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
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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.
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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.
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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
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
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
Returns if the ‘MVar` is currently empty.
# File lib/concurrent-ruby/concurrent/mvar.rb, line 195 def empty? @mutex.synchronize { @value == EMPTY } end
Returns if the ‘MVar` currently contains a value.
# File lib/concurrent-ruby/concurrent/mvar.rb, line 200 def full? !empty? end
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
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 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
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
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
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
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
# File lib/concurrent-ruby/concurrent/mvar.rb, line 206 def synchronize(&block) @mutex.synchronize(&block) end
Private Instance Methods
# File lib/concurrent-ruby/concurrent/mvar.rb, line 212 def unlocked_empty? @value == EMPTY end
# File lib/concurrent-ruby/concurrent/mvar.rb, line 216 def unlocked_full? ! unlocked_empty? end
# File lib/concurrent-ruby/concurrent/mvar.rb, line 224 def wait_for_empty(timeout) wait_while(@empty_condition, timeout) { unlocked_full? } end
# File lib/concurrent-ruby/concurrent/mvar.rb, line 220 def wait_for_full(timeout) wait_while(@full_condition, timeout) { unlocked_empty? } end
# 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