Implement primitive procedure application

src/parser.rs

@@ -1,8 +1,6 @@
 use crate::lexer::Token;
 use crate::lexer::Tokens;
 
-
-
 #[derive(Debug)]
 pub enum Datum {
     Boolean(bool),
@@ -10,6 +8,13 @@ pub enum Datum {
     Symbol(String),
     List(Vec<Datum>),
     Procedure(fn(Vec<Datum>) -> Datum),
+    None,
+}
+
+impl Default for Datum {
+    fn default() -> Self {
+        Datum::None
+    }
 }
 
 pub fn parse(tokens: Tokens) -> Datum {
@@ -23,14 +28,14 @@ fn parse_datum(tokens: &Tokens, ix: usize) -> (Datum, usize) {
         Token::Boolean(b) => (Datum::Boolean(*b), ix + 1),
         Token::Number(n) => (Datum::Number(*n), ix + 1),
         Token::LeftRoundBracket => parse_list(tokens, ix + 1),
-        _ => panic!("Unexpected token {:?}", tokens[ix])
+        _ => panic!("Unexpected token {:?}", tokens[ix]),
     }
 }
 
 pub fn parse_list(tokens: &Tokens, mut ix: usize) -> (Datum, usize) {
     let mut datums = vec![];
 
-    // FIXME: will crash when RightRoundBracket is missing 
+    // FIXME: will crash when RightRoundBracket is missing
     while tokens[ix] != Token::RightRoundBracket {
         let (datum, new_ix) = parse_datum(tokens, ix);
         datums.push(datum);