optional combinator

src/combinators.rs

@@ -1,5 +1,16 @@
 use crate::parser::Parser;
 
+pub fn optional<P, I, O, E>(parser: P) -> impl Parser<I, Option<O>, E>
+where
+    P: Parser<I, O, E>,
+    I: Clone,
+{
+    move |input: I| match parser.parse(input.clone()) {
+        Ok((output, rest)) => Ok((Some(output), rest)),
+        Err(_e) => Ok((None, input)),
+    }
+}
+
 pub fn map<P, F, I, O1, O2, E>(parser: P, map_fn: F) -> impl Parser<I, O2, E>
 where
     P: Parser<I, O1, E>,

src/lib.rs

@@ -12,7 +12,7 @@ pub use parser::{ParseResult, Parser};
 mod tests {
     use super::*;
     use crate::choice::choice;
-    use crate::primitives::{any_char, literal, literal_char, pred};
+    use crate::primitives::{any_char, literal, literal_char, one_of, pred};
     use crate::sequence::seq;
     use std::collections::HashMap;
 
@@ -48,6 +48,8 @@ mod tests {
             literal("false").to(JsonValue::Bool(false)),
         ));
 
+        let digit = one_of("1234567890");
+
         let json_string = seq((
             literal_char('"'),
             pred(any_char, |ch| *ch != '"'),

src/parser.rs

@@ -43,6 +43,16 @@ pub trait Parser<I, O, E> {
     {
         BoxedParser::new(crate::sequence::tuple2(self, next_parser))
     }
+
+    fn optional<'a>(self) -> BoxedParser<'a, I, Option<O>, E>
+    where
+        I: Clone + 'a,
+        O: 'a,
+        E: 'a,
+        Self: Sized + 'a,
+    {
+        BoxedParser::new(crate::combinators::optional(self))
+    }
 }
 
 pub struct BoxedParser<'a, I, O, E> {