class Concurrent::MVar

An ‘MVar` is a synchronized single element container. They are empty or contain one item. Taking a value from an empty `MVar` blocks, as does putting a value into a full one. You can either think of them as blocking queue of length one, or a special kind of mutable variable.

On top of the fundamental ‘#put` and `#take` operations, we also provide a `#mutate` that is atomic with respect to operations on the same instance. These operations all support timeouts.

We also support non-blocking operations ‘#try_put!` and `#try_take!`, a `#set!` that ignores existing values, a `#value` that returns the value without removing it or returns `MVar::EMPTY`, and a `#modify!` that yields `MVar::EMPTY` if the `MVar` is empty and can be used to set `MVar::EMPTY`. You shouldn’t use these operations in the first instance.

‘MVar` is a [Dereferenceable](Dereferenceable).

‘MVar` is related to M-structures in Id, `MVar` in Haskell and `SyncVar` in Scala.

Note that unlike the original Haskell paper, our ‘#take` is blocking. This is how Haskell and Scala do it today.

@!macro copy_options

## See Also

    1. Barth, R. Nikhil, and Arvind. [M-Structures: Extending a parallel, non- strict, functional language with state](dl.acm.org/citation.cfm?id=652538). In Proceedings of the 5th

    ACM Conference on Functional Programming Languages and Computer Architecture (FPCA), 1991.

    1. Peyton Jones, A. Gordon, and S. Finne. [Concurrent Haskell](dl.acm.org/citation.cfm?id=237794).

    In Proceedings of the 23rd Symposium on Principles of Programming Languages (PoPL), 1996.

Constants

EMPTY

Unique value that represents that an ‘MVar` was empty

TIMEOUT

Unique value that represents that an ‘MVar` timed out before it was able to produce a value.

Public Class Methods

new(value = EMPTY, opts = {}) click to toggle source

Create a new ‘MVar`, either empty or with an initial value.

@param [Hash] opts the options controlling how the future will be processed

@!macro deref_options

# File lib/concurrent-ruby/concurrent/mvar.rb, line 54
def initialize(value = EMPTY, opts = {})
  @value = value
  @mutex = Mutex.new
  @empty_condition = ConditionVariable.new
  @full_condition = ConditionVariable.new
  set_deref_options(opts)
end

Public Instance Methods

borrow(timeout = nil) { |value| ... } click to toggle source

acquires lock on the from an ‘MVAR`, yields the value to provided block, and release lock. A timeout can be set to limit the time spent blocked, in which case it returns `TIMEOUT` if the time is exceeded. @return [Object] the value returned by the block, or `TIMEOUT`

# File lib/concurrent-ruby/concurrent/mvar.rb, line 86
def borrow(timeout = nil)
  @mutex.synchronize do
    wait_for_full(timeout)

    # if we timeoud out we'll still be empty
    if unlocked_full?
      yield @value
    else
      TIMEOUT
    end
  end
end
empty?() click to toggle source

Returns if the ‘MVar` is currently empty.

# File lib/concurrent-ruby/concurrent/mvar.rb, line 195
def empty?
  @mutex.synchronize { @value == EMPTY }
end
full?() click to toggle source

Returns if the ‘MVar` currently contains a value.

# File lib/concurrent-ruby/concurrent/mvar.rb, line 200
def full?
  !empty?
end
modify(timeout = nil) { |value| ... } click to toggle source

Atomically ‘take`, yield the value to a block for transformation, and then `put` the transformed value. Returns the transformed value. A timeout can be set to limit the time spent blocked, in which case it returns `TIMEOUT` if the time is exceeded. @return [Object] the transformed value, or `TIMEOUT`

# File lib/concurrent-ruby/concurrent/mvar.rb, line 123
def modify(timeout = nil)
  raise ArgumentError.new('no block given') unless block_given?

  @mutex.synchronize do
    wait_for_full(timeout)

    # If we timed out we'll still be empty
    if unlocked_full?
      value = @value
      @value = yield value
      @full_condition.signal
      apply_deref_options(value)
    else
      TIMEOUT
    end
  end
end
modify!() { |value| ... } click to toggle source

Non-blocking version of ‘modify` that will yield with `EMPTY` if there is no value yet.

# File lib/concurrent-ruby/concurrent/mvar.rb, line 179
def modify!
  raise ArgumentError.new('no block given') unless block_given?

  @mutex.synchronize do
    value = @value
    @value = yield value
    if unlocked_empty?
      @empty_condition.signal
    else
      @full_condition.signal
    end
    apply_deref_options(value)
  end
end
put(value, timeout = nil) click to toggle source

Put a value into an ‘MVar`, blocking if there is already a value until it is empty. A timeout can be set to limit the time spent blocked, in which case it returns `TIMEOUT` if the time is exceeded. @return [Object] the value that was put, or `TIMEOUT`

# File lib/concurrent-ruby/concurrent/mvar.rb, line 103
def put(value, timeout = nil)
  @mutex.synchronize do
    wait_for_empty(timeout)

    # If we timed out we won't be empty
    if unlocked_empty?
      @value = value
      @full_condition.signal
      apply_deref_options(value)
    else
      TIMEOUT
    end
  end
end
set!(value) click to toggle source

Non-blocking version of ‘put` that will overwrite an existing value.

# File lib/concurrent-ruby/concurrent/mvar.rb, line 169
def set!(value)
  @mutex.synchronize do
    old_value = @value
    @value = value
    @full_condition.signal
    apply_deref_options(old_value)
  end
end
take(timeout = nil) click to toggle source

Remove the value from an ‘MVar`, leaving it empty, and blocking if there isn’t a value. A timeout can be set to limit the time spent blocked, in which case it returns ‘TIMEOUT` if the time is exceeded. @return [Object] the value that was taken, or `TIMEOUT`

# File lib/concurrent-ruby/concurrent/mvar.rb, line 66
def take(timeout = nil)
  @mutex.synchronize do
    wait_for_full(timeout)

    # If we timed out we'll still be empty
    if unlocked_full?
      value = @value
      @value = EMPTY
      @empty_condition.signal
      apply_deref_options(value)
    else
      TIMEOUT
    end
  end
end
try_put!(value) click to toggle source

Non-blocking version of ‘put`, that returns whether or not it was successful.

# File lib/concurrent-ruby/concurrent/mvar.rb, line 156
def try_put!(value)
  @mutex.synchronize do
    if unlocked_empty?
      @value = value
      @full_condition.signal
      true
    else
      false
    end
  end
end
try_take!() click to toggle source

Non-blocking version of ‘take`, that returns `EMPTY` instead of blocking.

# File lib/concurrent-ruby/concurrent/mvar.rb, line 142
def try_take!
  @mutex.synchronize do
    if unlocked_full?
      value = @value
      @value = EMPTY
      @empty_condition.signal
      apply_deref_options(value)
    else
      EMPTY
    end
  end
end

Protected Instance Methods

synchronize(&block) click to toggle source
# File lib/concurrent-ruby/concurrent/mvar.rb, line 206
def synchronize(&block)
  @mutex.synchronize(&block)
end

Private Instance Methods

unlocked_empty?() click to toggle source
# File lib/concurrent-ruby/concurrent/mvar.rb, line 212
def unlocked_empty?
  @value == EMPTY
end
unlocked_full?() click to toggle source
# File lib/concurrent-ruby/concurrent/mvar.rb, line 216
def unlocked_full?
  ! unlocked_empty?
end
wait_for_empty(timeout) click to toggle source
# File lib/concurrent-ruby/concurrent/mvar.rb, line 224
def wait_for_empty(timeout)
  wait_while(@empty_condition, timeout) { unlocked_full? }
end
wait_for_full(timeout) click to toggle source
# File lib/concurrent-ruby/concurrent/mvar.rb, line 220
def wait_for_full(timeout)
  wait_while(@full_condition, timeout) { unlocked_empty? }
end
wait_while(condition, timeout) click to toggle source
# File lib/concurrent-ruby/concurrent/mvar.rb, line 228
def wait_while(condition, timeout)
  if timeout.nil?
    while yield
      condition.wait(@mutex)
    end
  else
    stop = Concurrent.monotonic_time + timeout
    while yield && timeout > 0.0
      condition.wait(@mutex, timeout)
      timeout = stop - Concurrent.monotonic_time
    end
  end
end