Predicate

src/lib.rs

@@ -59,6 +59,22 @@ where
     }
 }
 
+fn pred<P, F, I, O>(parser: P, pred_fn: F) -> impl Parser<I, O, I>
+where
+    P: Parser<I, O, I>,
+    F: Fn(&O) -> bool,
+{
+    move |input| {
+        parser.parse(input).and_then(|(result, rest)| {
+            if pred_fn(&result) {
+                Ok((result, rest))
+            } else {
+                Err(rest)
+            }
+        })
+    }
+}
+
 fn zero_or_more<P, I, O>(parser: P) -> impl Parser<I, Vec<O>, I>
 where
     P: Parser<I, O, I>,
@@ -114,6 +130,13 @@ fn identifier(input: &str) -> ParseResult<&str, String, &str> {
     Ok((buf, &input[next_index..]))
 }
 
+fn any_char(input: &str) -> ParseResult<&str, char, &str> {
+    match input.chars().next() {
+        Some(ch) => Ok((ch, &input[ch.len_utf8()..])),
+        None => Err(input),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -152,4 +175,10 @@ mod tests {
         let input = "bongo ecks";
         assert_matches!(p.parse(input), Ok((v, "ecks")) if v.len() == 1);
     }
+
+    #[test]
+    fn test_pred() {
+        let p = pred(any_char, |c| *c == 'f');
+        assert_eq!(p.parse("frog"), Ok(('f', "rog")));
+    }
 }