diff --git a/src/func.rs b/src/func.rs
index ebf5d21..52d81f4 100644
--- a/src/func.rs
+++ b/src/func.rs
@@ -199,7 +199,8 @@ pub trait MonadFail<E>: Monad {
     /// Equivalent of Haskell's `fail`.
     fn fail<'a, A: 'a>(e: E) -> Self::F<'a, A>
     where
-        Self: 'a;
+        Self: 'a,
+        E: 'a;
 }
 
 /// Equivalent of Haskell's `Alternative`.
@@ -238,6 +239,14 @@ pub trait LocalFunctor: WeakFunctor {
 pub trait MonadFailAny {
     /// [`MonadFail`] for a specific error type.
     type W<E>: MonadFail<E>;
+
+    /// Equivalent of [`Result::map_err`].
+    fn map_err<'a, A: 'a, E0: 'a, E1: 'a>(
+        wa: <Self::W<E0> as WeakFunctor>::F<'a, A>,
+        f: impl 'a + FnOnce(E0) -> E1,
+    ) -> <Self::W<E1> as WeakFunctor>::F<'a, A>
+    where
+        Self: 'a;
 }
 
 /// Represents a (collection of) [Monad]\(s),
diff --git a/src/func/classes/composition.rs b/src/func/classes/composition.rs
index 7a7c5ca..fbcb62c 100644
--- a/src/func/classes/composition.rs
+++ b/src/func/classes/composition.rs
@@ -125,6 +125,7 @@ impl<E, U: Monad, V: MonadFail<E> + LocalFunctor> MonadFail<E> for CompositionCl
     fn fail<'a, A: 'a>(e: E) -> Self::F<'a, A>
     where
         Self: 'a,
+        E: 'a,
     {
         U::pure(V::fail(e))
     }
diff --git a/src/func/classes/result.rs b/src/func/classes/result.rs
index d4ad469..8d63020 100644
--- a/src/func/classes/result.rs
+++ b/src/func/classes/result.rs
@@ -163,6 +163,16 @@ pub struct ResultFailAny;
 
 impl MonadFailAny for ResultFailAny {
     type W<E> = ResultClass<E>;
+
+    fn map_err<'a, A: 'a, E0: 'a, E1: 'a>(
+        wa: <Self::W<E0> as WeakFunctor>::F<'a, A>,
+        f: impl 'a + FnOnce(E0) -> E1,
+    ) -> <Self::W<E1> as WeakFunctor>::F<'a, A>
+    where
+        Self: 'a,
+    {
+        wa.map_err(f)
+    }
 }
 
 impl MonadFailOver<classes::solo::SoloClass> for ResultFailAny {
@@ -191,6 +201,16 @@ pub struct ResultFailOver<T: Monad>(T);
 
 impl<T: Monad> MonadFailAny for ResultFailOver<T> {
     type W<E> = super::composition::CompositionClass<T, ResultClass<E>>;
+
+    fn map_err<'a, A: 'a, E0: 'a, E1: 'a>(
+        wa: <Self::W<E0> as WeakFunctor>::F<'a, A>,
+        f: impl 'a + FnOnce(E0) -> E1,
+    ) -> <Self::W<E1> as WeakFunctor>::F<'a, A>
+    where
+        Self: 'a,
+    {
+        T::fmap(|a| a.map_err(f), wa)
+    }
 }
 
 impl<T: Monad> MonadFailOver<T> for ResultFailOver<T> {