feat(compose): add trampoline()

packages/compose/src/index.ts

@@ -8,3 +8,4 @@ export * from "./juxt";
 export * from "./partial";
 export * from "./thread-first";
 export * from "./thread-last";
+export * from "./trampoline";

packages/compose/src/trampoline.ts

@@ -0,0 +1,35 @@
+import { Fn0 } from "@thi.ng/api";
+
+/**
+ * Takes a function returning either a no-arg function (thunk) or its
+ * already realized (non-function) result. Re-executes thunk for as long
+ * as it returns another function/thunk. Once a non-function result has
+ * been produced, `trampoline` returns that value itself. If the final
+ * result should be function, it needs to wrapped (e.g. as a 1-elem
+ * array).
+ *
+ * This function should be used for non-stack consuming recursion. I.e.
+ * a trampoline is a form of continuation passing style and only ever
+ * consumes max. 2 extra stack frames, independent from recursion depth.
+ *
+ * ```
+ * const countdown = (acc, x) =>
+ *   x >= 0 ?
+ *     () => (acc.push(x), countdown(acc, x-1)) :
+ *     acc;
+ *
+ * trampoline(countdown([], 4))
+ * // [ 4, 3, 2, 1, 0 ]
+ *
+ * trampoline(countdown([], -1))
+ * // []
+ * ```
+ *
+ * @param f
+ */
+export const trampoline = <T>(f: T | Fn0<T | Fn0<T>>) => {
+    while (typeof f === "function") {
+        f = (<any>f)();
+    }
+    return <T>f;
+};