Allow empty returns

examples/hello_world.sb

@@ -1,6 +1,6 @@
 fn main() {
     let x = "Hello World";
-    return x;
+    return;
 }
 
 fn fib() {}

src/parser/mod.rs

@@ -169,7 +169,7 @@ impl Parser {
                 state
             }
             TokenKind::Keyword(Keyword::Return) => {
-                let state = Statement::Return(self.parse_expression()?);
+                let state = self.parse_return()?;
                 self.match_token(TokenKind::SemiColon)?;
 
                 Ok(state)
@@ -178,6 +178,15 @@ impl Parser {
         }
     }
 
+    fn parse_return(&mut self) -> Result<Statement, String> {
+        // TODO: Replace unwrap with make_error
+        let peeked = self.peek().unwrap();
+        match peeked.kind {
+            TokenKind::SemiColon => Ok(Statement::Return(None)),
+            _ => Ok(Statement::Return(Some(self.parse_expression()?))),
+        }
+    }
+
     fn parse_expression(&mut self) -> Result<Expression, String> {
         let token = self.next_token();
         match token.kind {

src/parser/node_type.rs

@@ -19,7 +19,7 @@ pub struct Variable {
 #[derive(Debug, Eq, PartialEq)]
 pub enum Statement {
     Declare(Variable, Option<Expression>),
-    Return(Expression),
+    Return(Option<Expression>),
     If(Expression, Box<Statement>, Option<Box<Statement>>),
     While(Expression, Box<Statement>),
     Exp(Expression),