rlox/src/interpreter/eval.rs

use std::fmt::Display;

use crate::error::RuntimeError;
use crate::parser::{BinaryOp, Expr, Literal, Stmt, UnaryOp};

use super::environment::Environment;

pub type EvalResult<T> = Result<T, RuntimeError>;

/*====================================================================================================================*/

pub fn execute(statement: Stmt, env: &mut Environment) -> Result<(), RuntimeError> {
    statement.eval(env)
}

/*====================================================================================================================*/

#[derive(Debug, Clone, PartialEq)]
pub enum Value {
    String(String),
    Number(f64),
    Bool(bool),
    Nil,
}

impl Value {
    fn is_truthy(&self) -> bool {
        match self {
            Value::Bool(false) | Value::Nil => false,
            _ => true,
        }
    }
}

impl Display for Value {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Value::String(s) => write!(f, "\"{s}\""),
            Value::Number(num) => write!(f, "{num}"),
            Value::Bool(b) => write!(f, "{b}"),
            Value::Nil => write!(f, "nil"),
        }
    }
}

/*====================================================================================================================*/

/* trait Eval {
    fn eval(self, env: &mut Environment) -> EvalResult;
} */

impl Literal {
    fn eval(self, _env: &mut Environment) -> EvalResult<Value> {
        match self {
            Literal::String(s) => Ok(Value::String(s)),
            Literal::Number(num) => Ok(Value::Number(num)),
            Literal::Bool(b) => Ok(Value::Bool(b)),
            Literal::Nil => Ok(Value::Nil),
        }
    }
}

impl Expr {
    fn eval(self, env: &mut Environment) -> EvalResult<Value> {
        match self {
            Expr::Literal { literal } => literal.eval(env),
            Expr::Unary { op, expr } => {
                let arg = expr.eval(env)?;

                match (op, arg) {
                    (UnaryOp::Negate, Value::Number(num)) => Ok(Value::Number(-num)),
                    (UnaryOp::Not, Value::Bool(b)) => Ok(Value::Bool(!b)),
                    (UnaryOp::Not, primitive) => Ok(Value::Bool(!primitive.is_truthy())),
                    (op, arg) => Err(RuntimeError::UnaryOpInvalidArgument { op, arg }),
                }
            }
            Expr::Binary { left, op, right } => {
                use Value::{Bool, Number, String};

                let left = left.eval(env)?;
                let right = right.eval(env)?;

                match (left, op, right) {
                    (Number(left), BinaryOp::Add, Number(right)) => Ok(Number(left + right)),
                    (Number(left), BinaryOp::Subtract, Number(right)) => Ok(Number(left - right)),
                    (Number(left), BinaryOp::Multiply, Number(right)) => Ok(Number(left * right)),
                    (Number(left), BinaryOp::Divide, Number(right)) => {
                        if right == 0.0 {
                            return Err(RuntimeError::DivisionByZero);
                        }
                        Ok(Number(left / right))
                    }

                    (String(left), BinaryOp::Add, String(right)) => Ok(String(left + &right)),

                    (left, BinaryOp::Equal, right) => Ok(Bool(left == right)),
                    (left, BinaryOp::NotEqual, right) => Ok(Bool(left != right)),

                    (Number(left), BinaryOp::Less, Number(right)) => Ok(Bool(left < right)),
                    (Number(left), BinaryOp::LessEqual, Number(right)) => Ok(Bool(left <= right)),
                    (Number(left), BinaryOp::Greater, Number(right)) => Ok(Bool(left > right)),
                    (Number(left), BinaryOp::GreaterEqual, Number(right)) => Ok(Bool(left >= right)),

                    (String(left), BinaryOp::Less, String(right)) => Ok(Bool(left < right)),
                    (String(left), BinaryOp::LessEqual, String(right)) => Ok(Bool(left <= right)),
                    (String(left), BinaryOp::Greater, String(right)) => Ok(Bool(left > right)),
                    (String(left), BinaryOp::GreaterEqual, String(right)) => Ok(Bool(left >= right)),

                    (left, op, right) => Err(RuntimeError::BinaryOpInvalidArguments { left, op, right }),
                }
            }
            Expr::Grouping { expr } => expr.eval(env),
            Expr::Variable { name } => env.get(&name).cloned(),
            Expr::Assignment { name, value } => {
                let value = value.eval(env)?;
                env.assign(&name, value.clone())?;
                Ok(value)
            }
        }
    }
}

impl Stmt {
    fn eval(self, env: &mut Environment) -> EvalResult<()> {
        match self {
            Stmt::ExprStmt { expr } => {
                // expr.eval(env)?;
                // Ok(Value::Nil)
                expr.eval(env)?;
                Ok(())
            }
            Stmt::Print { expr } => {
                match expr.eval(env)? {
                    // special case: when printing a string, drop the surrounding ""
                    Value::String(s) => println!("{s}"),
                    val => println!("{val}"),
                }

                Ok(())
            }
            Stmt::VarDecl { name, initializer } => {
                let initializer = initializer.eval(env)?;
                env.define(name, initializer);
                Ok(())
            }
            Stmt::Block { statements } => {
                env.enter_scope();
                for statement in statements {
                    // on error the current frame gets destroyed anyways, so no need to exit scope
                    statement.eval(env)?;
                }
                env.exit_scope();
                Ok(())
            }
        }
    }
}