TODO about using Results

src/lexer/mod.rs

@@ -78,6 +78,9 @@ impl Lexer {
 }
 
 impl Lexer {
+    // TODO: Use std::result::Result for these state_* methods.
+    // https://doc.rust-lang.org/1.14.0/core/result/enum.Result.html
+
     /// Handle self.state == State::Initial
     fn state_initial(&mut self, c: char, token: &mut Option<Token>) {
         if c.is_left_paren() {