crafting-interpreters/rust/rox/src/interpreter.rs

use thiserror::Error;
use tracing::error;

use crate::{
    environment::{Environment, EnvironmentError},
    expression::Expression,
    statement::Statement,
    token::{Literal, Token, TokenType},
    value::Value,
};

#[derive(Error, Debug)]
pub enum InterpreterError {
    #[error("[line {0}] MINUS unary expression expects a number on the right")]
    UnaryExpressionNotANumber(usize),
    #[error("[line {0}] unknown unary operator: {1}")]
    UnaryOperatorUnknown(usize, String),
    #[error("[line {0}] unknown binary operator: {1}")]
    BinaryOperatorUnknown(usize, String),
    #[error("[line {0}] left or right is not a number.")]
    BinaryExpressionNeedsNumber(usize),
    #[error("[line {0}] left or right is neither a number nor string.")]
    BinaryExpressionNeedsNumberOrString(usize),
    #[error("{0}")]
    UndefinedVariable(EnvironmentError),
}

/// Interpreter for the Lox language.
#[derive(Default, Debug)]
pub struct Interpreter {
    environment: 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) {
                Ok(_) => {}
                Err(e) => error!("{e}"),
            };
        }

        Ok(())
    }

    ///Execute a statement.
    fn execute(&mut self, statement: Statement) -> Result<(), InterpreterError> {
        match statement {
            Statement::Block(statements) => {
                let sub_env = Environment::with_enclosing(self.environment.clone());
                self.block(statements, sub_env)?
            }
            Statement::Print(expression) => self.print_statement(expression)?,
            Statement::Expression(expression) => {
                self.evaluate(expression)?;
            }
            Statement::Var { name, initializer } => self.var_statement(name, *initializer)?,
        };

        Ok(())
    }

    /// Execute all statements within a block, using a new environment (with the old one as the
    /// enclosing one).
    fn block(
        &mut self,
        statements: Vec<Statement>,
        environment: Environment,
    ) -> Result<(), InterpreterError> {
        let prev_env = &self.environment.clone();
        self.environment = environment;

        for stmt in statements {
            if let Err(e) = self.execute(stmt) {
                error!("{e}");
            }
        }

        self.environment = prev_env.clone();

        Ok(())
    }

    /// Evaluate an expression and return its value.
    fn evaluate(&mut self, expression: Expression) -> Result<Value, InterpreterError> {
        match expression {
            Expression::Literal { value } => self.literal(value),
            Expression::Grouping { expression } => self.grouping(*expression),
            Expression::Unary {
                operator: 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),
        }
    }

    /// Evaluate an expression and print its value to stdout.
    fn print_statement(&mut self, expression: Expression) -> Result<(), InterpreterError> {
        let value = self.evaluate(expression)?;
        println!("{value}");

        Ok(())
    }

    /// Initialize a variable with an initializer expression or nil.
    fn var_statement(
        &mut self,
        name: Token,
        initializer: Option<Expression>,
    ) -> Result<(), InterpreterError> {
        let value = if let Some(initializer) = initializer {
            self.evaluate(initializer)
        } else {
            Ok(Value::Nil)
        }?;

        self.environment.define(name.lexeme, value);

        Ok(())
    }

    /// Assign the value of an expression to a variable.
    fn assign(&mut self, name: &Token, value: Expression) -> Result<Value, InterpreterError> {
        let value = self.evaluate(value)?;

        self.environment
            .assign(name, value.clone())
            .map_err(InterpreterError::UndefinedVariable)?;
        Ok(value)
    }

    /// Convert the literal value into a Value.
    fn literal(&self, literal: Literal) -> Result<Value, InterpreterError> {
        Ok(literal.into())
    }

    /// Evaluate the inner expression.
    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> {
        let right = self.evaluate(right)?;

        match op.token_type {
            TokenType::Minus => {
                if let Value::Number(val) = right {
                    Ok(Value::Number(-val))
                } else {
                    Err(InterpreterError::UnaryExpressionNotANumber(op.line))
                }
            }
            TokenType::Bang => Ok(Value::Boolean(!right.is_truthy())),
            _ => Err(InterpreterError::UnaryOperatorUnknown(op.line, op.lexeme)),
        }
    }

    /// Get the value of a variable.
    fn var_expression(&mut self, name: &Token) -> Result<Value, InterpreterError> {
        self.environment
            .get(name)
            .map_err(InterpreterError::UndefinedVariable)
    }

    /// Calculate number operations.
    fn number_op(&self, left: f64, op: TokenType, right: f64) -> f64 {
        match op {
            TokenType::Minus => left - right,
            TokenType::Plus => left + right,
            TokenType::Slash => left / right,
            TokenType::Star => left * right,
            _ => unreachable!(),
        }
    }

    /// Calculate boolean operations.
    fn boolean_op(&self, left: f64, op: TokenType, right: f64) -> bool {
        match op {
            TokenType::Greater => left > right,
            TokenType::GreaterEqual => left >= right,
            TokenType::Less => left < right,
            TokenType::LessEqual => left <= right,
            _ => unreachable!(),
        }
    }

    /// Evaluate the left and right expressions (in that order) and then combine them with the
    /// specified operator.
    fn binary(
        &mut self,
        left: Expression,
        op: Token,
        right: Expression,
    ) -> Result<Value, InterpreterError> {
        let left = self.evaluate(left)?;
        let right = self.evaluate(right)?;

        match op.token_type {
            TokenType::Minus | TokenType::Slash | TokenType::Star | TokenType::Plus => {
                if let (Value::Number(left), Value::Number(right)) = (left.clone(), right.clone()) {
                    Ok(Value::Number(self.number_op(left, op.token_type, right)))
                } else if let (Value::String(left), Value::String(right)) = (left, right) {
                    Ok(Value::String(format!("{}{}", left.clone(), right.clone())))
                } else {
                    Err(InterpreterError::BinaryExpressionNeedsNumberOrString(
                        op.line,
                    ))
                }
            }
            TokenType::Greater
            | TokenType::GreaterEqual
            | TokenType::Less
            | TokenType::LessEqual => {
                if let (Value::Number(left), Value::Number(right)) = (left, right) {
                    Ok(Value::Boolean(self.boolean_op(left, op.token_type, right)))
                } else {
                    Err(InterpreterError::BinaryExpressionNeedsNumber(op.line))
                }
            }
            TokenType::BangEqual => Ok(Value::Boolean(left != right)),
            TokenType::EqualEqual => Ok(Value::Boolean(left == right)),
            _ => Err(InterpreterError::BinaryOperatorUnknown(op.line, op.lexeme)),
        }
    }
}