diff --git a/include/ftl/unit.h b/include/ftl/unit.h
new file mode 100644
index 0000000..e38230b
--- /dev/null
+++ b/include/ftl/unit.h
@@ -0,0 +1,61 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <type_traits>
+#include <utility>
+
+namespace android::ftl {
+
+// The unit type, and its only value.
+constexpr struct Unit {
+} unit;
+
+constexpr bool operator==(Unit, Unit) {
+  return true;
+}
+
+constexpr bool operator!=(Unit, Unit) {
+  return false;
+}
+
+// Adapts a function object F to return Unit. The return value of F is ignored.
+//
+// As a practical use, the function passed to ftl::Optional<T>::transform is not allowed to return
+// void (cf. https://wg21.link/P0798R8#mapping-functions-returning-void), but may return Unit if
+// only its side effects are meaningful:
+//
+//   ftl::Optional opt = "food"s;
+//   opt.transform(ftl::unit_fn([](std::string& str) { str.pop_back(); }));
+//   assert(opt == "foo"s);
+//
+template <typename F>
+struct UnitFn {
+  F f;
+
+  template <typename... Args>
+  Unit operator()(Args&&... args) {
+    return f(std::forward<Args>(args)...), unit;
+  }
+};
+
+template <typename F>
+constexpr auto unit_fn(F&& f) -> UnitFn<std::decay_t<F>> {
+  return {std::forward<F>(f)};
+}
+
+}  // namespace android::ftl