value. #[derive(Clone, Debug)] pub enum Value { Literal(Literal), Callable(Callable), } impl PartialEq for Value { fn eq(&self, other: &Self) -> bool { match (self, other) { (Value::Literal(lhs), Value::Literal(rhs)) => lhs == rhs, // functions do not have equality _ => false, } } } impl From for Value { fn from(lit: Literal) -> Value { Value::Literal(lit) } } impl Value { fn expect_literal(self) -> Result { match self { Value::Literal(lit) => Ok(lit), _ => unimplemented!(), // which error? which line? } } } #[derive(Debug, Default)] struct Environment { enclosing: Option>>, values: HashMap, } impl Environment { fn define(&mut self, name: &scanner::Token, value: Value) -> Result<(), Error> { let ident = identifier_str(name)?; self.values.insert(ident.into(), value); Ok(()) } fn get(&self, name: &parser::Variable) -> Result { let ident = identifier_str(&name.0)?; self.values .get(ident) .map(Clone::clone) .ok_or_else(|| Error { line: name.0.line, kind: ErrorKind::UndefinedVariable(ident.into()), }) .or_else(|err| { if let Some(parent) = &self.enclosing { parent.read().unwrap().get(name) } else { Err(err) } }) } fn assign(&mut self, name: &scanner::Token, value: Value) -> Result<(), Error> { let ident = identifier_str(name)?; match self.values.get_mut(ident) { Some(target) => { *target = value; Ok(()) } None => { if let Some(parent) = &self.enclosing { return parent.write().unwrap().assign(name, value); } Err(Error { line: name.line, kind: ErrorKind::UndefinedVariable(ident.into()), }) } } } } fn identifier_str(name: &scanner::Token) -> Result<&str, Error> { if let TokenKind::Identifier(ident) = &name.kind { Ok(ident) } else { Err(Error { line: name.line, kind: ErrorKind::InternalError("unexpected identifier kind".into()), }) } } #[derive(Debug)] pub struct Interpreter { env: Rc>, } impl Interpreter { /// Create a new interpreter and configure the initial global /// variable set. pub fn create() -> Self { let mut globals = HashMap::new(); globals.insert( "clock".into(), Value::Callable(Callable::Builtin(&builtins::Clock {})), ); Interpreter { env: Rc::new(RwLock::new(Environment { enclosing: None, values: globals, })), } } // Environment modification helpers fn define_var(&mut self, name: &scanner::Token, value: Value) -> Result<(), Error> { self.env .write() .expect("environment lock is poisoned") .define(name, value) } fn assign_var(&mut self, name: &scanner::Token, value: Value) -> Result<(), Error> { self.env .write() .expect("environment lock is poisoned") .assign(name, value) } fn get_var(&mut self, var: &parser::Variable) -> Result { self.env .read() .expect("environment lock is poisoned") .get(var) } fn set_enclosing(&mut self, parent: Rc>) { self.env .write() .expect("environment lock is poisoned") .enclosing = Some(parent); } // Interpreter itself pub fn interpret(&mut self, program: &Block) -> Result { let mut value = Value::Literal(Literal::Nil); for stmt in program { value = self.interpret_stmt(stmt)?; } Ok(value) } fn interpret_stmt(&mut self, stmt: &Statement) -> Result { let value = match stmt { Statement::Expr(expr) => self.eval(expr)?, Statement::Print(expr) => { let result = self.eval(expr)?; let output = format!("{:?}", result); println!("{}", output); Value::Literal(Literal::String(output)) } Statement::Var(var) => return self.interpret_var(var), Statement::Block(block) => return self.interpret_block(Default::default(), block), Statement::If(if_stmt) => return self.interpret_if(if_stmt), Statement::While(while_stmt) => return self.interpret_while(while_stmt), Statement::Function(func) => return self.interpret_function(func.clone()), Statement::Return(ret) => { return Err(Error { line: 0, kind: ErrorKind::FunctionReturn(self.eval(&ret.value)?), }) } }; Ok(value) } fn interpret_var(&mut self, var: &parser::Var) -> Result { let init = var.initialiser.as_ref().ok_or_else(|| Error { line: var.name.line, kind: ErrorKind::InternalError("missing variable initialiser".into()), })?; let value = self.eval(init)?; self.define_var(&var.name, value.clone())?; Ok(value) } fn interpret_block( &mut self, env: Rc>, block: &parser::Block, ) -> Result { // Initialise a new environment and point it at the parent // (this is a bit tedious because we need to wrap it in and // out of the Rc). // // TODO(tazjin): Refactor this to use Rc on the interpreter itself. let previous = std::mem::replace(&mut self.env, env); self.set_enclosing(previous.clone()); let result = self.interpret(block); // Swap it back, discarding the child env. self.env = previous; return result; } fn interpret_if(&mut self, if_stmt: &parser::If) -> Result { let condition = self.eval(&if_stmt.condition)?; if eval_truthy(&condition) { self.interpret_stmt(&if_stmt.then_branch) } else if let Some(else_branch) = &if_stmt.else_branch { self.interpret_stmt(else_branch) } else { Ok(Value::Literal(Literal::Nil)) } } fn interpret_while(&mut self, stmt: &parser::While) -> Result { let mut value = Value::Literal(Literal::Nil); while eval_truthy(&self.eval(&stmt.condition)?) { value = self.interpret_stmt(&stmt.body)?; } Ok(value) } fn interpret_function(&mut self, stmt: Rc) -> Result { let name = stmt.name.clone(); let value = Value::Callable(Callable::Function(stmt)); self.define_var(&name, value.clone())?; Ok(value) } fn eval(&mut self, expr: &Expr) -> Result { match expr { Expr::Assign(assign) => self.eval_assign(assign), Expr::Literal(lit) => Ok(lit.clone().into()), Expr::Grouping(grouping) => self.eval(&*grouping.0), Expr::Unary(unary) => self.eval_unary(unary), Expr::Binary(binary) => self.eval_binary(binary), Expr::Variable(var) => self.get_var(var), Expr::Logical(log) => self.eval_logical(log), Expr::Call(call) => self.eval_call(call), } } fn eval_unary(&mut self, expr: &parser::Unary) -> Result { let right = self.eval(&*expr.right)?; match (&expr.operator.kind, right) { (TokenKind::Minus, Value::Literal(Literal::Number(num))) => { Ok(Literal::Number(-num).into()) } (TokenKind::Bang, right) => Ok(Literal::Boolean(!eval_truthy(&right)).into()), (op, right) => Err(Error { line: expr.operator.line, kind: ErrorKind::TypeError(format!( "Operator '{:?}' can not be called with argument '{:?}'", op, right )), }), } } fn eval_binary(&mut self, expr: &parser::Binary) -> Result { let left = self.eval(&*expr.left)?.expect_literal()?; let right = self.eval(&*expr.right)?.expect_literal()?; let result = match (&expr.operator.kind, left, right) { // Numeric (TokenKind::Minus, Literal::Number(l), Literal::Number(r)) => Literal::Number(l - r), (TokenKind::Slash, Literal::Number(l), Literal::Number(r)) => Literal::Number(l / r), (TokenKind::Star, Literal::Number(l), Literal::Number(r)) => Literal::Number(l * r), (TokenKind::Plus, Literal::Number(l), Literal::Number(r)) => Literal::Number(l + r), // Strings (TokenKind::Plus, Literal::String(l), Literal::String(r)) => { Literal::String(format!("{}{}", l, r)) } // Comparators (on numbers only?) (TokenKind::Greater, Literal::Number(l), Literal::Number(r)) => Literal::Boolean(l > r), (TokenKind::GreaterEqual, Literal::Number(l), Literal::Number(r)) => { Literal::Boolean(l >= r) } (TokenKind::Less, Literal::Number(l), Literal::Number(r)) => Literal::Boolean(l < r), (TokenKind::LessEqual, Literal::Number(l), Literal::Number(r)) => { Literal::Boolean(l <= r) } // Equality (TokenKind::Equal, l, r) => Literal::Boolean(l == r), (TokenKind::BangEqual, l, r) => Literal::Boolean(l != r), (op, left, right) => { return Err(Error { line: expr.operator.line, kind: ErrorKind::TypeError(format!( "Operator '{:?}' can not be called with arguments '({:?}, {:?})'", op, left, right )), }) } }; Ok(result.into()) } fn eval_assign(&mut self, assign: &parser::Assign) -> Result { let value = self.eval(&assign.value)?; self.assign_var(&assign.name, value.clone())?; Ok(value) } fn eval_logical(&mut self, logical: &parser::Logical) -> Result { let left = eval_truthy(&self.eval(&logical.left)?); let right = eval_truthy(&self.eval(&logical.right)?); match &logical.operator.kind { TokenKind::And => Ok(Literal::Boolean(left && right).into()), TokenKind::Or => Ok(Literal::Boolean(left || right).into()), kind => Err(Error { line: logical.operator.line, kind: ErrorKind::InternalError(format!("Invalid logical operator: {:?}", kind)), }), } } fn eval_call(&mut self, call: &parser::Call) -> Result { let callable = match self.eval(&call.callee)? { Value::Callable(c) => c, Value::Literal(v) => { return Err(Error { line: call.paren.line, kind: ErrorKind::RuntimeError(format!("not callable: {:?}", v)), }) } }; let mut args = vec![]; for arg in &call.args { args.push(self.eval(arg)?); } if callable.arity() != args.len() { return Err(Error { line: call.paren.line, kind: ErrorKind::RuntimeError(format!( "Expected {} arguments, but got {}", callable.arity(), args.len(), )), }); } callable.call(self, args) } } // Interpreter functions not dependent on interpreter-state. fn eval_truthy(lit: &Value) -> bool { if let Value::Literal(lit) = lit { match lit { Literal::Nil => false, Literal::Boolean(b) => *b, _ => true, } } else { false } }