choice combinator

src/lib.rs

@@ -19,6 +19,17 @@ trait Parser<I, O, E> {
         BoxedParser::new(map(self, map_fn))
     }
 
+    fn to<'a, O2>(self, item: O2) -> BoxedParser<'a, I, O2, E>
+    where
+        Self: Sized + 'a,
+        I: 'a,
+        O: 'a,
+        O2: Clone + 'a,
+        E: 'a,
+    {
+        self.map(move |_| item.clone())
+    }
+
     fn then<'a, P, O2>(self, next_parser: P) -> BoxedParser<'a, I, (O, O2), E>
     where
         Self: Sized + 'a,
@@ -182,6 +193,18 @@ fn any_char(input: &str) -> ParseResult<&str, char, &str> {
     }
 }
 
+fn choice<P1, P2, I, O, E>(parser1: P1, parser2: P2) -> impl Parser<I, O, E>
+where
+    P1: Parser<I, O, E>,
+    P2: Parser<I, O, E>,
+    I: Copy,
+{
+    move |input| match parser1.parse(input) {
+        ok @ Ok(..) => ok,
+        Err(_e) => parser2.parse(input),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -229,4 +252,10 @@ mod tests {
         let p = pred(any_char, |c| *c == 'f');
         assert_eq!(p.parse("frog"), Ok(('f', "rog")));
     }
+
+    #[test]
+    fn test_choice() {
+        let p = choice(literal("gnostika").to(1), one_or_more(literal(" ")).to(2));
+        assert_eq!(p.parse("gnostika twentynine"), Ok((1, " twentynine")));
+    }
 }