feat(task): add prepend as well

Author: tdreyno

docs/task-instance.md

@@ -442,7 +442,7 @@ type forward = <S2>(value: S2) => Task<E, S2>;
 
 ## append
 
-Given a successful Task, join it with an additional value. Useful for threading a value along with a task. Like `zip`, but when one of the values is not a Task.
+Given a successful Task, join it before an additional value. Useful for threading a value along with a task. Like `zip`, but when one of the values is not a Task.
 
 {% tabs %}
 {% tab title="Usage" %}
@@ -462,6 +462,28 @@ type append = <S2>(this: Task<E, S>, value: S2) => Task<E, [S, S2]>;
 {% endtab %}
 {% endtabs %}
 
+## prepend
+
+Given a successful Task, join it after an additional value. Useful for threading a value along with a task. Like `zip`, but when one of the values is not a Task.
+
+{% tabs %}
+{% tab title="Usage" %}
+
+```typescript
+const task: Task<unknown, [number, number]> = Task.of(5).prepend(10);
+```
+
+{% endtab %}
+
+{% tab title="Type Definition" %}
+
+```typescript
+type prepend = <S2>(this: Task<E, S>, value: S2) => Task<E, [S2, S]>;
+```
+
+{% endtab %}
+{% endtabs %}
+
 ## ap
 
 The applicative. If you know what that means, you'll be excited. If not, it is fine. This is a low level tool that helps build more complex features.

src/Task/Task.ts

@@ -130,6 +130,21 @@ export class Task<E, S> implements PromiseLike<S> {
     return map<E, S, any[]>(a => [a, ...items], this);
   }
 
+  public prepend<A, B, C, D, E>(
+    a: A,
+    b: B,
+    c: C,
+    d: D,
+    e: E
+  ): Task<E, [S, A, B, C, D, E]>;
+  public prepend<A, B, C, D>(a: A, b: B, c: C, d: D): Task<E, [A, B, C, D, S]>;
+  public prepend<A, B, C>(a: A, b: B, c: C): Task<E, [A, B, C, S]>;
+  public prepend<A, B>(a: A, b: B): Task<E, [A, B, S]>;
+  public prepend<A>(a: A): Task<E, [A, S]>;
+  public prepend(...items: any[]): Task<E, any[]> {
+    return map<E, S, any[]>(a => [...items, a], this);
+  }
+
   public tap(fn: (result: S) => void): Task<E, S> {
     return tap(fn, this);
   }

src/Task/__tests__/prepend.spec.ts

@@ -0,0 +1,15 @@
+import { succeed } from "../Task";
+
+describe("prepend", () => {
+  test("should prepend some number of constants to the current task value ", () => {
+    const resolve = jest.fn();
+    const reject = jest.fn();
+
+    succeed(5)
+      .prepend(10, "15", 20)
+      .fork(reject, resolve);
+
+    expect(resolve).toBeCalledWith([10, "15", 20, 5]);
+    expect(reject).not.toBeCalled();
+  });
+});