impl Opt combinator

1 files changed, 30 insertions, 0 deletions

diff --git a/src/lib.rs b/src/lib.rs
index e0be671..2878185 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -770,6 +770,36 @@ where
     Not { parser }
 }
 
+struct Opt<P> {
+    parser: P,
+}
+
+impl<I, P> Parser<I> for Opt<P>
+where
+    I: Input,
+    P: Parser<I>,
+{
+    type Output = Option<P::Output>;
+
+    fn run<OM: Mode, EM: Mode, Tracer: Trace<I>>(
+        &mut self,
+        it: InputIter<I>,
+    ) -> ParserResult<I, Self::Output, OM, EM> {
+        match self.parser.run::<OM, EM, Tracer>(it.clone()) {
+            Ok((rest, output)) => Ok((rest, OM::map(output, |o| Some(o)))),
+            Err(_) => Ok((it, OM::bind(|| None))),
+        }
+    }
+}
+
+pub fn opt<I, P>(parser: P) -> impl Parser<I, Output = Option<P::Output>>
+where
+    I: Input,
+    P: Parser<I>,
+{
+    Opt { parser }
+}
+
 pub fn alpha<I>() -> impl Parser<I, Output = I>
 where
     I: Input,