diff --git a/src/combinator/multi.rs b/src/combinator/multi.rs
index d0636287..0c686942 100644
--- a/src/combinator/multi.rs
+++ b/src/combinator/multi.rs
@@ -842,14 +842,14 @@ where
 pub fn separated_foldl1<I, O, O2, E, P, S, Op>(
     mut parser: P,
     mut sep: S,
-    op: Op,
+    mut op: Op,
 ) -> impl Parser<I, O, E>
 where
     I: Stream,
     P: Parser<I, O, E>,
     S: Parser<I, O2, E>,
     E: ParserError<I>,
-    Op: Fn(O, O2, O) -> O,
+    Op: FnMut(O, O2, O) -> O,
 {
     trace("separated_foldl1", move |i: &mut I| {
         let mut ol = parser.parse_next(i)?;
@@ -911,14 +911,14 @@ where
 pub fn separated_foldr1<I, O, O2, E, P, S, Op>(
     mut parser: P,
     mut sep: S,
-    op: Op,
+    mut op: Op,
 ) -> impl Parser<I, O, E>
 where
     I: Stream,
     P: Parser<I, O, E>,
     S: Parser<I, O2, E>,
     E: ParserError<I>,
-    Op: Fn(O, O2, O) -> O,
+    Op: FnMut(O, O2, O) -> O,
 {
     trace("separated_foldr1", move |i: &mut I| {
         let ol = parser.parse_next(i)?;
diff --git a/src/combinator/parser.rs b/src/combinator/parser.rs
index c95f1f6f..55f8b99d 100644
--- a/src/combinator/parser.rs
+++ b/src/combinator/parser.rs
@@ -35,7 +35,7 @@ where
 pub struct Map<F, G, I, O, O2, E>
 where
     F: Parser<I, O, E>,
-    G: Fn(O) -> O2,
+    G: FnMut(O) -> O2,
 {
     parser: F,
     map: G,
@@ -48,7 +48,7 @@ where
 impl<F, G, I, O, O2, E> Map<F, G, I, O, O2, E>
 where
     F: Parser<I, O, E>,
-    G: Fn(O) -> O2,
+    G: FnMut(O) -> O2,
 {
     #[inline(always)]
     pub(crate) fn new(parser: F, map: G) -> Self {
@@ -66,7 +66,7 @@ where
 impl<F, G, I, O, O2, E> Parser<I, O2, E> for Map<F, G, I, O, O2, E>
 where
     F: Parser<I, O, E>,
-    G: Fn(O) -> O2,
+    G: FnMut(O) -> O2,
 {
     #[inline]
     fn parse_next(&mut self, i: &mut I) -> PResult<O2, E> {
@@ -393,7 +393,7 @@ where
 pub struct Verify<F, G, I, O, O2, E>
 where
     F: Parser<I, O, E>,
-    G: Fn(&O2) -> bool,
+    G: FnMut(&O2) -> bool,
     I: Stream,
     O: Borrow<O2>,
     O2: ?Sized,
@@ -410,7 +410,7 @@ where
 impl<F, G, I, O, O2, E> Verify<F, G, I, O, O2, E>
 where
     F: Parser<I, O, E>,
-    G: Fn(&O2) -> bool,
+    G: FnMut(&O2) -> bool,
     I: Stream,
     O: Borrow<O2>,
     O2: ?Sized,
@@ -432,7 +432,7 @@ where
 impl<F, G, I, O, O2, E> Parser<I, O, E> for Verify<F, G, I, O, O2, E>
 where
     F: Parser<I, O, E>,
-    G: Fn(&O2) -> bool,
+    G: FnMut(&O2) -> bool,
     I: Stream,
     O: Borrow<O2>,
     O2: ?Sized,
diff --git a/src/parser.rs b/src/parser.rs
index 8a1040dc..48e8be0c 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -382,7 +382,7 @@ pub trait Parser<I, O, E> {
     #[inline(always)]
     fn map<G, O2>(self, map: G) -> Map<Self, G, I, O, O2, E>
     where
-        G: Fn(O) -> O2,
+        G: FnMut(O) -> O2,
         Self: core::marker::Sized,
     {
         Map::new(self, map)
@@ -581,7 +581,7 @@ pub trait Parser<I, O, E> {
     fn verify<G, O2>(self, filter: G) -> Verify<Self, G, I, O, O2, E>
     where
         Self: core::marker::Sized,
-        G: Fn(&O2) -> bool,
+        G: FnMut(&O2) -> bool,
         I: Stream,
         O: crate::lib::std::borrow::Borrow<O2>,
         O2: ?Sized,