diff --git a/CHANGELOG.md b/CHANGELOG.md
index 91225eba9..217cbf6c7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]:
 - Added `ToMut` trait, which allows borrowing mutably from a Coproduct or HList.
+- Added `FuncMut` and `PolyMut`, which parallel `Func` and `Poly`.
 
 ## [0.2.2] - 2018-10-21
 - Added support for [transmogrifying (recursively sculpting)](https://docs.rs/frunk/0.2.2/frunk/labelled/trait.Transmogrifier.html) one data type into another
diff --git a/core/src/coproduct.rs b/core/src/coproduct.rs
index 3cb6033b0..71f86c534 100644
--- a/core/src/coproduct.rs
+++ b/core/src/coproduct.rs
@@ -73,7 +73,7 @@
 
 use hlist::{HCons, HNil};
 use indices::{Here, There};
-use traits::{Func, Poly, ToMut, ToRef};
+use traits::{Func, FuncMut, Poly, PolyMut, ToMut, ToRef};
 
 /// Enum type representing a Coproduct. Think of this as a Result, but capable
 /// of supporting any arbitrary number of types instead of just 2.
@@ -506,9 +506,10 @@ impl<Head, Tail> Coproduct<Head, Tail> {
     ///
     /// * An `hlist![]` of closures (one for each type, in order).
     /// * A single closure (for a Coproduct that is homogenous).
-    /// * A single [`Poly`].
+    /// * A single [`Poly`] or [`PolyMut`].
     ///
     /// [`Poly`]: ../traits/struct.Poly.html
+    /// [`PolyMut`]: ../traits/struct.PolyMut.html
     ///
     /// # Example
     ///
@@ -745,6 +746,20 @@ where
     }
 }
 
+impl<P, R, CH, CTail> CoproductFoldable<PolyMut<P>, R> for Coproduct<CH, CTail>
+where
+    P: FuncMut<CH, Output = R>,
+    CTail: CoproductFoldable<PolyMut<P>, R>,
+{
+    fn fold(self, mut f: PolyMut<P>) -> R {
+        use self::Coproduct::*;
+        match self {
+            Inl(r) => f.0.call(r),
+            Inr(rest) => rest.fold(f),
+        }
+    }
+}
+
 impl<F, R, FTail, CH, CTail> CoproductFoldable<HCons<F, FTail>, R> for Coproduct<CH, CTail>
 where
     F: FnOnce(CH) -> R,
diff --git a/core/src/hlist.rs b/core/src/hlist.rs
index 7928eadb6..98f31a99f 100644
--- a/core/src/hlist.rs
+++ b/core/src/hlist.rs
@@ -57,7 +57,7 @@
 //! ```
 
 use indices::{Here, Suffixed, There};
-use traits::{Func, IntoReverse, Poly, ToMut, ToRef};
+use traits::{Func, FuncMut, IntoReverse, Poly, PolyMut, ToMut, ToRef};
 
 use std::ops::Add;
 
@@ -352,9 +352,10 @@ macro_rules! gen_inherent_methods {
             ///
             /// * An `hlist![]` of closures (one for each element).
             /// * A single closure (for mapping an HList that is homogenous).
-            /// * A single [`Poly`].
+            /// * A single [`Poly`] or [`PolyMut`].
             ///
             /// [`Poly`]: ../traits/struct.Poly.html
+            /// [`PolyMut`]: ../traits/struct.PolyMut.html
             ///
             /// # Examples
             ///
@@ -908,6 +909,20 @@ where
     }
 }
 
+impl<P, H, Tail> HMappable<PolyMut<P>> for HCons<H, Tail>
+where
+    P: FuncMut<H>,
+    Tail: HMappable<PolyMut<P>>,
+{
+    type Output = HCons<<P as FuncMut<H>>::Output, <Tail as HMappable<PolyMut<P>>>::Output>;
+    fn map(self, mut poly: PolyMut<P>) -> Self::Output {
+        HCons {
+            head: poly.0.call(self.head),
+            tail: self.tail.map(poly),
+        }
+    }
+}
+
 /// Trait for mapping over an HList
 ///
 /// This trait is part of the implementation of the inherent method
@@ -943,12 +958,12 @@ impl<F> HMappable<F> for HNil {
 
 impl<F, R, H, Tail> HMappable<F> for HCons<H, Tail>
 where
-    F: Fn(H) -> R,
+    F: FnMut(H) -> R,
     Tail: HMappable<F>,
 {
     type Output = HCons<R, <Tail as HMappable<F>>::Output>;
 
-    fn map(self, f: F) -> Self::Output {
+    fn map(self, mut f: F) -> Self::Output {
         let HCons { head, tail } = self;
         HCons {
             head: f(head),
@@ -1143,11 +1158,11 @@ where
 impl<F, H, Tail, Acc> HFoldLeftable<F, Acc> for HCons<H, Tail>
 where
     Tail: HFoldLeftable<F, Acc>,
-    F: Fn(Acc, H) -> Acc,
+    F: FnMut(Acc, H) -> Acc,
 {
     type Output = <Tail as HFoldLeftable<F, Acc>>::Output;
 
-    fn foldl(self, f: F, acc: Acc) -> Self::Output {
+    fn foldl(self, mut f: F, acc: Acc) -> Self::Output {
         let HCons { head, tail } = self;
         let acc = f(acc, head);
         tail.foldl(f, acc)
diff --git a/core/src/traits.rs b/core/src/traits.rs
index 19f0ed528..923898b4a 100644
--- a/core/src/traits.rs
+++ b/core/src/traits.rs
@@ -58,6 +58,13 @@ pub trait IntoReverse {
 #[derive(Debug, Copy, Clone, Default)]
 pub struct Poly<T>(pub T);
 
+/// A wrapper like [`Poly`] for [`FuncMut`].
+///
+/// [`FuncMut`]: trait.FuncMut.html
+/// [`Poly`]: trait.Poly.html
+#[derive(Debug, Copy, Clone, Default)]
+pub struct PolyMut<T>(pub T);
+
 /// This is a simple, user-implementable alternative to `Fn`.
 ///
 /// Might not be necessary if/when Fn(Once, Mut) traits are implementable
@@ -74,3 +81,45 @@ pub trait Func<Input> {
     /// use cases.
     fn call(i: Input) -> Self::Output;
 }
+
+/// This is a user-implementable alternative to `FnMut`.
+///
+/// Not necessary if/when the FnMut trait is implementable in stable Rust.
+///
+/// ```
+/// # #[macro_use]
+/// # extern crate frunk;
+/// # use frunk::{FuncMut, PolyMut};
+/// # use std::fmt::{Debug, Write};
+///
+/// struct ConcatDebug<'a>(&'a mut String);
+///
+/// impl<'a, T: Debug> FuncMut<T> for ConcatDebug<'a> {
+///     type Output = ();
+///     fn call(&mut self, t: T) {
+///         write!(self.0, "{:?}", t).unwrap();
+///     }
+/// }
+///
+/// fn main() {
+///     let mut string = String::new();
+///
+///     {
+///         let mut concatter = ConcatDebug(&mut string);
+///         let l = hlist![
+///             23,
+///             "foo",
+///             (1, "two", 3.0)
+///         ];
+///         l.map(PolyMut(concatter));
+///     }
+///
+///     assert_eq!(string, r#"23"foo"(1, "two", 3.0)"#);
+/// }
+/// ```
+pub trait FuncMut<Input> {
+    type Output;
+
+    /// Call the `FuncMut`.
+    fn call(&mut self, i: Input) -> Self::Output;
+}
diff --git a/src/lib.rs b/src/lib.rs
index b423a9643..deb89e192 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -238,9 +238,9 @@ pub use hlist::HCons;
 #[doc(no_inline)]
 pub use hlist::HNil;
 #[doc(no_inline)]
-pub use traits::Func;
+pub use traits::{Func, FuncMut};
 #[doc(no_inline)]
-pub use traits::Poly;
+pub use traits::{Poly, PolyMut};
 #[doc(no_inline)]
 pub use traits::{ToMut, ToRef}; // useful for where bounds