diff --git a/src/func/instances/effect.rs b/src/func/instances/effect.rs
index 9610f90..0fec3ca 100644
--- a/src/func/instances/effect.rs
+++ b/src/func/instances/effect.rs
@@ -159,3 +159,89 @@ impl<'a, E: 'a + Send> LocalFunctor<'a> for EffectInstance<E> {
         })
     }
 }
+
+#[cfg(test)]
+mod effect_tests {
+    use super::{test_suite, tests, Effect, EffectInstance, WithEffect};
+
+    #[derive(Debug, PartialEq)]
+    enum TestEffect {
+        Pure,
+        Sample,
+        Parallel(Box<Self>, Box<Self>),
+        Sequential(Box<Self>, Box<Self>),
+    }
+
+    impl TestEffect {
+        fn is_pure(&self) -> bool {
+            matches!(self, Self::Pure)
+        }
+
+        fn simplify(self) -> Self {
+            match self {
+                Self::Pure => Self::Pure,
+                Self::Sample => Self::Sample,
+                Self::Parallel(a, b) if a.is_pure() => *b,
+                Self::Parallel(a, b) if b.is_pure() => *a,
+                Self::Sequential(a, b) if a.is_pure() => *b,
+                Self::Sequential(a, b) if b.is_pure() => *a,
+                Self::Parallel(a, b) => match *a {
+                    Self::Parallel(aa, ab) => {
+                        Self::Parallel(aa, Self::Parallel(ab, b).into()).simplify()
+                    }
+                    a => Self::Parallel(a.simplify().into(), b.simplify().into()),
+                },
+                Self::Sequential(a, b) => match *a {
+                    TestEffect::Sequential(aa, ab) => {
+                        Self::Sequential(aa, Self::Sequential(ab, b).into()).simplify()
+                    }
+                    a => Self::Sequential(a.simplify().into(), b.simplify().into()),
+                },
+            }
+        }
+    }
+
+    impl Effect for TestEffect {
+        fn e_pure() -> Self {
+            Self::Pure
+        }
+
+        fn e_parallel(el: Self, er: Self) -> Self {
+            Self::Parallel(el.into(), er.into())
+        }
+
+        fn e_after(self, effect: Self) -> Self {
+            Self::Sequential(effect.into(), self.into())
+        }
+    }
+
+    type T = EffectInstance<TestEffect>;
+
+    impl<'a> tests::Eqr<'a> for T {
+        fn eqr<A: 'a + Send + PartialEq + std::fmt::Debug>(
+            name: &'a str,
+            left: Self::F<A>,
+            right: Self::F<A>,
+        ) -> tests::R {
+            tests::eqr(
+                name,
+                (left.value, left.effect.simplify()),
+                (right.value, right.effect.simplify()),
+            )
+        }
+    }
+
+    impl<'a> test_suite::FunctorTestSuite<'a> for T {
+        fn sample<A: 'a + Send, F: FnMut(&'a (dyn Send + Sync + Fn(A) -> Self::F<A>))>(mut f: F) {
+            f(&|a| WithEffect {
+                value: a,
+                effect: TestEffect::Sample,
+            });
+        }
+    }
+
+    #[test]
+    fn monad_follows_laws() {
+        test_suite::monad_follows_laws::<T>().unwrap();
+    }
+}