feat: add Task.prototype.failIf

src/Task/Task.ts

@@ -753,6 +753,18 @@ export const retryWithExponentialBackoff = <E, S>(
 export const flatten = <E, S>(task: Task<E, Task<E, S>>): Task<E, S> =>
   task.chain(identity)
 
+/**
+ * Given a predicate, if it returns true, error the task with a given value.
+ * @param pred Run this on a successful task, return true to fail the task.
+ * @param error If the predicate succeeded, run this function to get the error result.
+ */
+export const failIf = <E, S, E2>(
+  pred: (result: S) => boolean,
+  error: (result: S) => E2,
+  task: Task<E, S>,
+): Task<E | E2, S> =>
+  task.chain(result => (pred(result) ? fail(error(result)) : of(result)))
+
 /**
  * Create a new task.
  * @param computation A function which will be run when the task starts.
@@ -963,6 +975,13 @@ export class Task<E, S> implements PromiseLike<S> {
   public flatten<S2>(this: Task<E, Task<E, S2>>): Task<E, S2> {
     return flatten(this)
   }
+
+  public failIf<E2>(
+    pred: (result: S) => boolean,
+    error: (result: S) => E2,
+  ): Task<E | E2, S> {
+    return failIf(pred, error, this)
+  }
 }
 
 /**

src/Task/__tests__/failIf.spec.ts

@@ -0,0 +1,31 @@
+import { of } from "../Task"
+
+describe("failIf", () => {
+  test("should fail with new error type", () => {
+    const resolve = jest.fn()
+    const reject = jest.fn()
+
+    const lt5 = (v: number) => v < 5
+
+    of(4)
+      .failIf(lt5, v => `${v} < 5`)
+      .fork(reject, resolve)
+
+    expect(resolve).not.toBeCalled()
+    expect(reject).toBeCalledWith("4 < 5")
+  })
+
+  test("should succeed if predicate fails", () => {
+    const resolve = jest.fn()
+    const reject = jest.fn()
+
+    const lt5 = (v: number) => v < 5
+
+    of(10)
+      .failIf(lt5, v => `${v} < 5`)
+      .fork(reject, resolve)
+
+    expect(reject).not.toBeCalled()
+    expect(resolve).toBeCalledWith(10)
+  })
+})