refactor: Nil, more helpful help

Cargo.lock

@@ -3,5 +3,5 @@
 version = 3
 
 [[package]]
-name = "vy"
+name = "vyc"
 version = "0.1.0"

Cargo.toml

@@ -1,5 +1,5 @@
 [package]
-name = "vy"
+name = "vyc"
 version = "0.1.0"
 edition = "2021"
 

src/main.rs

@@ -3,13 +3,24 @@ use std::{process::exit, fs::read_to_string};
 pub mod parser;
 
 const EXECUTABLE_NAME: &str = env!("CARGO_PKG_NAME");
+const HELP_MESSAGE: &str = "\
+    -h, --help
+        Print this help message and exit.
+    -v, --version
+        Print version information and exit.
+    -c FILE, --compile FILE
+        Compile the given file and exit.\
+";
 
 fn main() {
     let args = std::env::args().collect::<Vec<String>>();
     let mut args_index: usize = 0;
     match args.len() {
         // No argument provided
-        1 => { display_help(1); },
+        1 => {
+            println!("No argument provided.");
+            display_help(1);
+        },
         _ => {
             while args.len() > args_index {
                 let arg: &str = &args[args_index];
@@ -29,6 +40,7 @@ fn main() {
                                 println!("{:?}", node);
                             }
                         } else {
+                            println!("No file provided.");
                             display_help(1);
                         }
                     }
@@ -40,6 +52,7 @@ fn main() {
 }
 
 fn display_help(exit_code: i32) {
-    println!("Usage: {} <file>", EXECUTABLE_NAME);
+    println!("Usage: {} [OPTIONS]", EXECUTABLE_NAME);
+    println!("{}", HELP_MESSAGE);
     exit(exit_code);
 }

src/parser.rs

@@ -10,6 +10,10 @@ pub struct List {
     pub value: Option<Rc<RefCell<Cons>>>,
 }
 
+impl List {
+    pub const NIL: List = List { value: None };
+}
+
 /// Cons -> (car, [cdr])
 #[derive(Debug, Clone)]
 pub struct Cons {
@@ -26,10 +30,11 @@ pub enum Value {
     Int(i64), Float(f64),
 
     String(String), Symbol(String),    
-    
     List(List),
+}
 
-    Nil,
+impl Value {
+    pub const NIL: Value = Value::List(List::NIL);
 }
 
 #[derive(Debug, Clone)]
@@ -261,7 +266,7 @@ fn read<'a>(
                         if let Tree::List { vec, quote } = &finished {
                             if vec.is_empty() {
                                 finished = Tree::Atom {
-                                    atom: Value::Nil,
+                                    atom: Value::NIL,
                                     quote: *quote,
                                 };
                             }
@@ -300,7 +305,7 @@ fn read_atom(token: &str) -> Value {
     match lower.as_str() {
         "true" => Value::True,
         "false" => Value::False,
-        "nil" => Value::Nil,
+        "nil" => Value::NIL,
         _ => {
             // Parse number
             if let Ok(int) = token.parse::<i64>() { Value::Int(int) }