feat: Add the produce parser

CHANGELOG.md

@@ -1,3 +1,9 @@
+<a name=""></a>
+##  (2019-12-24)
+
+
+
+
 <a name="4.0.0-beta.2"></a>
 ## 4.0.0-beta.2 (2019-12-19)
 

src/lib.rs

@@ -217,7 +217,9 @@ pub use crate::parser::{
         sep_end_by1, skip_count, skip_count_min_max, skip_many, skip_many1,
     },
     sequence::between,
-    token::{any, eof, none_of, one_of, position, satisfy, satisfy_map, token, tokens, value},
+    token::{
+        any, eof, none_of, one_of, position, produce, satisfy, satisfy_map, token, tokens, value,
+    },
 };
 
 #[doc(inline)]

src/parser/token.rs

@@ -611,6 +611,44 @@ where
     Value(v, PhantomData)
 }
 
+#[derive(Copy, Clone)]
+pub struct Produce<Input, F>(F, PhantomData<fn(Input) -> Input>);
+impl<Input, F, R> Parser<Input> for Produce<Input, F>
+where
+    Input: Stream,
+    F: FnMut() -> R,
+{
+    type Output = R;
+    type PartialState = ();
+    #[inline]
+    fn parse_lazy(&mut self, _: &mut Input) -> ParseResult<R, Input::Error> {
+        PeekOk((self.0)())
+    }
+}
+
+/// Always returns the value produced by calling `f`.
+///
+/// Can be used when `value` is unable to be used for lack of `Clone` implementation on the value.
+///
+/// ```
+/// # use combine::*;
+/// # fn main() {
+/// #[derive(Debug, PartialEq)]
+/// struct NoClone;
+/// let result = produce(|| vec![NoClone])
+///     .parse("hello world")
+///     .map(|x| x.0);
+/// assert_eq!(result, Ok(vec![NoClone]));
+/// # }
+/// ```
+pub fn produce<Input, F, R>(f: F) -> Produce<Input, F>
+where
+    Input: Stream,
+    F: FnMut() -> R,
+{
+    Produce(f, PhantomData)
+}
+
 #[derive(Copy, Clone)]
 pub struct Eof<Input>(PhantomData<Input>);
 impl<Input> Parser<Input> for Eof<Input>