add return statement and refactor let

src/ast.rs

@@ -4,13 +4,9 @@ use crate::token::Token;
 
 pub trait Node: Debug {}
 
-pub trait Statement: Node {
-    fn statement_node(&self);
-}
+pub trait Statement: Node {}
 
-pub trait Expression: Node {
-    fn expression_node(&self);
-}
+pub trait Expression: Node {}
 
 pub struct Program {
     pub statements: Vec<Rc<dyn Statement>>,
@@ -32,19 +28,31 @@ impl Debug for Program {
     }
 }
 
+// FIXME: rename ExampleStatement structs to Example.
+// we already know it's a statement since it impls
+// `Statement`, this isn't Java world.
+
 #[derive(Debug)]
-pub struct LetStatement {
+pub struct Let {
     pub token: Token,
     pub name: Identifier,
     pub value: Rc<dyn Expression>,
 }
 
-impl Node for LetStatement {}
+impl Node for Let {}
 
-impl Statement for LetStatement {
-    fn statement_node(&self) {}
+impl Statement for Let {}
+
+#[derive(Debug)]
+pub struct Return {
+    pub token: Token,
+    pub value: Rc<dyn Expression>,
 }
 
+impl Statement for Return {}
+
+impl Node for Return {}
+
 #[derive(Debug)]
 pub struct Identifier {
     pub token: Token,
@@ -52,17 +60,11 @@ pub struct Identifier {
 
 impl Node for Identifier {}
 
-impl Expression for Identifier {
-    fn expression_node(&self) {}
-}
+impl Expression for Identifier {}
 
 #[derive(Debug)]
 pub struct DummyExpression {}
 
 impl Node for DummyExpression {}
 
-impl Expression for DummyExpression {
-    fn expression_node(&self) {
-        panic!("this is dummy");
-    }
-}
+impl Expression for DummyExpression {}

src/parser.rs

@@ -1,7 +1,7 @@
 use std::{fmt, rc::Rc};
 
 use crate::{
-    ast::{DummyExpression, Expression, Identifier, LetStatement, Program, Statement},
+    ast::{DummyExpression, Expression, Identifier, Let, Program, Statement},
     lexer::Lexer,
     token::Token,
 };
@@ -102,7 +102,7 @@ impl Parser {
 
         let value = self.parse_expression()?;
 
-        Some(Rc::new(LetStatement { token, name, value }))
+        Some(Rc::new(Let { token, name, value }))
     }
 
     fn parse_expression(&mut self) -> Option<Rc<dyn Expression>> {
@@ -204,7 +204,7 @@ mod tests {
     fn test_let_statement(stmt: Rc<dyn Statement>, name: &str) {
         assert_eq!(
             format!("{stmt:?}"),
-            format!("LetStatement {{ token: Let, name: Identifier {{ token: Ident(\"{name}\") }}, value: DummyExpression }}"),
+            format!("Let {{ token: Let, name: Identifier {{ token: Ident(\"{name}\") }}, value: DummyExpression }}"),
         );
     }