feat: add Task.fromPromises and fix Task.prototype.chain error handling

Author: tdreyno

docs/task-static.md

@@ -260,6 +260,30 @@ type fromPromise = <S>(maybePromise: S | Promise<S>) => Task<unknown, S>
 {% endtab %}
 {% endtabs %}
 
+## fromPromises
+
+Converts an array of Promises into an array of Tasks and joins them with Task.all
+
+Promise's do not track an error type (one of the reasons Tasks are more powerful) so the resulting Task is unable to infer the error type as well. It is recommended to pass it in as a generic.
+
+{% tabs %}
+{% tab title="Usage" %}
+
+```typescript
+const task: Task<unknown, Response> = Task.fromPromises(fetch(URL))
+```
+
+{% endtab %}
+
+{% tab title="Type Definition" %}
+
+```typescript
+type fromPromises = <S>(promises: Array<Promise<S>>) => Task<unknown, S[]>
+```
+
+{% endtab %}
+{% endtabs %}
+
 ## fromLazyPromise
 
 Given a function which returns a Promise, turn that into a Task. This allows the Promise not to start until the Task forks (following the lazy philosophy of the rest of the library). This also means if two tasks chain from this one, the promise creating function will be called twice. See `onlyOnce` if you wish to avoid this.

src/Task/Task.ts

@@ -211,6 +211,14 @@ export const fromPromise = <S>(
     ? new Task((reject, resolve) => maybePromise.then(resolve, reject))
     : of(maybePromise)
 
+/**
+ * Given an array of promises, create a Task which relies on it.
+ * @param promise The promises we will gather the success from.
+ */
+export const fromPromises = <S>(
+  promises: Array<Promise<S>>,
+): Task<unknown, S[]> => all(promises.map(fromPromise))
+
 /**
  * Take a function which generates a promise and lazily execute it.
  * @param getPromise The getter function
@@ -690,6 +698,7 @@ export class Task<E, S> implements PromiseLike<S> {
   public static firstSuccess = firstSuccess
   public static never = never
   public static fromPromise = fromPromise
+  public static fromPromises = fromPromises
   public static fromLazyPromise = fromLazyPromise
   public static race = race
   public static external = external
@@ -756,7 +765,7 @@ export class Task<E, S> implements PromiseLike<S> {
     return this.toPromise().then(onfulfilled, onrejected)
   }
 
-  public chain<S2>(fn: (result: S) => Task<E, S2>): Task<E, S2> {
+  public chain<E2, S2>(fn: (result: S) => Task<E2, S2>): Task<E | E2, S2> {
     return chain(fn, this)
   }
 

src/Task/__tests__/fromPromises.spec.ts

@@ -0,0 +1,36 @@
+import { fromPromises } from "../Task"
+import { ERROR_RESULT, SUCCESS_RESULT } from "./util"
+
+describe("fromPromisea", () => {
+  test("should succeed when all promises succeeds", async () => {
+    const resolve = jest.fn()
+    const reject = jest.fn()
+
+    const promiseA = Promise.resolve(SUCCESS_RESULT)
+    const promiseB = Promise.resolve(SUCCESS_RESULT)
+
+    fromPromises([promiseA, promiseB]).fork(reject, resolve)
+
+    await promiseA.catch(() => void 0)
+    await promiseB.catch(() => void 0)
+
+    expect(resolve).toBeCalledWith([SUCCESS_RESULT, SUCCESS_RESULT])
+    expect(reject).not.toBeCalled()
+  })
+
+  test("should fail when a promise fails", async () => {
+    const resolve = jest.fn()
+    const reject = jest.fn()
+
+    const promiseA = Promise.resolve(SUCCESS_RESULT)
+    const promiseB = Promise.reject(ERROR_RESULT)
+
+    fromPromises([promiseA, promiseB]).fork(reject, resolve)
+
+    await promiseA.catch(() => void 0)
+    await promiseB.catch(() => void 0)
+
+    expect(resolve).not.toBeCalled()
+    expect(reject).toBeCalledWith(ERROR_RESULT)
+  })
+})