feat: add optional modifier.

src/parser.rs

@@ -3,7 +3,7 @@ mod primitive_parsers;
 
 use crate::parser::modified_parsers::{
     BindParser, ConvertParser, LitervalCountParser, LitervalParser, LookAheadParser, MapParser,
-    NextParser, ReverseParser, TryParser,
+    NextParser, OptionalParser, ReverseParser, TryParser,
 };
 use crate::parser::primitive_parsers::{
     AnyParser, FailParser, FailWithMessageParser, SatifiedMapParser, SatisfyParser, SkipParser,
@@ -200,6 +200,13 @@ where
             phantom_data: PhantomData,
         }
     }
+
+    fn optional(self) -> OptionalParser<Self>
+    where
+        Self: Sized,
+    {
+        OptionalParser { parser: self }
+    }
 }
 
 impl<'a, TToken, T: 'a, TContext, F> Parser<'a, TToken, T, TContext> for F

src/parser/modified_parsers.rs

@@ -240,6 +240,27 @@ where
     }
 }
 
+pub struct OptionalParser<P> {
+    pub(crate) parser: P,
+}
+
+impl<'a, TToken, T: 'a, TContext, P> Parser<'a, TToken, Option<T>, TContext> for OptionalParser<P>
+where
+    TToken: Debug + Clone,
+    P: Parser<'a, TToken, T, TContext>,
+{
+    fn parse(
+        &self,
+        context: Rc<RefCell<ParserContext<TToken, TContext>>>,
+        input: &'a [TToken],
+    ) -> ParserResult<'a, TToken, Option<T>> {
+        match self.parser.parse(context, input) {
+            Ok(r) => Ok((r.0, Some(r.1))),
+            Err(r) => Ok((r.input, None)),
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;
@@ -414,4 +435,16 @@ mod test {
             char_parser('a').reverse(()).parse(c, i)
         });
     }
+
+    #[test]
+    fn optional_test() {
+        parser_test_helper(
+            ParserContext::new_with_str("abc", ()),
+            &Some('a'),
+            |c, i| char_parser('a').optional().parse(c, i),
+        );
+        parser_test_helper(ParserContext::new_with_str("bc", ()), &None, |c, i| {
+            char_parser('a').optional().parse(c, i)
+        });
+    }
 }