Class: Concurrent::Promise — Concurrent

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# Class: Concurrent::Promise

Inherits:
  IVar
  - Object
  - Synchronization::LockableObject
  - IVar
  - Concurrent::Promise
  show all
Defined in:
  lib/concurrent/promise.rb

## Overview

Promises are inspired by the JavaScript Promises/A and Promises/A+ specifications.

> A promise represents the eventual value returned from the single completion of an operation.

Promises are similar to futures and share many of the same behaviours. Promises are far more robust, however. Promises can be chained in a tree structure where each promise may have zero or more children. Promises are chained using the `then` method. The result of a call to `then` is always another promise. Promises are resolved asynchronously (with respect to the main thread) but in a strict order: parents are guaranteed to be resolved before their children, children before their younger siblings. The `then` method takes two parameters: an optional block to be executed upon parent resolution and an optional callable to be executed upon parent failure. The result of each promise is passed to each of its children upon resolution. When a promise is rejected all its children will be summarily rejected and will receive the reason.

Promises have several possible states: :unscheduled, :pending, :processing, :rejected, or :fulfilled. These are also aggregated as `#incomplete?` and `#complete?`. When a Promise is created it is set to :unscheduled. Once the `#execute` method is called the state becomes :pending. Once a job is pulled from the thread pool's queue and is given to a thread for processing (often immediately upon `#post`) the state becomes :processing. The future will remain in this state until processing is complete. A future that is in the :unscheduled, :pending, or :processing is considered `#incomplete?`. A `#complete?` Promise is either :rejected, indicating that an exception was thrown during processing, or :fulfilled, indicating success. If a Promise is :fulfilled its `#value` will be updated to reflect the result of the operation. If :rejected the `reason` will be updated with a reference to the thrown exception. The predicate methods `#unscheduled?`, `#pending?`, `#rejected?`, and `#fulfilled?` can be called at any time to obtain the state of the Promise, as can the `#state` method, which returns a symbol.

Retrieving the value of a promise is done through the `value` (alias: `deref`) method. Obtaining the value of a promise is a potentially blocking operation. When a promise is rejected a call to `value` will return `nil` immediately. When a promise is fulfilled a call to `value` will immediately return the current value. When a promise is pending a call to `value` will block until the promise is either rejected or fulfilled. A timeout value can be passed to `value` to limit how long the call will block. If `nil` the call will block indefinitely. If `0` the call will not block. Any other integer or float value will indicate the maximum number of seconds to block.

Promises run on the global thread pool.

## Copy Options

Object references in Ruby are mutable. This can lead to serious problems when the Concern::Obligation#value of an object is a mutable reference. Which is always the case unless the value is a `Fixnum`, `Symbol`, or similar "primitive" data type. Each instance can be configured with a few options that can help protect the program from potentially dangerous operations. Each of these options can be optionally set when the object instance is created:

- `:dup_on_deref` When true the object will call the `#dup` method on the `value` object every time the `#value` method is called (default: false)
- `:freeze_on_deref` When true the object will call the `#freeze` method on the `value` object every time the `#value` method is called (default: false)
- `:copy_on_deref` When given a `Proc` object the `Proc` will be run every time the `#value` method is called. The `Proc` will be given the current `value` as its only argument and the result returned by the block will be the return value of the `#value` call. When `nil` this option will be ignored (default: nil)

When multiple deref options are set the order of operations is strictly defined. The order of deref operations is:

- `:copy_on_deref`
- `:dup_on_deref`
- `:freeze_on_deref`

Because of this ordering there is no need to `#freeze` an object created by a provided `:copy_on_deref` block. Simply set `:freeze_on_deref` to `true`. Setting both `:dup_on_deref` to `true` and `:freeze_on_deref` to `true` is as close to the behavior of a "pure" functional language (like Erlang, Clojure, or Haskell) as we are likely to get in Ruby.

### Examples

Start by requiring promises

    require 'concurrent'

Then create one

    p = Concurrent::Promise.execute do
          # do something
          42
        end

Promises can be chained using the `then` method. The `then` method accepts a block and an executor, to be executed on fulfillment, and a callable argument to be executed on rejection. The result of the each promise is passed as the block argument to chained promises.

    p = Concurrent::Promise.new{10}.then{|x| x * 2}.then{|result| result - 10 }.execute

And so on, and so on, and so on...

    p = Concurrent::Promise.fulfill(20).
        then{|result| result - 10 }.
        then{|result| result * 3 }.
        then(executor: different_executor){|result| result % 5 }.execute

The initial state of a newly created Promise depends on the state of its parent:

- if parent is unscheduled the child will be unscheduled
- if parent is pending the child will be pending
- if parent is fulfilled the child will be pending
- if parent is rejected the child will be pending (but will ultimately be rejected)

Promises are executed asynchronously from the main thread. By the time a child Promise finishes intialization it may be in a different state than its parent (by the time a child is created its parent may have completed execution and changed state). Despite being asynchronous, however, the order of execution of Promise objects in a chain (or tree) is strictly defined.

There are multiple ways to create and execute a new `Promise`. Both ways provide identical behavior:

    # create, operate, then execute
    p1 = Concurrent::Promise.new{ "Hello World!" }
    p1.state #=> :unscheduled
    p1.execute

    # create and immediately execute
    p2 = Concurrent::Promise.new{ "Hello World!" }.execute

    # execute during creation
    p3 = Concurrent::Promise.execute{ "Hello World!" }

Once the `execute` method is called a `Promise` becomes `pending`:

    p = Concurrent::Promise.execute{ "Hello, world!" }
    p.state    #=> :pending
    p.pending? #=> true

Wait a little bit, and the promise will resolve and provide a value:

    p = Concurrent::Promise.execute{ "Hello, world!" }
    sleep(0.1)

    p.state      #=> :fulfilled
    p.fulfilled? #=> true
    p.value      #=> "Hello, world!"

If an exception occurs, the promise will be rejected and will provide a reason for the rejection:

    p = Concurrent::Promise.execute{ raise StandardError.new("Here comes the Boom!") }
    sleep(0.1)

    p.state     #=> :rejected
    p.rejected? #=> true
    p.reason    #=> "#<StandardError: Here comes the Boom!>"

#### Rejection

When a promise is rejected all its children will be rejected and will receive the rejection `reason` as the rejection callable parameter:

    p = [ Concurrent::Promise.execute{ Thread.pass; raise StandardError } ]

    c1 = p.then(Proc.new{ |reason| 42 })
    c2 = p.then(Proc.new{ |reason| raise 'Boom!' })

    sleep(0.1)

    c1.state  #=> :rejected
    c2.state  #=> :rejected

Once a promise is rejected it will continue to accept children that will receive immediately rejection (they will be executed asynchronously).

#### Aliases

The `then` method is the most generic alias: it accepts a block to be executed upon parent fulfillment and a callable to be executed upon parent rejection. At least one of them should be passed. The default block is `{ |result| result }` that fulfills the child with the parent value. The default callable is `{ |reason| raise reason }` that rejects the child with the parent reason.

- `on_success { |result| ... }` is the same as `then {|result| ... }`
- `rescue { |reason| ... }` is the same as `then(Proc.new { |reason| ... } )`
- `rescue` is aliased by `catch` and `on_error`

## Class Method Summary (collapse)

- + (Boolean) all?(*promises)

  Aggregates a collection of promises and executes the `then` condition if all aggregated promises succeed.

- + (Promise) any?(*promises)

  Aggregates a collection of promises and executes the `then` condition if any aggregated promises succeed.

- + (Promise) execute(opts = {}, &block)

  Create a new `Promise` object with the given block, execute it, and return the `:pending` object.

- + (Promise) fulfill(value, opts = {})

  Create a new `Promise` and fulfill it immediately.

- + (Promise) reject(reason, opts = {})

  Create a new `Promise` and reject it immediately.

- + (Promise<Array>) zip(*promises)

  Builds a promise that produces the result of promises in an Array and fails if any of them fails.

## Instance Method Summary (collapse)

- - (Promise) execute

  Execute an `:unscheduled` `Promise`.

- - (IVar) fail(reason = StandardError.new)

  Set the `IVar` to failed due to some error and wake or notify all threads waiting on it.

- - (Promise) flat_map(&block)

  Yield the successful result to the block that returns a promise.

- - (Promise) initialize(opts = {}) { ... } constructor

  Initialize a new Promise with the provided options.

- - (Promise) on_success { ... }

  Chain onto this promise an action to be undertaken on success (fulfillment).

- - (Promise) rescue { ... } (also: #catch, #on_error)

  Chain onto this promise an action to be undertaken on failure (rejection).

- - (IVar) set(value = NULL) { ... }

  Set the `IVar` to a value and wake or notify all threads waiting on it.

- - (Promise) then(rescuer = nil, executor = @executor) { ... }

  Chain a new promise off the current promise.

- - (Promise<Array>) zip(*others)

  Builds a promise that produces the result of self and others in an Array and fails if any of them fails.

## Constructor Details

### - (`Promise`) initialize(opts = {}) { ... }

Initialize a new Promise with the provided options.

Parameters:

- opts (`Hash`) (defaults to: `{}`) —

  the options used to define the behavior at update and deref and to specify the executor on which to perform actions

Options Hash (`opts`):

- :executor (`Executor`) —

  when set use the given `Executor` instance. Three special values are also supported: `:task` returns the global task pool, `:operation` returns the global operation pool, and `:immediate` returns a new `ImmediateExecutor` object.

- :dup_on_deref (`Boolean`) — default: `false` —

  Call `#dup` before returning the data from Concern::Obligation#value

- :freeze_on_deref (`Boolean`) — default: `false` —

  Call `#freeze` before returning the data from Concern::Obligation#value

- :copy_on_deref (`Proc`) — default: `nil` —

  When calling the Concern::Obligation#value method, call the given proc passing the internal value as the sole argument then return the new value returned from the proc.

- :parent (`Promise`) —

  the parent `Promise` when building a chain/tree

- :on_fulfill (`Proc`) —

  fulfillment handler

- :on_reject (`Proc`) —

  rejection handler

- :args (`object`, `Array`) —

  zero or more arguments to be passed the task block on execution

Yields:

- The block operation to be performed asynchronously.

Raises:

- (`ArgumentError`) —

  if no block is given

See Also:

- http://wiki.commonjs.org/wiki/Promises/A
- http://promises-aplus.github.io/promises-spec/

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    # File 'lib/concurrent/promise.rb', line 210

    def initialize(opts = {}, &block)
      opts.delete_if { |k, v| v.nil? }
      super(NULL, opts.merge(__promise_body_from_block__: block), &nil)
    end

## Class Method Details

### + (`Boolean`) all?(*promises)

Aggregates a collection of promises and executes the `then` condition if all aggregated promises succeed. Executes the `rescue` handler with a `Concurrent::PromiseExecutionError` if any of the aggregated promises fail. Upon execution will execute any of the aggregate promises that were not already executed.

Returns:

- (`Boolean`)

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    # File 'lib/concurrent/promise.rb', line 431

    def self.all?(*promises)
      aggregate(:all?, *promises)
    end

### + (`Promise`) any?(*promises)

Aggregates a collection of promises and executes the `then` condition if any aggregated promises succeed. Executes the `rescue` handler with a `Concurrent::PromiseExecutionError` if any of the aggregated promises fail. Upon execution will execute any of the aggregate promises that were not already executed.

The returned promise will not yet have been executed. Additional `#then` and `#rescue` handlers may still be provided. Once the returned promise is execute the aggregate promises will be also be executed (if they have not been executed already). The results of the aggregate promises will be checked upon completion. The necessary `#then` and `#rescue` blocks on the aggregating promise will then be executed as appropriate. If the `#rescue` handlers are executed the raises exception will be `Concurrent::PromiseExecutionError`.

Parameters:

- promises (`Array`) —

  Zero or more promises to aggregate

Returns:

- (`Promise`) —

  an unscheduled (not executed) promise that aggregates the promises given as arguments

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    # File 'lib/concurrent/promise.rb', line 442

    def self.any?(*promises)
      aggregate(:any?, *promises)
    end

### + (`Promise`) execute(opts = {}, &block)

Create a new `Promise` object with the given block, execute it, and return the `:pending` object.

Examples:

    promise = Concurrent::Promise.execute{ sleep(1); 42 }
    promise.state #=> :pending

Parameters:

- opts (`Hash`) (defaults to: `{}`) —

  the options used to define the behavior at update and deref and to specify the executor on which to perform actions

Options Hash (`opts`):

- :executor (`Executor`) —

  when set use the given `Executor` instance. Three special values are also supported: `:task` returns the global task pool, `:operation` returns the global operation pool, and `:immediate` returns a new `ImmediateExecutor` object.

- :dup_on_deref (`Boolean`) — default: `false` —

  Call `#dup` before returning the data from Concern::Obligation#value

- :freeze_on_deref (`Boolean`) — default: `false` —

  Call `#freeze` before returning the data from Concern::Obligation#value

- :copy_on_deref (`Proc`) — default: `nil` —

  When calling the Concern::Obligation#value method, call the given proc passing the internal value as the sole argument then return the new value returned from the proc.

- :parent (`Promise`) —

  the parent `Promise` when building a chain/tree

- :on_fulfill (`Proc`) —

  fulfillment handler

- :on_reject (`Proc`) —

  rejection handler

- :args (`object`, `Array`) —

  zero or more arguments to be passed the task block on execution

Returns:

- (`Promise`) —

  the newly created `Promise` in the `:pending` state

Raises:

- (`ArgumentError`) —

  if no block is given

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    # File 'lib/concurrent/promise.rb', line 295

    def self.execute(opts = {}, &block)
      new(opts, &block).execute
    end

### + (`Promise`) fulfill(value, opts = {})

Create a new `Promise` and fulfill it immediately.

Parameters:

- opts (`Hash`) (defaults to: `{}`) —

  the options used to define the behavior at update and deref and to specify the executor on which to perform actions

Options Hash (`opts`):

- :executor (`Executor`) —

  when set use the given `Executor` instance. Three special values are also supported: `:task` returns the global task pool, `:operation` returns the global operation pool, and `:immediate` returns a new `ImmediateExecutor` object.

- :dup_on_deref (`Boolean`) — default: `false` —

  Call `#dup` before returning the data from Concern::Obligation#value

- :freeze_on_deref (`Boolean`) — default: `false` —

  Call `#freeze` before returning the data from Concern::Obligation#value

- :copy_on_deref (`Proc`) — default: `nil` —

  When calling the Concern::Obligation#value method, call the given proc passing the internal value as the sole argument then return the new value returned from the proc.

- :parent (`Promise`) —

  the parent `Promise` when building a chain/tree

- :on_fulfill (`Proc`) —

  fulfillment handler

- :on_reject (`Proc`) —

  rejection handler

- :args (`object`, `Array`) —

  zero or more arguments to be passed the task block on execution

Returns:

- (`Promise`) —

  the newly created `Promise`

Raises:

- (`ArgumentError`) —

  if no block is given

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    # File 'lib/concurrent/promise.rb', line 224

    def self.fulfill(value, opts = {})
      Promise.new(opts).tap { |p| p.send(:synchronized_set_state!, true, value, nil) }
    end

### + (`Promise`) reject(reason, opts = {})

Create a new `Promise` and reject it immediately.

Parameters:

- opts (`Hash`) (defaults to: `{}`) —

  the options used to define the behavior at update and deref and to specify the executor on which to perform actions

Options Hash (`opts`):

- :executor (`Executor`) —

  when set use the given `Executor` instance. Three special values are also supported: `:task` returns the global task pool, `:operation` returns the global operation pool, and `:immediate` returns a new `ImmediateExecutor` object.

- :dup_on_deref (`Boolean`) — default: `false` —

  Call `#dup` before returning the data from Concern::Obligation#value

- :freeze_on_deref (`Boolean`) — default: `false` —

  Call `#freeze` before returning the data from Concern::Obligation#value

- :copy_on_deref (`Proc`) — default: `nil` —

  When calling the Concern::Obligation#value method, call the given proc passing the internal value as the sole argument then return the new value returned from the proc.

- :parent (`Promise`) —

  the parent `Promise` when building a chain/tree

- :on_fulfill (`Proc`) —

  fulfillment handler

- :on_reject (`Proc`) —

  rejection handler

- :args (`object`, `Array`) —

  zero or more arguments to be passed the task block on execution

Returns:

- (`Promise`) —

  the newly created `Promise`

Raises:

- (`ArgumentError`) —

  if no block is given

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    # File 'lib/concurrent/promise.rb', line 237

    def self.reject(reason, opts = {})
      Promise.new(opts).tap { |p| p.send(:synchronized_set_state!, false, nil, reason) }
    end

### + (`Promise<Array>`) zip(*promises)

Builds a promise that produces the result of promises in an Array and fails if any of them fails.

Parameters:

- promises (`Array<Promise>`)

Returns:

- (`Promise<Array>`)

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    # File 'lib/concurrent/promise.rb', line 389

    def self.zip(*promises)
      zero = fulfill([], executor: ImmediateExecutor.new)

      promises.reduce(zero) do |p1, p2|
        p1.flat_map do |results|
          p2.then do |next_result|
            results << next_result
          end
        end
      end
    end

## Instance Method Details

### - (`Promise`) execute

Execute an `:unscheduled` `Promise`. Immediately sets the state to `:pending` and passes the block to a new thread/thread pool for eventual execution. Does nothing if the `Promise` is in any state other than `:unscheduled`.

Returns:

- (`Promise`) —

  a reference to `self`

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    # File 'lib/concurrent/promise.rb', line 246

    def execute
      if root?
        if compare_and_set_state(:pending, :unscheduled)
          set_pending
          realize(@promise_body)
        end
      else
        @parent.execute
      end
      self
    end

### - (`IVar`) fail(reason = StandardError.new)

Set the `IVar` to failed due to some error and wake or notify all threads waiting on it.

Parameters:

- reason (`Object`) (defaults to: `StandardError.new`) —

  for the failure

Returns:

- (`IVar`) —

  self

Raises:

- (`Concurrent::MultipleAssignmentError`) —

  if the `IVar` has already been set or otherwise completed

- (`Concurrent::PromiseExecutionError`) —

  if not the root promise

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    # File 'lib/concurrent/promise.rb', line 277

    def fail(reason = StandardError.new)
      set { raise reason }
    end

### - (`Promise`) flat_map(&block)

Yield the successful result to the block that returns a promise. If that promise is also successful the result is the result of the yielded promise. If either part fails the whole also fails.

Examples:

    Promise.execute { 1 }.flat_map { |v| Promise.execute { v + 2 } }.value! #=> 3

Returns:

- (`Promise`)

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    # File 'lib/concurrent/promise.rb', line 362

    def flat_map(&block)
      child = Promise.new(
        parent: self,
        executor: ImmediateExecutor.new,
      )

      on_error { |e| child.on_reject(e) }
      on_success do |result1|
        begin
          inner = block.call(result1)
          inner.execute
          inner.on_success { |result2| child.on_fulfill(result2) }
          inner.on_error { |e| child.on_reject(e) }
        rescue => e
          child.on_reject(e)
        end
      end

      child
    end

### - (`Promise`) on_success { ... }

Chain onto this promise an action to be undertaken on success (fulfillment).

Yields:

- The block to execute

Returns:

- (`Promise`) —

  self

Raises:

- (`ArgumentError`)

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    # File 'lib/concurrent/promise.rb', line 336

    def on_success(&block)
      raise ArgumentError.new('no block given') unless block_given?
      self.then(&block)
    end

### - (`Promise`) rescue { ... } Also known as: catch, on_error

Chain onto this promise an action to be undertaken on failure (rejection).

Yields:

- The block to execute

Returns:

- (`Promise`) —

  self

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    # File 'lib/concurrent/promise.rb', line 347

    def rescue(&block)
      self.then(block)
    end

### - (`IVar`) set(value = NULL) { ... }

Set the `IVar` to a value and wake or notify all threads waiting on it.

Parameters:

- value (`Object`) (defaults to: `NULL`) —

  the value to store in the `IVar`

Yields:

- A block operation to use for setting the value

Returns:

- (`IVar`) —

  self

Raises:

- (`ArgumentError`) —

  if both a value and a block are given

- (`Concurrent::MultipleAssignmentError`) —

  if the `IVar` has already been set or otherwise completed

- (`Concurrent::PromiseExecutionError`) —

  if not the root promise

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    # File 'lib/concurrent/promise.rb', line 261

    def set(value = NULL, &block)
      raise PromiseExecutionError.new('supported only on root promise') unless root?
      check_for_block_or_value!(block_given?, value)
      synchronize do
        if @state != :unscheduled
          raise MultipleAssignmentError
        else
          @promise_body = block || Proc.new { |result| value }
        end
      end
      execute
    end

### - (`Promise`) then(rescuer = nil, executor = @executor) { ... }

Chain a new promise off the current promise.

in the new Promise

Parameters:

- rescuer (`Proc`) (defaults to: `nil`) —

  An optional rescue block to be executed if the promise is rejected.

- executor (`ThreadPool`) (defaults to: `@executor`) —

  An optional thread pool executor to be used

Yields:

- The block operation to be performed asynchronously.

Returns:

- (`Promise`) —

  the new promise

Raises:

- (`ArgumentError`)

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    # File 'lib/concurrent/promise.rb', line 310

    def then(rescuer = nil, executor = @executor, &block)
      raise ArgumentError.new('rescuers and block are both missing') if rescuer.nil? && !block_given?
      block = Proc.new { |result| result } unless block_given?
      child = Promise.new(
        parent: self,
        executor: executor,
        on_fulfill: block,
        on_reject: rescuer
      )

      synchronize do
        child.state = :pending if @state == :pending
        child.on_fulfill(apply_deref_options(@value)) if @state == :fulfilled
        child.on_reject(@reason) if @state == :rejected
        @children << child
      end

      child
    end

### - (`Promise<Array>`) zip(*others)

Builds a promise that produces the result of self and others in an Array and fails if any of them fails.

Parameters:

- others (`Array<Promise>`)

Returns:

- (`Promise<Array>`)

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    # File 'lib/concurrent/promise.rb', line 407

    def zip(*others)
      self.class.zip(self, *others)
    end