feat: display function

src/evaluator/env.rs

@@ -253,6 +253,19 @@ pub fn core_environment() -> Rc<Environment> {
                 }))
             }),
         ),
+        // IO
+        (
+            "display",
+            Expression::NativeFunc(|args| {
+                if args.len() != 1 {
+                    Err(Error::MismatchedArgCount(1, args.len()))?
+                }
+
+                print!("{}", args[0]);
+
+                Ok(Expression::Void.into())
+            }),
+        ),
     ]
     .into_iter()
     .map(|(k, v)| (k.to_string(), Rc::new(v)));

src/evaluator/mod.rs

@@ -16,6 +16,7 @@ pub enum Expression {
     Keyword(String),
     String(String),
     Nil,
+    Void,
 
     // Collections
     Vector(Vec<Rc<Expression>>),
@@ -39,6 +40,7 @@ impl std::fmt::Display for Expression {
             Expression::Keyword(val) => write!(f, ":{}", val),
             Expression::String(val) => write!(f, "{}", val),
             Expression::Nil => write!(f, "()"),
+            Expression::Void => write!(f, ""),
 
             Expression::Vector(vec) => {
                 let s = vec