Class: Concurrent::MVar
- Inherits:
-
Synchronization::Object
- Object
- Synchronization::Object
- Concurrent::MVar
- Includes:
- Concern::Dereferenceable
- Defined in:
- lib/concurrent/mvar.rb
Overview
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.
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.
Copy Options
Object references in Ruby are mutable. This can lead to serious
problems when the Concern::Dereferenceable#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_derefWhen true the object will call the#dupmethod on thevalueobject every time the#valuemethod is called (default: false):freeze_on_derefWhen true the object will call the#freezemethod on thevalueobject every time the#valuemethod is called (default: false):copy_on_derefWhen given aProcobject theProcwill be run every time the#valuemethod is called. TheProcwill be given the currentvalueas its only argument and the result returned by the block will be the return value of the#valuecall. Whennilthis 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.
See Also
P. Barth, R. Nikhil, and Arvind. M-Structures: Extending a parallel, non- strict, functional language with state. In Proceedings of the 5th ACM Conference on Functional Programming Languages and Computer Architecture (FPCA), 1991.
S. Peyton Jones, A. Gordon, and S. Finne. Concurrent Haskell. In Proceedings of the 23rd Symposium on Principles of Programming Languages (PoPL), 1996.
Constant Summary
- EMPTY =
Unique value that represents that an
MVarwas empty Object.new
- TIMEOUT =
Unique value that represents that an
MVartimed out before it was able to produce a value. Object.new
Instance Method Summary (collapse)
-
- (Object) borrow(timeout = nil)
acquires lock on the from an
MVAR, yields the value to provided block, and release lock. -
- (Boolean) empty?
Returns if the
MVaris currently empty. -
- (Boolean) full?
Returns if the
MVarcurrently contains a value. -
- (MVar) initialize(value = EMPTY, opts = {})
constructor
Create a new
MVar, either empty or with an initial value. -
- (Object) modify(timeout = nil)
Atomically
take, yield the value to a block for transformation, and thenputthe transformed value. -
- (undocumented) modify!
Non-blocking version of
modifythat will yield withEMPTYif there is no value yet. -
- (Object) put(value, timeout = nil)
Put a value into an
MVar, blocking if there is already a value until it is empty. -
- (undocumented) set!(value)
Non-blocking version of
putthat will overwrite an existing value. -
- (Object) take(timeout = nil)
Remove the value from an
MVar, leaving it empty, and blocking if there isn't a value. -
- (undocumented) try_put!(value)
Non-blocking version of
put, that returns whether or not it was successful. -
- (undocumented) try_take!
Non-blocking version of
take, that returnsEMPTYinstead of blocking. -
- (Object) value
(also: #deref)
included
from Concern::Dereferenceable
Return the value this object represents after applying the options specified by the
#set_deref_optionsmethod.
Constructor Details
- (MVar) initialize(value = EMPTY, opts = {})
Create a new MVar, either empty or with an initial value.
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# File 'lib/concurrent/mvar.rb', line 54 def initialize(value = EMPTY, opts = {}) @value = value @mutex = Mutex.new @empty_condition = ConditionVariable.new @full_condition = ConditionVariable.new (opts) end |
Instance Method Details
- (Object) borrow(timeout = nil)
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.
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# File 'lib/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 |
- (Boolean) empty?
Returns if the MVar is currently empty.
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# File 'lib/concurrent/mvar.rb', line 195 def empty? @mutex.synchronize { @value == EMPTY } end |
- (Boolean) full?
Returns if the MVar currently contains a value.
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# File 'lib/concurrent/mvar.rb', line 200 def full? !empty? end |
- (Object) modify(timeout = nil)
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.
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# File 'lib/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 (value) else TIMEOUT end end end |
- (undocumented) modify!
Non-blocking version of modify that will yield with EMPTY if there is no value yet.
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# File 'lib/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 (value) end end |
- (Object) put(value, timeout = nil)
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.
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# File 'lib/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 (value) else TIMEOUT end end end |
- (undocumented) set!(value)
Non-blocking version of put that will overwrite an existing value.
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# File 'lib/concurrent/mvar.rb', line 169 def set!(value) @mutex.synchronize do old_value = @value @value = value @full_condition.signal (old_value) end end |
- (Object) take(timeout = nil)
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.
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# File 'lib/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 (value) else TIMEOUT end end end |
- (undocumented) try_put!(value)
Non-blocking version of put, that returns whether or not it was successful.
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# File 'lib/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 |
- (undocumented) try_take!
Non-blocking version of take, that returns EMPTY instead of blocking.
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# File 'lib/concurrent/mvar.rb', line 142 def try_take! @mutex.synchronize do if unlocked_full? value = @value @value = EMPTY @empty_condition.signal (value) else EMPTY end end end |
- (Object) value Also known as: deref Originally defined in module Concern::Dereferenceable
Return the value this object represents after applying the options specified
by the #set_deref_options method.