Class: Concurrent::Promise

Inherits:
IVar
  • Object
show all
Defined in:
lib/concurrent/promise.rb

Overview

Promises are inspired by the JavaScript [Promises/A](wiki.commonjs.org/wiki/Promises/A) and [Promises/A+](promises-aplus.github.io/promises-spec/) 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 four possible states: unscheduled, pending, rejected, and fulfilled. A Promise created using ‘.new` will be unscheduled. It is scheduled by calling the `execute` method. Upon execution the Promise and all its children will be set to pending. When a promise is pending it will remain in that state until processing is complete. A completed Promise is either rejected, indicating that an exception was thrown during processing, or fulfilled, indicating it succeeded. 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. A Promise created using `.execute` will be pending, a Promise created using `.fulfill(value)` will be fulfilled with the given value and a Promise created using `.reject(reason)` will be rejected with the given reason.

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 “primative” 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 oject instance is created:

  • ‘:dup_on_deref` When true the object will call the `#dup` method on the `value` object every time the `#value` methid 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

“‘ruby require ’concurrent’ “‘

Then create one

“‘ruby p = Concurrent::Promise.execute do

  # do something
  42
end

“‘

Promises can be chained using the ‘then` method. The `then` method accepts a block, 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.

“‘ruby p = Concurrent::Promise.new10.then{|x| x * 2}.then{|result| result - 10 }.execute “`

And so on, and so on, and so on…

“‘ruby p = Concurrent::Promise.fulfill(20).

then{|result| result - 10 }.
then{|result| result * 3 }.
then{|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 nitialization it may be in a different state that 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:

“‘ruby # 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`:

“‘ruby 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:

“‘ruby 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:

“‘ruby 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:

“‘ruby 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

Instance Method Summary collapse

Methods inherited from IVar

#add_observer, #try_set

Methods included from Concern::Observable

#add_observer, #count_observers, #delete_observer, #delete_observers, #with_observer

Methods included from Concern::Obligation

#complete?, #exception, #fulfilled?, #incomplete?, #pending?, #reason, #rejected?, #state, #unscheduled?, #value, #value!, #wait, #wait!

Methods included from Concern::Dereferenceable

#value

Constructor Details

#initialize(opts = {}) { ... } ⇒ Promise

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:



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# File 'lib/concurrent/promise.rb', line 208

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

.all?(*promises) ⇒ Boolean

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 426

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

.any?(*promises) ⇒ Promise

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 437

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

.execute(opts = {}, &block) ⇒ Promise

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 293

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

.fulfill(value, opts = {}) ⇒ Promise

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 222

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

.reject(reason, opts = {}) ⇒ Promise

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 235

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

.zip(*promises) ⇒ Promise<Array>

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

Parameters:

Returns:



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# File 'lib/concurrent/promise.rb', line 384

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

#execute ⇒ Promise

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 244

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

#fail(reason = StandardError.new) ⇒ IVar

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:

Raises:



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# File 'lib/concurrent/promise.rb', line 275

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

#flat_map(&block) ⇒ Promise

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:



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# File 'lib/concurrent/promise.rb', line 357

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

#on_success { ... } ⇒ Promise

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

Yields:

  • The block to execute

Returns:

Raises:

  • (ArgumentError) 


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# File 'lib/concurrent/promise.rb', line 331

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

#rescue { ... } ⇒ Promise Also known as: catch, on_error

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

Yields:

  • The block to execute

Returns:



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# File 'lib/concurrent/promise.rb', line 342

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

#set(value = NULL) { ... } ⇒ IVar

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:

Raises:



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# File 'lib/concurrent/promise.rb', line 259

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

#then(rescuer = nil) { ... } ⇒ Promise

Chain a new promise off the current promise.

Parameters:

  • rescuer (Proc) (defaults to: nil) —

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

Yields:

  • The block operation to be performed asynchronously.

Returns:

Raises:

  • (ArgumentError) 


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# File 'lib/concurrent/promise.rb', line 305

def then(rescuer = nil, &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

#zip(*others) ⇒ Promise<Array>

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

Parameters:

Returns:



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# File 'lib/concurrent/promise.rb', line 402

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