diff --git a/src/func/classes.rs b/src/func/classes.rs
index c81f35b..14678ff 100644
--- a/src/func/classes.rs
+++ b/src/func/classes.rs
@@ -20,6 +20,7 @@
 use crate::func::*;
 
 pub mod composition;
+pub mod effect;
 pub mod future;
 pub mod lazy;
 pub mod option;
diff --git a/src/func/classes/effect.rs b/src/func/classes/effect.rs
new file mode 100644
index 0000000..760a431
--- /dev/null
+++ b/src/func/classes/effect.rs
@@ -0,0 +1,185 @@
+use crate::func::*;
+
+pub trait Effect {
+    fn pure() -> Self;
+
+    fn seq(lhs: Self, rhs: Self) -> Self;
+
+    fn after(self, effect: Self) -> Self;
+}
+
+#[derive(Clone)]
+pub struct WithEffect<A, E> {
+    value: A,
+    effect: E,
+}
+
+impl<A, E: Effect> WithEffect<A, E> {
+    fn after(self, effect: E) -> Self {
+        WithEffect {
+            value: self.value,
+            effect: self.effect.after(effect),
+        }
+    }
+}
+
+#[derive(SharedFunctor, CovariantFunctor)]
+pub struct EffectClass<E>(E);
+
+impl<E> WeakFunctor for EffectClass<E> {
+    type F<'a, A: 'a> = WithEffect<A, E>
+    where
+        Self: 'a;
+}
+
+impl<E> Functor for EffectClass<E> {
+    fn fmap<'a, A: 'a, B: 'a>(f: impl 'a + FnOnce(A) -> B, fa: Self::F<'a, A>) -> Self::F<'a, B>
+    where
+        Self: 'a,
+    {
+        WithEffect {
+            value: f(fa.value),
+            effect: fa.effect,
+        }
+    }
+
+    fn replace<'a, A: 'a, B: 'a>(fa: Self::F<'a, A>, b: B) -> Self::F<'a, B>
+    where
+        Self: 'a,
+    {
+        drop(fa.value);
+        WithEffect {
+            value: b,
+            effect: fa.effect,
+        }
+    }
+}
+
+impl<E: Effect> Pure for EffectClass<E> {
+    fn pure<'a, A: 'a>(a: A) -> Self::F<'a, A>
+    where
+        Self: 'a,
+    {
+        WithEffect {
+            value: a,
+            effect: E::pure(),
+        }
+    }
+}
+
+impl<E: Effect> ApplicativeSeq for EffectClass<E> {
+    fn seq<'a, A: 'a, B: 'a>(
+        ff: Self::F<'a, impl 'a + FnOnce(A) -> B>,
+        fa: Self::F<'a, A>,
+    ) -> Self::F<'a, B>
+    where
+        Self: 'a,
+    {
+        WithEffect {
+            value: (ff.value)(fa.value),
+            effect: E::seq(ff.effect, fa.effect),
+        }
+    }
+}
+
+impl<E: Effect> ApplicativeLA2 for EffectClass<E> {
+    fn la2<'a, A: 'a, B: 'a, C: 'a>(
+        f: impl 'a + FnOnce(A, B) -> C,
+        fa: Self::F<'a, A>,
+        fb: Self::F<'a, B>,
+    ) -> Self::F<'a, C>
+    where
+        Self: 'a,
+    {
+        WithEffect {
+            value: f(fa.value, fb.value),
+            effect: E::seq(fa.effect, fb.effect),
+        }
+    }
+}
+
+impl<E: Effect> ApplicativeTuple for EffectClass<E> {
+    fn tuple<'a, A: 'a, B: 'a>((fa, fb): (Self::F<'a, A>, Self::F<'a, B>)) -> Self::F<'a, (A, B)>
+    where
+        Self: 'a,
+    {
+        WithEffect {
+            value: (fa.value, fb.value),
+            effect: E::seq(fa.effect, fb.effect),
+        }
+    }
+}
+
+impl<E: Effect> Applicative for EffectClass<E> {
+    fn discard_first<'a, A: 'a, B: 'a>(fa: Self::F<'a, A>, fb: Self::F<'a, B>) -> Self::F<'a, B>
+    where
+        Self: 'a,
+    {
+        drop(fa.value);
+        WithEffect {
+            value: fb.value,
+            effect: E::seq(fa.effect, fb.effect),
+        }
+    }
+
+    fn discard_second<'a, A: 'a, B: 'a>(fa: Self::F<'a, A>, fb: Self::F<'a, B>) -> Self::F<'a, A>
+    where
+        Self: 'a,
+    {
+        drop(fb.value);
+        WithEffect {
+            value: fa.value,
+            effect: E::seq(fa.effect, fb.effect),
+        }
+    }
+}
+
+impl<E: Effect> Monad for EffectClass<E> {
+    fn bind<'a, A: 'a, B: 'a>(
+        fa: Self::F<'a, A>,
+        f: impl 'a + FnOnce(A) -> Self::F<'a, B>,
+    ) -> Self::F<'a, B>
+    where
+        Self: 'a,
+    {
+        f(fa.value).after(fa.effect)
+    }
+
+    fn iterate<'a, B: 'a>(mut f: impl Iterative<'a, T = Self, B = B>) -> Self::F<'a, B>
+    where
+        Self: 'a,
+    {
+        let mut effect = E::pure();
+        loop {
+            let fa = f.next();
+            effect = fa.effect.after(effect);
+            match fa.value {
+                ControlFlow::Continue(next_f) => f = next_f,
+                ControlFlow::Break(b) => return WithEffect { value: b, effect },
+            }
+        }
+    }
+
+    fn join<'a, A: 'a>(ffa: Self::F<'a, Self::F<'a, A>>) -> Self::F<'a, A>
+    where
+        Self::F<'a, A>: 'a,
+        Self: 'a,
+    {
+        ffa.value.after(ffa.effect)
+    }
+}
+
+impl<E> LocalFunctor for EffectClass<E> {
+    fn stuff<'a, A: 'a, T: 'a + Pure>(fa: Self::F<'a, T::F<'a, A>>) -> T::F<'a, Self::F<'a, A>>
+    where
+        Self: 'a,
+    {
+        T::fmap(
+            |a| WithEffect {
+                value: a,
+                effect: fa.effect,
+            },
+            fa.value,
+        )
+    }
+}