Added Tree walker evaluator

1. Added a tree walker interpreter
2. Added tests to support evaluator

Cargo.toml

@@ -7,4 +7,3 @@ edition = "2018"
 [dependencies]
 lazy_static = "1.4.0"
 itertools = "0.8.2"
-

src/evaluator/mod.rs

@@ -0,0 +1,135 @@
+use crate::parser::ast::Node;
+pub mod tree_walker;
+
+pub trait Evaluator {
+    fn eval(&self, node: Node) -> Option<Object>;
+}
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub enum Object {
+    Integer(i64),
+    Boolean(bool),
+    Null,
+}
+
+const NULL: Object = Object::Null;
+const TRUE: Object = Object::Boolean(true);
+const FALSE: Object = Object::Boolean(false);
+
+impl Object {
+    pub fn inspect(&self) -> String {
+        match self {
+            Object::Integer(v) => v.to_string(),
+            Object::Boolean(v) => v.to_string(),
+            Object::Null => "NULL".into(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{
+        evaluator::{tree_walker::TreeWalker, Evaluator, Object},
+        lexer::Lexer,
+        parser::{ast::Node, Parser},
+    };
+
+    #[test]
+    fn eval_integer_expression() {
+        let test_cases = [
+            ("5", Some(Object::Integer(5))),
+            ("10", Some(Object::Integer(10))),
+        ];
+
+        for test in test_cases.iter() {
+            let lexer = Lexer::new(test.0);
+            let mut parser = Parser::new(lexer);
+            let program = parser.parse_program();
+            assert!(program.is_some());
+            let program = program.unwrap();
+            let evaluator = TreeWalker::new();
+            let eval = evaluator.eval(Node::Program(program));
+            assert_eq!(eval, test.1);
+        }
+    }
+
+    #[test]
+    fn eval_boolean_expression() {
+        let test_cases = [
+            ("true", Some(Object::Boolean(true))),
+            ("false", Some(Object::Boolean(false))),
+            ("3 < 4", Some(Object::Boolean(true))),
+            ("3 > 4", Some(Object::Boolean(false))),
+            ("3 == 4", Some(Object::Boolean(false))),
+            ("3 != 4", Some(Object::Boolean(true))),
+            ("(1 < 2 ) == true", Some(Object::Boolean(true))),
+            ("(1 < 2 ) == false", Some(Object::Boolean(false))),
+            ("(1 > 2 ) == true", Some(Object::Boolean(false))),
+            ("(1 > 2 ) == false", Some(Object::Boolean(true))),
+        ];
+
+        for test in test_cases.iter() {
+            let lexer = Lexer::new(test.0);
+            let mut parser = Parser::new(lexer);
+            let program = parser.parse_program();
+            assert!(program.is_some());
+            let program = program.unwrap();
+            let evaluator = TreeWalker::new();
+            let eval = evaluator.eval(Node::Program(program));
+            assert_eq!(eval, test.1);
+        }
+    }
+
+    #[test]
+    fn eval_bang_operator() {
+        let test_cases = [
+            ("!5", Some(Object::Boolean(false))),
+            ("!!true", Some(Object::Boolean(true))),
+            ("!!false", Some(Object::Boolean(false))),
+            ("!!5", Some(Object::Boolean(true))),
+            ("!true", Some(Object::Boolean(false))),
+            ("!false", Some(Object::Boolean(true))),
+        ];
+        for test in test_cases.iter() {
+            let lexer = Lexer::new(test.0);
+            let mut parser = Parser::new(lexer);
+            let program = parser.parse_program();
+            assert!(program.is_some());
+            let program = program.unwrap();
+            let evaluator = TreeWalker::new();
+            let eval = evaluator.eval(Node::Program(program));
+            assert_eq!(eval, test.1);
+        }
+    }
+
+    #[test]
+    fn eval_integer_literal_expression() {
+        let test_cases = [
+            ("5", Some(Object::Integer(5))),
+            ("10", Some(Object::Integer(10))),
+            ("-5", Some(Object::Integer(-5))),
+            ("-10", Some(Object::Integer(-10))),
+            ("5 + 5 + 5 + 5 - 10", Some(Object::Integer(10))),
+            ("2 * 2 * 2 * 2 * 2", Some(Object::Integer(32))),
+            ("-50 + 100 + -50", Some(Object::Integer(0))),
+            ("5 * 2 + 10 ", Some(Object::Integer(20))),
+            ("5 + 2 * 10", Some(Object::Integer(25))),
+            ("20 + 2 * -10", Some(Object::Integer(0))),
+            ("50  / 2 * 2 + 10", Some(Object::Integer(60))),
+            ("2 * (5 + 10)", Some(Object::Integer(30))),
+            ("3 * (3 * 3) + 10", Some(Object::Integer(37))),
+            ("(5 + 10 * 2 + 15 / 3) * 2 + -10", Some(Object::Integer(50))),
+        ];
+
+        for test in test_cases.iter() {
+            let lexer = Lexer::new(test.0);
+            let mut parser = Parser::new(lexer);
+            let program = parser.parse_program();
+            assert!(program.is_some());
+            let program = program.unwrap();
+            let evaluator = TreeWalker::new();
+            let eval = evaluator.eval(Node::Program(program));
+            assert_eq!(eval, test.1);
+        }
+    }
+}

src/evaluator/tree_walker/mod.rs

@@ -0,0 +1,102 @@
+use crate::{
+    evaluator::{Evaluator, Object, FALSE, NULL, TRUE},
+    lexer::TokenType,
+    parser::ast::{Expression, ExpressionStatement, Node, Statement},
+};
+
+pub struct TreeWalker;
+
+impl TreeWalker {
+    pub fn new() -> impl Evaluator {
+        Self
+    }
+}
+
+impl Evaluator for TreeWalker {
+    fn eval(&self, node: Node) -> Option<Object> {
+        match node {
+            Node::Program(p) => self.eval_statements(p.statements),
+            Node::Statement(stmt) => match stmt {
+                Statement::ExpressionStatement(ExpressionStatement { expression, .. }) => {
+                    self.eval(Node::Expression(expression))
+                }
+                _ => None,
+            },
+            Node::Expression(expr) => match expr {
+                Expression::IntegerLiteral(il) => Some(Object::Integer(il.value)),
+                Expression::BooleanExpression(b) => Some(Object::Boolean(b.value)),
+                Expression::PrefixExpression(p) => {
+                    let expr = self.eval(Node::Expression(*p.right))?;
+                    self.eval_prefix_expression(p.operator, expr)
+                }
+                Expression::InfixExpression(ie) => {
+                    let left = self.eval(Node::Expression(*ie.left))?;
+                    let right = self.eval(Node::Expression(*ie.right))?;
+
+                    self.eval_infix_expression(left, ie.operator, right)
+                }
+                _ => None,
+            },
+        }
+    }
+}
+
+impl TreeWalker {
+    fn eval_statements(&self, stmts: Vec<Statement>) -> Option<Object> {
+        let mut out: Option<Object> = None;
+        for stmt in stmts {
+            out = self.eval(Node::Statement(stmt))
+        }
+        out
+    }
+
+    fn eval_prefix_expression(&self, operator: TokenType, expr: Object) -> Option<Object> {
+        match operator {
+            TokenType::Bang => Some(self.eval_bang_operator_expression(expr)),
+            TokenType::Minus => Some(self.eval_minus_prefix_operator_expression(expr)),
+            _ => None,
+        }
+    }
+
+    fn eval_infix_expression(
+        &self,
+        left: Object,
+        operator: TokenType,
+        right: Object,
+    ) -> Option<Object> {
+        Some(match (left, right) {
+            (Object::Integer(l), Object::Integer(r)) => match operator {
+                TokenType::Plus => Object::Integer(l + r),
+                TokenType::Minus => Object::Integer(l - r),
+                TokenType::Asterisk => Object::Integer(l * r),
+                TokenType::Slash => Object::Integer(l / r),
+                TokenType::Equals => Object::Boolean(l == r),
+                TokenType::NotEquals => Object::Boolean(l != r),
+                TokenType::GreaterThan => Object::Boolean(l > r),
+                TokenType::LessThan => Object::Boolean(l < r),
+                _ => NULL,
+            },
+            _ => match operator {
+                TokenType::Equals => Object::Boolean(left == right),
+                TokenType::NotEquals => Object::Boolean(left != right),
+                _ => NULL,
+            },
+        })
+    }
+
+    fn eval_bang_operator_expression(&self, expr: Object) -> Object {
+        match expr {
+            TRUE => FALSE,
+            FALSE => TRUE,
+            NULL => TRUE,
+            _ => FALSE,
+        }
+    }
+
+    fn eval_minus_prefix_operator_expression(&self, expr: Object) -> Object {
+        match expr {
+            Object::Integer(v) => Object::Integer(-v),
+            _ => NULL,
+        }
+    }
+}

src/main.rs

@@ -1,6 +1,7 @@
 #[macro_use]
 extern crate lazy_static;
 
+mod evaluator;
 mod lexer;
 mod parser;
 mod repl;

src/parser/ast/mod.rs

@@ -16,6 +16,13 @@ pub struct Program {
     pub statements: Vec<Statement>,
 }
 
+#[derive(Debug)]
+pub enum Node {
+    Program(Program),
+    Statement(Statement),
+    Expression(Expression),
+}
+
 impl Display for Program {
     fn fmt(&self, f: &mut Formatter) -> FmtResult {
         let mut out = String::new();
@@ -143,7 +150,7 @@ impl Display for ReturnStatement {
 #[derive(Debug, PartialEq)]
 pub struct ExpressionStatement {
     token: Token,
-    expression: Expression,
+    pub expression: Expression,
 }
 
 impl ExpressionStatement {
@@ -287,7 +294,7 @@ impl Display for Identifier {
 
 #[derive(Debug, PartialEq)]
 pub struct IntegerLiteral {
-    value: i64,
+    pub value: i64,
 }
 
 impl IntegerLiteral {
@@ -310,8 +317,8 @@ impl IntegerLiteral {
 
 #[derive(Debug, PartialEq)]
 pub struct PrefixExpression {
-    operator: TokenType,
-    right: Box<Expression>,
+    pub operator: TokenType,
+    pub right: Box<Expression>,
 }
 
 impl PrefixExpression {
@@ -344,9 +351,9 @@ impl Display for PrefixExpression {
 
 #[derive(Debug, PartialEq)]
 pub struct InfixExpression {
-    left: Box<Expression>,
-    operator: TokenType,
-    right: Box<Expression>,
+    pub left: Box<Expression>,
+    pub operator: TokenType,
+    pub right: Box<Expression>,
 }
 
 impl InfixExpression {
@@ -381,7 +388,7 @@ impl Display for InfixExpression {
 #[derive(Debug, PartialEq)]
 pub struct BooleanExpression {
     token: TokenType,
-    value: bool,
+    pub value: bool,
 }
 
 impl BooleanExpression {

src/repl.rs

@@ -1,6 +1,7 @@
 use crate::{
+    evaluator::{tree_walker::TreeWalker, Evaluator},
     lexer::Lexer,
-    parser::{Error as ParserError, Parser},
+    parser::{ast::Node, Error as ParserError, Parser},
 };
 use std::io::{self, BufRead, Result as IoResult, Write};
 
@@ -29,11 +30,13 @@ fn start<R: BufRead, W: Write>(mut ip: R, mut out: W) {
             print_parser_errors(&mut out, &parser.errors).unwrap();
             continue;
         }
-        if let Some(program) = program {
-            for stmt in &program.statements {
-                out.write_fmt(format_args!("{}\n", stmt)).unwrap();
+        let program = program.unwrap();
+        let evaluator = TreeWalker::new();
+        let obj = evaluator.eval(Node::Program(program));
+        if let Some(node) = obj {
+            out.write_fmt(format_args!("{}\n", node.inspect())).unwrap();
+            out.flush().unwrap();
         }
-        };
     }
 }