impl Verify combinator

1 files changed, 37 insertions, 0 deletions

diff --git a/src/lib.rs b/src/lib.rs
index 6f16148..686ce6f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -170,6 +170,16 @@ pub trait Parser<I: Input>: Sized {
             delimiter,
         }
     }
+
+    fn verify<C>(self, checker: C) -> impl Parser<I, Output = Self::Output>
+    where
+        C: Parser<I>,
+    {
+        Verify {
+            parser: self,
+            checker,
+        }
+    }
 }
 
 impl<I, O, F> Parser<I> for F
@@ -812,6 +822,33 @@ where
     Opt { parser }
 }
 
+struct Verify<P, C> {
+    parser: P,
+    checker: C,
+}
+
+impl<I, P, C> Parser<I> for Verify<P, C>
+where
+    I: Input,
+    P: Parser<I>,
+    C: Parser<I>,
+{
+    type Output = 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::<Emit, Emit, Tracer>(it.clone()) {
+            Ok((rest, output)) if self.checker.check(it.clone()).is_ok() => {
+                Ok((rest, OM::bind(|| output)))
+            }
+            Err(rest) => Err(EM::bind(|| rest)),
+            _ => Err(EM::bind(|| it)),
+        }
+    }
+}
+
 pub fn alpha<I>() -> impl Parser<I, Output = I>
 where
     I: Input,