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Showing content from http://ruby-concurrency.github.io/concurrent-ruby/master/Concurrent/Promise.html below:

Class: Concurrent::Promise — Concurrent Ruby

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.

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.

Start by requiring promises

require 'concurrent/promise'

Then create one

p = Concurrent::Promise.execute do
            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 initialization 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:

p1 = Concurrent::Promise.new{ "Hello World!" }
p1.state p1.execute

p2 = Concurrent::Promise.new{ "Hello World!" }.execute

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

Once the execute method is called a Promise becomes pending:

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

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

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

p.state      p.fulfilled? p.value      

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     p.rejected? p.reason    

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(-> reason { 42 })
c2 = p.then(-> reason { raise 'Boom!' })

c1.wait.state  c1.value       c2.wait.state  c2.reason      

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

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

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.

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 464

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

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.

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 475

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

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

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 296

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

Create a new Promise and fulfill it immediately.

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

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

Create a new Promise and reject it immediately.

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

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

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

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 409

def self.zip(*promises)
  opts = promises.last.is_a?(::Hash) ? promises.pop.dup : {}
  opts[:executor] ||= ImmediateExecutor.new
  zero = if !opts.key?(:execute) || opts.delete(:execute)
    fulfill([], opts)
  else
    Promise.new(opts) { [] }
  end

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

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.

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

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

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

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 278

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

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.

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 375

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

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

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 349

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

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

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 360

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

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

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 262

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

Chain a new promise off the current promise.

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 314

def then(*args, &block)
  if args.last.is_a?(::Hash)
    executor = args.pop[:executor]
    rescuer = args.first
  else
    rescuer, executor = args
  end

  executor ||= @executor

  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

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

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# File 'lib/concurrent-ruby/concurrent/promise.rb', line 440

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

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