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; // Tree-walk interpreter // Representation of an in-language value. #[derive(Clone, Debug)] enum Value { Literal(Literal), } impl From<Literal> for Value { fn from(lit: Literal) -> Value { Value::Literal(lit) } } impl Value { fn expect_literal(self) -> Result<Literal, Error> { match self { Value::Literal(lit) => Ok(lit), _ => unimplemented!(), // which error? which line? } } } #[derive(Debug, Default)] struct Environment { enclosing: Option<Rc<RwLock<Environment>>>, values: HashMap<String, Value>, } 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<Value, Error> { 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<'a>(name: &'a scanner::Token) -> Result<&'a 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, Default)] pub struct Interpreter { env: Rc<RwLock<Environment>>, } impl Interpreter { // 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<Value, Error> { self.env .read() .expect("environment lock is poisoned") .get(var) } fn set_enclosing(&mut self, parent: Rc<RwLock<Environment>>) { self.env .write() .expect("environment lock is poisoned") .enclosing = Some(parent); } // Interpreter itself pub fn interpret<'a>(&mut self, program: &Block<'a>) -> Result<(), Error> { for stmt in program { self.interpret_stmt(stmt)?; } Ok(()) } fn interpret_stmt<'a>(&mut self, stmt: &Statement<'a>) -> Result<(), Error> { match stmt { Statement::Expr(expr) => { self.eval(expr)?; } Statement::Print(expr) => { let result = self.eval(expr)?; println!("{:?}", result) } Statement::Var(var) => return self.interpret_var(var), Statement::Block(block) => return self.interpret_block(block), Statement::If(if_stmt) => return self.interpret_if(if_stmt), Statement::While(while_stmt) => return self.interpret_while(while_stmt), } Ok(()) } fn interpret_var<'a>(&mut self, var: &parser::Var<'a>) -> Result<(), Error> { 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)?; return Ok(()); } fn interpret_block<'a>(&mut self, block: &parser::Block<'a>) -> Result<(), Error> { // 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, Default::default()); 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<'a>(&mut self, if_stmt: &parser::If<'a>) -> Result<(), Error> { 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(()) } } fn interpret_while<'a>(&mut self, stmt: &parser::While<'a>) -> Result<(), Error> { while eval_truthy(&self.eval(&stmt.condition)?) { self.interpret_stmt(&stmt.body)?; } Ok(()) } fn eval<'a>(&mut self, expr: &Expr<'a>) -> Result<Value, Error> { 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(_) => unimplemented!(), } } fn eval_unary<'a>(&mut self, expr: &parser::Unary<'a>) -> Result<Value, Error> { 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<'a>(&mut self, expr: &parser::Binary<'a>) -> Result<Value, Error> { 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<'a>(&mut self, assign: &parser::Assign<'a>) -> Result<Value, Error> { let value = self.eval(&assign.value)?; self.assign_var(&assign.name, value.clone())?; Ok(value) } fn eval_logical<'a>(&mut self, logical: &parser::Logical<'a>) -> Result<Value, Error> { 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)), }), } } } // 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 } }