diff --git a/lib/source.ts b/lib/source.ts
new file mode 100644
index 0000000..c975a69
--- /dev/null
+++ b/lib/source.ts
@@ -0,0 +1,5 @@
+export type Cancel = () => void;
+export type Source<T> = {
+  (): T;
+  (callback: (value: T) => void): Cancel;
+};