fix: nil and () are identical

src/evaluator/mod.rs

@@ -39,7 +39,7 @@ impl std::fmt::Display for Expression {
             Expression::Boolean(false) => write!(f, "false"),
             Expression::Keyword(val) => write!(f, ":{}", val),
             Expression::String(val) => write!(f, "{}", val),
-            Expression::Nil => write!(f, "nil"),
+            Expression::Nil => write!(f, "()"),
 
             Expression::Vector(vec) => {
                 let s = vec
@@ -93,6 +93,12 @@ pub fn eval(env: Rc<Environment>, ast: Vec<Node>) -> Result<Vec<Rc<Expression>>>
 fn eval_ast_node(env: Rc<Environment>, ast_node: Node) -> Result<Rc<Expression>> {
     let expr = match ast_node {
         Node::List(list) => {
+            // Empty lists are nil in scheme
+            if list.is_empty() {
+                return Ok(NIL.with(|v| v.clone()));
+            }
+
+            // We always assume lists evaluate
             let mut list = list.into_iter();
             let operator = eval_ast_node(env.clone(), list.next().ok_or(Error::InvalidOperator)?)?;
 
@@ -135,10 +141,13 @@ mod test {
     use rstest::rstest;
 
     #[rstest]
-    // Basic test  of raw values
+    // Raw values
     #[case("1", "1")]
     #[case("\"uwu\"", "uwu")]
     #[case(":owo", ":owo")]
+    #[case("()", "()")]
+    #[case("nil", "()")]
+    // Arithmetic
     #[case("(+ 1 2)", "3")]
     #[case("(- 5 1)", "4")]
     #[case("(* 8 9)", "72")]
@@ -161,7 +170,7 @@ mod test {
     // Environment manipulation
     #[case("(define! asdf (+ 2 2)) (+ asdf 2)", "4\n6")]
     #[case("(let* (a 2) (+ a 2))", "4")]
-    // If-else
+    // Branching
     #[case("(if true true false)", "true")]
     #[case("(if nil true false)", "true")]
     #[case("(if false true false)", "false")]