interpreter in rust

rust/rox/src/interpreter.rs

@@ -0,0 +1,138 @@
+use thiserror::Error;
+
+use crate::{
+    expression::Expression,
+    token::{Literal, Token, TokenType},
+};
+
+#[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),
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum Value {
+    String(String),
+    Number(f64),
+    Boolean(bool),
+    Nil,
+}
+
+impl Value {
+    fn is_truthy(&self) -> bool {
+        match self {
+            Value::Nil => false,
+            Value::Boolean(b) => *b,
+            _ => true,
+        }
+    }
+}
+
+impl From<Literal> for Value {
+    fn from(value: Literal) -> Self {
+        match value {
+            Literal::String(x) => Value::String(x),
+            Literal::Number(x) => Value::Number(x),
+            Literal::Boolean(x) => Value::Boolean(x),
+            Literal::Nil => Value::Nil,
+        }
+    }
+}
+
+/// Try to evaluate an expression and return its result.
+pub fn evaluate(expression: Expression) -> Result<Value, InterpreterError> {
+    match expression {
+        Expression::Literal { value } => literal(value),
+        Expression::Grouping { expression } => grouping(*expression),
+        Expression::Unary {
+            operator: op,
+            right,
+        } => unary(op, *right),
+        Expression::Binary {
+            left,
+            operator,
+            right,
+        } => binary(*left, operator, *right),
+    }
+}
+
+/// Convert the literal value into a Value.
+fn literal(literal: Literal) -> Result<Value, InterpreterError> {
+    Ok(literal.into())
+}
+
+/// Evaluate the inner expression.
+fn grouping(inner: Expression) -> Result<Value, InterpreterError> {
+    evaluate(inner)
+}
+
+/// Evaluate the expression on the right and use its result when evaluating the unary operator.
+fn unary(op: Token, right: Expression) -> Result<Value, InterpreterError> {
+    let right = 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)),
+    }
+}
+
+/// Calculate number operations.
+fn number_op(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(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(left: Expression, op: Token, right: Expression) -> Result<Value, InterpreterError> {
+    let left = evaluate(left)?;
+    let right = evaluate(right)?;
+
+    match op.token_type {
+        TokenType::Minus | TokenType::Slash | TokenType::Star | TokenType::Plus => {
+            if let (Value::Number(left), Value::Number(right)) = (left, right) {
+                Ok(Value::Number(number_op(left, op.token_type, right)))
+            } else {
+                todo!()
+            }
+        }
+        TokenType::Greater | TokenType::GreaterEqual | TokenType::Less | TokenType::LessEqual => {
+            if let (Value::Number(left), Value::Number(right)) = (left, right) {
+                Ok(Value::Boolean(boolean_op(left, op.token_type, right)))
+            } else {
+                todo!()
+            }
+        }
+        TokenType::BangEqual => Ok(Value::Boolean(left != right)),
+        TokenType::EqualEqual => Ok(Value::Boolean(left == right)),
+        _ => Err(InterpreterError::BinaryOperatorUnknown(op.line, op.lexeme)),
+    }
+}

rust/rox/src/lib.rs

@@ -11,6 +11,7 @@ use tracing::{error, info};
 
 pub mod cli;
 pub mod expression;
+pub mod interpreter;
 pub mod keywords;
 pub mod parser;
 pub mod scanner;
@@ -30,11 +31,7 @@ pub mod tokenizer {
 /// Read the source code in a file and scan it to tokens.
 pub fn compile(source: &Path) -> Result<(), io::Error> {
     let input = fs::read_to_string(source)?;
-    let tokens = scanner::tokenize(&input);
-    match parser::generate_ast(tokens) {
-        Ok(ast) => info!("{ast:?}"),
-        Err(e) => error!("{e}"),
-    }
+    run(&input);
 
     Ok(())
 }
@@ -51,10 +48,18 @@ pub fn repl() {
             Err(e) => error!("{}", e),
         }
         let input = input.trim().to_string();
-        let tokens = scanner::tokenize(&input);
-        match parser::generate_ast(tokens) {
-            Ok(ast) => info!("{ast:?}"),
-            Err(e) => error!("{e}"),
-        }
+        run(&input);
+    }
+}
+
+/// Evaluate a Lox input string and print errors or output.
+fn run(input: &str) {
+    let tokens = scanner::tokenize(input);
+    match parser::generate_ast(tokens) {
+        Ok(ast) => match interpreter::evaluate(ast) {
+            Ok(value) => info!("{value:?}"),
+            Err(e) => error!("{e}"),
+        },
+        Err(e) => error!("{e}"),
     }
 }