feat(fio): add flattenM static method

Author: tusharmath

src/main/FIO.ts

@@ -256,6 +256,30 @@ export class FIO<E1 = unknown, A1 = unknown, R1 = NoEnv> {
     return fio.chain(Id)
   }
 
+  /**
+   * Takes in a effect-ful function that return a FIO and unwraps it.
+   * This is an alias to `FIO.flatten(UIO(fn))`
+   *
+   * ```ts
+   * // An impure function that creates mutable state but also returns a FIO.
+   * const FN = () => {
+   *   let count = 0
+   *
+   *   return FIO.try(() => count++)
+   * }
+   * // Using flatten
+   * FIO.flatten(UIO(FN))
+   *
+   * // Using flattenM
+   * FIO.flattenM(FN)
+   * ```
+   */
+  public static flattenM<E1, A1, R1>(
+    fio: () => FIO<E1, A1, R1>
+  ): FIO<E1, A1, R1> {
+    return FIO.flatten(UIO(fio))
+  }
+
   /**
    * Creates a new [[Fiber]] to run the given [[IO]].
    */
@@ -668,49 +692,69 @@ export class FIO<E1 = unknown, A1 = unknown, R1 = NoEnv> {
     c: (e1: A1, e2: A2) => C
   ): FIO<E1 | E2, C, R1 & R2> {
     // Create Caches
-    const cache = ExitRef<E1, A1>().zip(ExitRef<E2, A2>())
-
-    // Create a Counter and an Await
-    return Ref.of(0)
-      .zip(Await.of<never, boolean>())
-      .chain(({0: count, 1: done}) => {
-        // Cancels the provided fiber on exit status.
-        // tslint:disable-next-line:no-shadowed-variable
-        const coordinate = <E1, A1, E2, A2>(
-          exit: Either<E1, A1>,
-          fiber: Fiber<E2, A2>,
-          ref: Ref<Either<E1, A1>>
-        ): UIO<boolean> =>
-          ref
-            .set(exit)
-            .chain(e =>
-              e.fold(
-                FIO.of(false),
-                () => fiber.abort.and(done.set(FIO.of(true))),
-                () =>
-                  count
-                    .update(_ => _ + 1)
-                    .and(
-                      count.read.chain(value =>
-                        value === 2 ? done.set(FIO.of(true)) : FIO.of(false)
-                      )
-                    )
+    const Caches = ExitRef<E1, A1>().zip(ExitRef<E2, A2>())
+
+    // Maintains the count of results produced
+    const Counter = Ref.of(0)
+
+    // Is set only when the IO is completed.
+    const Done = Await.of<never, boolean>()
+
+    return Counter.zip(Done).chain(({0: count, 1: isDone}) => {
+      // Cancels the provided fiber on exit status.
+      const coordinate = <EE1, AA1, EE2, AA2>(
+        exit: Either<EE1, AA1>,
+        fiber: Fiber<EE2, AA2>,
+        cache: Ref<Either<EE1, AA1>>
+      ): UIO<boolean> => {
+        // Saves the result into a [[Ref]] instance
+        const cacheResult = cache.set(exit)
+
+        // Abort the other Fiber
+        const abortFiber = fiber.abort
+
+        // Set the result
+        const markAsDone = isDone.set(FIO.of(true))
+
+        // Increases the result count by 1
+        const incCount = count.update(_ => _ + 1)
+
+        return cacheResult.and(
+          exit.fold(
+            FIO.of(false),
+
+            // On failure —
+            // 1. Increase count
+            // 2. Abort fiber
+            // 3. Set final result
+            () => abortFiber.and(markAsDone),
+
+            // On success  —
+            // 1. Increase count
+            // 2. if count === 2 : Set final result
+            () =>
+              incCount.and(
+                count.read.chain(value =>
+                  value === 2 ? markAsDone : FIO.of(false)
+                )
               )
-            )
-
-        return cache.chain(({0: c1, 1: c2}) =>
-          this.raceWith(
-            that,
-            (exit, fiber) => coordinate(exit, fiber, c1).void,
-            (exit, fiber) => coordinate(exit, fiber, c2).void
           )
-            .and(done.get)
-            .and(
-              c1.read
-                .chain(FIO.fromEither)
-                .zipWith(c2.read.chain(FIO.fromEither), c)
-            )
         )
-      })
+      }
+
+      return Caches.chain(({0: cacheL, 1: cacheR}) =>
+        this.raceWith(
+          that,
+          (exit, fiber) => coordinate(exit, fiber, cacheL).void,
+          (exit, fiber) => coordinate(exit, fiber, cacheR).void
+        )
+          .and(isDone.get)
+          .and(
+            cacheL.read
+              .chain(FIO.fromEither)
+              .zipWith(cacheR.read.chain(FIO.fromEither), c)
+          )
+      )
+    })
   }
 }