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    # ............................................................................................... #
    #
    # Copyright (c) 2012-2018 Institut National des Sciences Appliquées de Lyon (INSA Lyon) and others
    #
    # This program and the accompanying materials are made available under the
    # terms of the Eclipse Public License 2.0 which is available at
    # http://www.eclipse.org/legal/epl-2.0.
    #
    # SPDX-License-Identifier: EPL-2.0
    #
    # ............................................................................................... #

    ----
    This module offers asynchronous programming helpers, especially execution context agnostic promises
    and futures. The provided APIs are orthogonal to the execution strategy: it is up to you to execute
    code from the same thread, from a separate thread, or by pushing new tasks to a service executor.

    The functions and augmentations in this module often delegate to Java classes from
    `gololang.concurrent.async`.
    ----
    module gololang.Async

    ----
    Returns a new promise. Promise objects have the following useful methods.

    * `set(value)`: sets the promise value. The value is ignored if the promise has already been set.
    * `fail(exception)`: set the value to an exception.
    * `get()`: returns the promise value. It returns a bogus `null` value if the promise is still
      undefined.
    * `blockingGet()`: waits until the promise is set or failed, and returns the value.
    * `future()`: returns a new future object on a promise.
    * `isResolved()` and `isFailed()` query the promise status.

    Future objects have the following methods.

    * `onSet(|v| {...})`: registers a callback when the value is set, or executes it right now if it
      has already been set.
    * `onFail(|e| {...})`: registers a callback when the corresponding promise fails with an exception.
    * `isResolved()`, `isFailed()` `get()` and `blockingGet()` delegate to the promise implementation.
    ----
    function promise = ->
      gololang.concurrent.async.Promise()

    ----
    Augmentation on the base `Promise` objects provided by the `gololang.concurrent.async.Promise` Java
    class.

    The promise initialize method takes one argument, a callback with two parameters, resolve and reject.
    Do something within the callback, then call resolve if everything worked, otherwise call reject:

        let myPromise = -> promise(): initialize(|resolve, reject| {
          if everythingTurnedOutFine is true {
            resolve("Stuff worked!")
          } else {
            reject(java.lang.Exception("Failed!"))
          }
        })

        myPromise()
          : onSet(|result| {
              println(result) # Stuff worked!
          })
          : onFail(|err| {
              println(err: getMessage()) # Failed!
          })
    ----
    augment gololang.concurrent.async.Promise {
      function initialize = |this, closure| {
        closure(|data| -> this: set(data), |err| -> this: fail(err))
        return this: future()
      }

      ----
      You can define a promise which runs "stuff" inside a Thread
      ----
      function initializeWithinThread = |this, closure| {
        Thread({
          this: initialize(closure)
        }): start()
        return this: future()
      }
    }

    ----
    Returns a future set to `value`.
    ----
    function setFuture = |value| ->
      gololang.concurrent.async.AssignedFuture.setFuture(value)

    ----
    Returns a failed future to a `throwable` exception.
    ----
    function failedFuture = |throwable| ->
      gololang.concurrent.async.AssignedFuture.failedFuture(throwable)

    ----
    Augmentation on the base `Future` objects provided by the `gololang.concurrent.async.Future` Java
    class.
    ----
    augment gololang.concurrent.async.Future {

      ----
      Returns a future whose value is mapped through the `fun` function.

      If this future is set to `v`, then the returned future is set to `fun(v)`. If it fails, the
      returned future is also failed with the same exception.
      ----
      function map = |this, fun| {
        let p = promise()
        this: onSet(|v| -> p: set(fun(v))): onFail(|t| -> p: fail(t))
        return p: future()
      }

      ----
      Similar to [`map`](#gololang.concurrent.async.Future.map_2),
      except that `fun` returns a future, not a value.
      ----
      function flatMap = |this, fun| {
        let p = promise()
        this: onSet(|v| ->
          fun(v): onSet(|r| -> p: set(r)): onFail(|t| -> p: fail(t))
        ): onFail(|t| -> p: fail(t))
        return p: future()
      }

      ----
      Returns a future that filters this future through the `pred` predicate function.

      Suppose that this future is set to `v`:

      * if `pred(v)` is `true`, then the result future is set to `v`,
      * if `pred(v)` is `false`, then the result is failed to a `java.util.NoSuchElementException`.

      If this future fails, so does the returned future.
      ----
      function filter = |this, pred| {
        let p = promise()
        this: onSet(|v| {
          if pred(v) {
            p: set(v)
          } else {
            p: fail(java.util.NoSuchElementException())
          }
        }): onFail(|t| -> p: fail(t))
        return p: future()
      }

      ----
      Returns a fallback future:

      * when this future is set, the returned future is set to the same value,
      * when it fails, the returned future matches the success or failure of `future`.
      ----
      function fallbackTo = |this, future| {
        let p = promise()
        this: onSet(|v| -> p: set(v)): onFail(|t| {
          future: onSet(|v| -> p: set(v)): onFail(|e| -> p: fail(e))
        })
        return p: future()
      }
    }

    ----
    Given a collection of futures, returns a future whose value is eventually a vector with the
    results of these futures.

    Given:

        all([ setFuture(1), failedFuture(e) ])

    this yields a future whose eventual value is:

        vector[1, e]

    Results are accumulated as futures get resolved. The last completed future triggers the calls to
    `onSet`-registered listeners on the same thread.
    ----
    function all = |futures| {
      let size = futures: size()
      let vector = java.util.ArrayList(size)
      let p = promise()
      let c = java.util.concurrent.atomic.AtomicInteger(0)
      let trigger = {
        if c: incrementAndGet() == size {
          p: set(vector)
        }
      }
      for (var i = 0, i < size, i = i + 1) {
        futures: get(i):
          onSet(|v| {
            vector: add(i, v)
            trigger()
          }):
          onFail(|e| {
            vector: add(i, e)
            trigger()
          })
      }
      return p: future()
    }

    ----
    Given a collection of futures, returns a future whose value is set to the first completing future.

    If all futures fail, then the returned future fails to a `java.util.NoSuchElementException`.
    ----
    function any = |futures| {
      let size = futures: size()
      let p = promise()
      let i = java.util.concurrent.atomic.AtomicInteger(0)
      foreach f in futures {
        f: onSet(|v| -> p: set(v)): onFail(|e| {
          if i: incrementAndGet() == size {
            p: fail(java.util.NoSuchElementException())
          }
        })
      }
      return p: future()
    }

    ----
    Returns a future whose value is set to the *reduction* of a collection of futures.

    * `futures` is a collection of futures, and
    * `init` is the initial value, and
    * `reducer` is the reducing function of the form `|acc, next| {...}`.

    If any future fails, then the result future fails, too. Otherwise, the returned future is set to the
    accumulation of the values. Listeners callbacks get executed on the thread of the completing future
    which is either the last successful future or the first future to fail.
    ----
    function reduce = |futures, init, reducer| {
      let p = promise()
      all(futures): onSet(|results| {
        var acc = init
        foreach result in results {
          if result oftype java.lang.Throwable.class {
            p: fail(result)
            return
          } else {
            acc = reducer(acc, result)
          }
        }
        p: set(acc)
      })
      return p: future()
    }

    ----
    Bridge structure to hold a reference to a Golo future and a Java future.

    Instances of this struct are being returned by the
    [`enqueue`](#java.util.concurrent.ExecutorService.enqueue_2) augmentation
    on `ExecutorService` instances. This essentially adds the ability to:

    * use the Golo future for its composability, and
    * use tha Java future to cancel a job.
    ----
    struct FutureBridge = {
      _goloFuture,
      _javaFuture
    }

    ----
    A set of forwarding augmentations for [`FutureBridge`](#FutureBridge) instances.

    The provided functions all forward to Golo futures, while
    [`cancel`](#gololang.Async.types.FutureBridge.cancel_2)
    forwards to a Java future.
    ----
    augment gololang.Async.types.FutureBridge {

      function onSet = |this, listener| ->
        this: _goloFuture(): onSet(listener)

      function onFail = |this, listener| ->
        this: _goloFuture(): onFail(listener)

      function map = |this, fun| ->
        this: _goloFuture(): map(fun)

      function flatMap = |this, fun| ->
        this: _goloFuture(): flatMap(fun)

      function filter = |this, pred| ->
        this: _goloFuture(): filter(pred)

      function fallbackTo = |this, future| ->
        this: _goloFuture(): fallbackTo(future)

      function cancel = |this, mayInterruptIfRunning| ->
        this: _javaFuture(): cancel(mayInterruptIfRunning)
    }

    ----
    Augmentations for `ExecutorService`.
    ----
    augment java.util.concurrent.ExecutorService {

      ----
      Submits a function `fun` to be executed by this scheduler, and returns a
      [`FutureBridge`](#FutureBridge).

      `fun` takes no parameters, and its return value is used as a future value.

      The returned [`FutureBridge`](#FutureBridge)` behaves both as a composable
      Golo future and as a Java future that can be cancelled.

      Here is a sample usage:

          # Enqueue some elaborated work
          let f = executor: enqueue({
            Thread.sleep(1000_L)
            return 666
          })

          # Watch what could happen
          f: onSet(|v| -> println(v)):
             onFail(|e| -> println(e: getMessage()))

          # ...but make it fail unless the CPU was too slow
          f: cancel(true)
      ----
      function enqueue = |this, fun| {
        let callable = fun: to(java.util.concurrent.Callable.class)
        let javaFuture = this: submit(callable)
        let result = promise()
        this: submit({
          try {
            result: set(javaFuture: get())
          } catch (e) {
            result: fail(e)
            if e oftype java.lang.InterruptedException.class {
              java.lang.Thread.currentThread(): interrupt()
            }
          }
        })
        return ImmutableFutureBridge(result: future(), javaFuture)
      }
    }