feat: Adds cachedFlatMap operator (#20)

.changeset/tame-geese-allow.md

@@ -0,0 +1,5 @@
+---
+"windpipe": minor
+---
+
+Adds the `cachedFlatMap` operator

src/stream/higher-order.ts

@@ -125,6 +125,54 @@ export class HigherOrderStream<T, E> extends StreamTransforms<T, E> {
         );
     }
 
+    /**
+     * Map over each value in the stream, produce a stream from it, cache the resultant stream
+     * and flatten all the value streams together
+     *
+     * @group Higher Order
+     */
+    cachedFlatMap<U>(
+        cb: (value: T) => MaybePromise<Stream<U, E>>,
+        keyFn: (value: T) => string | number | symbol,
+    ): Stream<U, E> {
+        const trace = this.trace("cachedFlatMap");
+
+        return this.consume(async function* (it) {
+            const cache = new Map<PropertyKey, Atom<U, E>[]>();
+
+            for await (const atom of it) {
+                if (!isOk(atom)) {
+                    yield atom;
+                    continue;
+                }
+
+                const key = keyFn(atom.value);
+                const cachedValues = cache.get(key);
+
+                if (cachedValues !== undefined) {
+                    yield* cachedValues;
+                    continue;
+                }
+
+                // Run the flat map handler
+                const streamAtom = await run(() => cb(atom.value), trace);
+
+                // If an error was emitted whilst initialising the new stream, return it
+                if (!isOk(streamAtom)) {
+                    yield streamAtom;
+                    continue;
+                }
+
+                // Otherwise, consume the iterator
+                const values = await streamAtom.value.toArray({ atoms: true });
+
+                cache.set(key, values);
+
+                yield* values;
+            }
+        });
+    }
+
     /**
      * Produce a new stream from the stream that has any nested streams flattened
      *

test/higher-order.test.ts

@@ -127,6 +127,111 @@ describe.concurrent("higher order streams", () => {
         });
     });
 
+    describe.concurrent("cachedFlatMap", () => {
+        test("lookup non-repeating strings returning single atom", async ({ expect }) => {
+            expect.assertions(2);
+
+            const lookup = vi.fn((param: string) => $.of(param));
+
+            const s = $.from(["a", "b", "c"]).cachedFlatMap(lookup, (v) => v);
+
+            expect(await s.toArray({ atoms: true })).toEqual([$.ok("a"), $.ok("b"), $.ok("c")]);
+            expect(lookup).toBeCalledTimes(3);
+        });
+
+        test("lookup repeating strings returning single atom", async ({ expect }) => {
+            expect.assertions(2);
+
+            const lookup = vi.fn((param: string) => $.of(param));
+
+            const s = $.from(["a", "b", "c", "a", "a"]).cachedFlatMap(lookup, (v) => v);
+
+            expect(await s.toArray({ atoms: true })).toEqual([
+                $.ok("a"),
+                $.ok("b"),
+                $.ok("c"),
+                $.ok("a"),
+                $.ok("a"),
+            ]);
+            expect(lookup).toBeCalledTimes(3);
+        });
+
+        test("lookup repeating numbers returning multiple atoms", async ({ expect }) => {
+            expect.assertions(2);
+
+            const lookup = vi.fn((n: number) => $.fromArray([n, n * 2, n * 4]));
+
+            const s = $.from([1, 100, 200, 1, 10]).cachedFlatMap(lookup, (v) => v);
+
+            expect(await s.toArray({ atoms: true })).toEqual([
+                $.ok(1),
+                $.ok(2),
+                $.ok(4),
+                $.ok(100),
+                $.ok(200),
+                $.ok(400),
+                $.ok(200),
+                $.ok(400),
+                $.ok(800),
+                $.ok(1),
+                $.ok(2),
+                $.ok(4),
+                $.ok(10),
+                $.ok(20),
+                $.ok(40),
+            ]);
+            expect(lookup).toBeCalledTimes(4);
+        });
+
+        test("lookup repeating numbers returning multiple atoms", async ({ expect }) => {
+            expect.assertions(2);
+
+            const oneHundredDividedBy = vi.fn((n: number) => {
+                if (n === 0) {
+                    throw "Cannot divide by zero!";
+                }
+
+                return $.of(100 / n);
+            });
+
+            const s = $.from([5, 0, 50, 5, 5]).cachedFlatMap(oneHundredDividedBy, (v) => v);
+
+            expect(await s.toArray({ atoms: true })).toEqual([
+                $.ok(20),
+                $.unknown("Cannot divide by zero!", ["cachedFlatMap"]),
+                $.ok(2),
+                $.ok(20),
+                $.ok(20),
+            ]);
+            expect(oneHundredDividedBy).toBeCalledTimes(3);
+        });
+
+        test("lookup repeating numbers, including an error, returning multiple atoms", async ({
+            expect,
+        }) => {
+            expect.assertions(2);
+
+            const lookup = vi.fn((n: number) => $.of(n));
+
+            const s = $.from<number, unknown>([
+                $.ok(1),
+                $.ok(2),
+                $.error("oh no!"),
+                $.ok(2),
+                $.ok(1),
+            ]).cachedFlatMap(lookup, (v) => v);
+
+            expect(await s.toArray({ atoms: true })).toEqual([
+                $.ok(1),
+                $.ok(2),
+                $.error("oh no!"),
+                $.ok(2),
+                $.ok(1),
+            ]);
+            expect(lookup).toBeCalledTimes(2);
+        });
+    });
+
     describe.concurrent("flatten", () => {
         test("simple nested stream", async ({ expect }) => {
             expect.assertions(1);