475:). It certainly is the case that implementations of the Actor model typically make use of these hardware capabilities. However, there is no reason that the model could not be implemented directly in hardware without exposing any hardware threads and locks. Also, there is no necessary relationship between the number of Actors, threads, and locks that might be involved in a computation. Implementations of the Actor model are free to make use of threads and locks in any way that is compatible with the laws for Actors.
22:
139:. One issue is what can be observed about Actor systems. The question does not have an obvious answer because it poses both theoretical and observational challenges similar to those that had arisen in constructing the foundations of quantum physics. In concrete terms for Actor systems, typically we cannot observe the details by which the arrival order of messages for an Actor is determined (see
441:
were widely used to model nondeterministic computation. However, they were widely acknowledged to have an important limitation: they modeled control flow but not data flow. Consequently, they were not readily composable, thereby limiting their modularity. Hewitt pointed out another difficulty with
263:
pioneered using message passing for computation, motivated by discrete event simulation applications. These applications had become large and unmodular in previous simulation languages. At each time step, a large central program would have to go through and update the state of each simulation object
110:
introduced situation variables in logic in the
Situational Calculus. In McCarthy and Hayes 1969, a situation is defined as "the complete state of the universe at an instant of time." In this respect, the situations of McCarthy are not suitable for use in the Actor model since it has no global states.
370:
The first (token) encountered (in a program) was looked up in the dynamic context, to determine the receiver of the subsequent message. The name lookup began with the class dictionary of the current activation. Failing there, it moved to the sender of that activation and so on up the sender chain.
376:
Thus the message-passing model in
Smalltalk-72 was closely tied to a particular machine model and programming-language syntax that did not lend itself to concurrency. Also, although the system was bootstrapped on itself, the language constructs were not formally defined as objects that respond to
114:
From the definition of an Actor, it can be seen that numerous events take place: local decisions, creating Actors, sending messages, receiving messages, and designating how to respond to the next message received. Partial orderings on such events have been axiomatized in the Actor model and their
450:
disappear from the input places of a transition and appear in the output places. The physical basis of using a primitive with this kind of simultaneity seemed questionable to him. Despite these apparent difficulties, Petri nets continue to be a popular approach to modelling concurrency, and are
429:. Meanwhile, the Actor efforts at MIT remained focused on developing the science and engineering of higher level concurrency. (See the paper by Jean-Pierre Briot for ideas that were developed later on how to incorporate some kinds of Actor concurrency into later versions of Smalltalk.)
98:
A fundamental challenge in defining the Actor model is that it did not provide for global states so that a computational step could not be defined as going from one global state to the next global state as had been done in all previous models of computation.
541:
messages, the program script of an Actor would itself have a program script (which in turn would have ...)! Another consideration was that some Actors would not use interpretation in their actual interpretation.
360:
model of computation used for teaching children to program. However, the message passing of
Smalltalk-72 was quite complex. Code in the language was viewed by the interpreter as simply a stream of tokens. As
236:
in which the values of parameters were substituted into the body of an invoked lambda expression. The substitution model is unsuitable for concurrency because it does not allow the capability of
248:
made use of a data structure called an environment so that the values of parameters did not have to be substituted into the body of an invoked lambda expression. This allowed for sharing of the
494:
argued against including such queues as an integral part of the Actor model. One consideration was that such queues could themselves be modeled as Actors that received messages to
425:
The
Smalltalk system went on to become very influential, innovating in bitmap displays, personal computing, the class browser interface, and many other ways. For details see Kay's
381:
messages (see discussion below). This led some to believe that a new mathematical model of concurrent computation based on message passing should be simpler than
Smalltalk-72.
514:
of communications that have been received by an Actor. But this sequence emerged only as the Actor operated. In fact the ordering of this sequence can be indeterminate (see
183:
127:
According to Hewitt (2006), the Actor model is based on physics in contrast with other models of computation that were based on mathematical logic, set theory, algebra,
525:: "Should interpretation be an integral part of the Actor model?" The idea of interpretation is that an Actor would be defined by how its program script processed
371:
When a binding was finally found for the token, its value became the receiver of a new message, and the interpreter activated the code for that object's class.
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the concurrency expressed in the Actor model. On the other hand, the Actor model is capable of expressing all of the parallelism in the lambda calculus.
698:
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314:-71. Hewitt was intrigued by Smalltalk-71 but was put off by the complexity of communication that included invocations with many fields including
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were identical in implementation." According to Hewitt, the lambda calculus is capable of expressing some kinds of parallelism but, in general,
873:
Hewitt, Carl; Bishop, Peter; Greif, Irene; Smith, Brian; Matson, Todd; Steiger, Richard (January 1974). "Actor induction and meta-evaluation".
43:
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of Simula in the message-passing structure of programs. However
Smalltalk-72 made primitives such as integers, floating point numbers,
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107:
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written in Lisp.) Hewitt argued against making interpretation integral to the Actor model. One consideration was that to process the
921:
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65:
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396:. The authors of Simula had considered making such primitives into objects but refrained largely for efficiency reasons.
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the communications. Another consideration was that some Actors would not use such queues in their actual implementation.
1516:
307:
148:
1472:
Fuchs, Christopher (2002). "Quantum mechanics as quantum information (and only a little more)". In A. Khrenikov (ed.).
1455:
Hewitt, Carl; Lieberman, Henry (November 1983). "Design Issues in
Parallel Architecture for Artificial Intelligence".
397:
1239:
Lieberman, Henry (May 1981). "Thinking About Lots of Things at Once without
Getting Confused: Parallelism in Act 1".
831:
McCarthy, John; Hayes, Patrick (1969). "Some
Philosophical Problems from the Standpoint of Artificial Intelligence".
408:
programming language (and later versions of Java, starting with Java 1.5) adopted the less elegant solution of using
36:
30:
724:
186:). For example, the lambda expression below implements a tree data structure when supplied with parameters for a
729:
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468:
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419:
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393:
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349:
277:
273:
245:
47:
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Hewitt, Carl; de Jong, Peter (August 1983). "Analyzing the Roles of
Descriptions and Actions in Open Systems".
1338:
Lieberman, Henry; Hewitt, Carl (June 1983). "A real Time Garbage Collector Based on the Lifetimes of Objects".
686:
281:
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at first used the expedient of having both primitive and object versions of integers, floating point numbers,
875:
Proceedings of the 1st annual ACM SIGACT-SIGPLAN symposium on Principles of programming languages - POPL '73
522:
464:
241:
103:
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Symposium on the Foundations of Modern Physics: 50 years of the Einstein-Podolsky-Rosen Gedanken experiment
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published a paper on specification and proof techniques for serializers providing an efficient solution to
280:
that would update its own local state based on messages from other objects. In addition they introduced a
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Using the laws of the Actor model, Hewitt and Baker proved that any Actor that behaves like a function is
487:
151:. Instead of observing the insides of arbitration processes of Actor computations, we await the outcomes.
87:
1427:
Jammer, M. (1985). "The EPR Problem in Its Historical Development". In P. Lahti, P. Mittelstaedt (ed.).
801:
563:
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an Actor might be implemented in hardware instead. Of course there is nothing wrong with interpretation
507:
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that changed depending on the state of whichever simulation objects it interacted with on that step.
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Proceedings of the 2nd ACM SIGACT-SIGPLAN symposium on Principles of programming languages - POPL '75
776:
179:
136:
1352:
1482:
1120:
Hewitt, Carl; Atkinson, Russ (January 1979). "Specification and Proof Techniques for Serializers".
883:
841:
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646:
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Aki Yonezawa published his specification and verification techniques for Actors. Russ Atkinson and
357:
1043:
Specification and Verification Techniques for Parallel Programs Based on Message Passing Semantics
1365:
1227:
1143:
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896:
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Nevertheless, progress developing the model was steady. In 1975, Irene Greif published the first
233:
116:
1171:
Hewitt, Carl; Attardi, Beppe; Lieberman, Henry (October 1979). "Delegation in Message Passing".
483:
An important challenge in defining the Actor model was to abstract away implementation details.
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859:
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Finally eight years after the first Actor publication, Will Clinger (building on the work of
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Subsequent versions of the Smalltalk language largely followed the path of using the virtual
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messages is more modular and extensible than the monolithic interpreter approach of Lisp.
265:
176:
168:
143:). Attempting to do so affects the results and can even push the indeterminacy elsewhere.
348:
In 1972 Kay visited MIT and discussed some of his ideas for Smalltalk-72 building on the
1073:
Hewitt, Carl (June 1977). "Viewing Control Structures as Patterns of Passing Messages".
1024:
Hewitt, Carl; Baker, Henry (August 1977). "Laws for Communicating Parallel Processes".
670:
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in which its communications are stored until received by the Actor to be processed?"
172:
1369:
1147:
948:
900:
1377:
Theriault, Daniel (June 1983). "Issues in the Design and Implementation of Act 2".
1231:
1016:
697:). Subsequently, Hewitt augmented the diagrams with arrival times to construct a
1313:
958:
Proceedings of the 4th International Joint Conference on Artificial Intelligence
678:
658:
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602:
491:
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Prior to the Actor model, concurrency was defined in low-level machine terms of
446:, the atomic step of computation in Petri nets is a transition in which tokens
362:
83:
1213:
622:
521:
Another example of abstracting away implementation detail was the question of
991:
Proceedings of the Symposium on Artificial Intelligence Programming Languages
585:
interpreter in which they concluded "we discovered that the 'actors' and the
1129:
931:
Greif, Irene; Hewitt, Carl (January 1975). "Actor semantics of PLANNER-73".
858:. Translated by A. J. Pomerans. New York: Harper & Row. pp. 63โ64.
793:
438:
311:
292:
909:
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developed the idea (first described in an IFIP workshop in 1967) of having
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messages. (In this way Actors would be defined in a manner analogous to
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was influenced by message passing in the pattern-directed invocation of
252:
of updating shared data structures but did not provide for concurrency.
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288:. Their innovations considerably improved the modularity of programs.
237:
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For example, consider the following question: "Does each Actor have a
510:
instead. Of course, there is a mathematical abstraction which is the
260:
1173:
Proceedings of First International Conference on Distributed Systems
1321:
Theriault, Daniel (April 1982). "A Primer for the Act-1 Language".
533:
which was "defined" by a meta-circular interpreter procedure named
232:
However, the semantics of the lambda calculus were expressed using
1403:
Conference of the American Association for Artificial Intelligence
1412:
Proceedings of the National Conference on Artificial Intelligence
794:"A Universal Modular Actor Formalism for Artificial Intelligence"
581:
then took an interest in Actors and published a paper on their
1310:
1294:
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1185:
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1109:
1062:
1059:
Very Large Address Space Modularly Extensible Computer Systems
1046:
974:
822:
McCarthy, John (1963). "Situations, actions and causal laws".
15:
240:
of changing resources. Inspired by the lambda calculus, the
826:(2). Stanford University Artificial Intelligence Laboratory.
131:
Physics influenced the Actor model in many ways, especially
956:
Hewitt, Carl (September 1975). "How to Use What You Know".
1483:"What is Commitment? Physical, Organizational, and Social"
159:
The Actor model builds on previous models of computation.
910:"Behavioral Semantics of Nonrecursive Control Structure"
1438:
The Shaky Game: Einstein Realism and the Quantum Theory
601:
Two years after Greif published her operational model,
1275:
Reasoning about Change in Knowledgeable Office Systems
86:, first published in 1973, is a mathematical model of
1256:
Lieberman, Henry (June 1981). "A Preview of Act 1".
681:
1979) published the first satisfactory mathematical
1026:
International Federation for Information Processing
792:Carl Hewitt; Peter Bishop; Richard Steiger (1973).
418:, a variant of which had been used earlier in some
1202:IEEE Transactions on Systems, Man, and Cybernetics
851:
987:"The Incremental Garbage Collection of Processes"
653:Mathematical characterization using domain theory
194:. When such a tree is given a parameter message
1431:. Singapore: World Scientific. pp. 129โ149.
854:Physics and Beyond: Encounters and Conversations
295:control structure instead of true concurrency.
213:ฮป(leftSubTree,rightSubTree) ฮป(message)
1474:Quantum Theory: Reconstruction of Foundations
1396:"An Object-Oriented Simulator for the Apiary"
1193:Kornfeld, Bill; Hewitt, Carl (January 1981).
971:Semantics of Communicating Parallel Professes
437:Prior to the development of the Actor model,
8:
914:Proceedings of Colloque Sur la Programmation
613:Proof of continuity of computable functions
985:Baker, Henry; Hewitt, Carl (August 1977).
835:(4). Edunburgh University Press: 463โ502.
1351:
1221:
1137:
1122:IEEE Transactions on Software Engineering
1094:
1006:
882:
840:
66:Learn how and when to remove this message
1440:. Chicago: University of Chicago Press.
29:This article includes a list of general
1106:Actor Systems for Real-Time Computation
741:
715:Actor model and process calculi history
550:. Also implementing interpreters using
516:Indeterminacy in concurrent computation
479:Abstracting away implementation details
182:(see Hewitt, Bishop, and Steiger 1973;
141:Indeterminacy in concurrent computation
810:
799:
699:technically simpler denotational model
455:Threads, locks, and buffers (channels)
451:still the subject of active research.
115:relationship to physics explored (see
1182:Automatic Verification of Serializers
7:
202:and likewise when given the message
1195:"The Scientific Community Metaphor"
1157:A Computational Theory of Animation
673:, Lehmann, and de Roever 1979, and
442:Petri nets: simultaneous action.
94:Event orderings versus global state
1075:Journal of Artificial Intelligence
701:that is easier to understand. See
35:it lacks sufficient corresponding
14:
720:History of denotational semantics
703:History of denotational semantics
693:in his dissertation in 1981 (see
506:an Actor might have a network of
1476:. Vรคxjo: Vรคxjo University Press.
1394:Lieberman, Henry (August 1983).
754:"The Early History of Smalltalk"
20:
1481:Hewitt, Carl (April 27, 2006).
609:published the Laws for Actors.
155:Models prior to the Actor model
1507:Actor model (computer science)
1307:Foundations of Actor Semantics
1291:Parallelism in Problem Solving
427:The Early History of Smalltalk
175:can be viewed as the earliest
1:
244:for the programming language
1087:10.1016/0004-3702(77)90033-9
149:metastability in electronics
908:Hewitt, Carl (April 1974).
850:Heisenberg, Werner (1971).
566:model in her dissertation.
1533:
725:Actor model middle history
665:1976, Michael Smyth 1978,
1340:Communications of the ACM
1214:10.1109/TSMC.1981.4308575
730:Actor model later history
633:Specifications and proofs
687:unbounded nondeterminism
621:in the sense defined by
225:(message == "getRight")
184:Abelson and Sussman 1985
102:In 1963 in the field of
1379:MIT AI Technical Report
1289:Kornfeld, Bill (1981).
1180:Atkinson, Russ (1980).
1130:10.1109/TSE.1979.234149
217:(message == "getLeft")
123:Relationship to physics
104:Artificial Intelligence
50:more precise citations.
1305:Clinger, Will (1981).
1273:Barber, Gerry (1981).
1057:Bishop, Peter (1977).
1041:Yonezawa, Aki (1977).
809:Cite journal requires
627:denotational semantics
88:concurrent computation
1362:10.1145/358141.358147
1104:Baker, Henry (1978).
999:10.1145/800228.806932
969:Greif, Irene (1975).
941:10.1145/512976.512984
893:10.1145/512927.512942
824:Technical Report Memo
773:10.1145/155360.155364
645:shared resources for
291:However, Simula used
234:variable substitution
1512:History of computing
877:. pp. 153โ168.
833:Machine Intelligence
685:model incorporating
365:later described it:
180:programming language
137:relativistic physics
1517:History of software
1405:. Washington, D. C.
761:ACM SIGPLAN Notices
647:concurrency control
1155:Kahn, Ken (1979).
935:. pp. 67โ77.
587:lambda expressions
117:Actor model theory
1436:Fine, A. (1986).
1175:. Huntsville, AL.
796:. IJCAI: 235โ245.
558:Operational model
422:implementations.
340:operator selector
284:for objects with
221:leftSubTree
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777:the original
767:(3): 69โ75.
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1465:1721.1/5653
1457:MIT AI Memo
1420:1721.1/5649
1387:1721.1/6940
1331:1721.1/5675
1323:MIT AI Memo
1314:Mathematics
1266:1721.1/6350
1258:MIT AI Memo
1249:1721.1/6351
1241:MIT AI Memo
1223:1721.1/5693
1139:1721.1/5756
1096:1721.1/6272
916:: 385โ407.
667:Henry Baker
659:Irene Greif
639:Carl Hewitt
607:Henry Baker
603:Carl Hewitt
564:operational
492:Carl Hewitt
363:Dan Ingalls
328:reply-style
286:inheritance
242:interpreter
206:it returns
200:leftSubTree
188:leftSubTree
84:Actor model
48:introducing
1501:Categories
1490:COIN@AAMAS
1346:(6): 419.
964:: 189โ198.
736:References
623:Dana Scott
619:continuous
579:Guy Steele
439:Petri nets
433:Petri nets
204:"getRight"
31:references
1348:CiteSeerX
1309:(Ph.D.).
1293:(Ph.D.).
1277:(Ph.D.).
1208:: 24โ33.
1184:(Ph.D.).
1159:(Ph.D.).
1124:: 10โ23.
1108:(Ph.D.).
1061:(Ph.D.).
1045:(Ph.D.).
993:: 55โ59.
973:(Ph.D.).
879:CiteSeerX
837:CiteSeerX
750:Kay, Alan
312:Smalltalk
299:Smalltalk
293:coroutine
261:Simula 67
196:"getLeft"
1370:14161480
1148:15272353
949:18178340
901:33611569
709:See also
512:sequence
508:arbiters
473:channels
416:unboxing
356:and the
352:work of
324:receiver
304:Alan Kay
276:on each
1459:(750).
1381:(728).
1325:(672).
1260:(625).
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1232:1322857
1017:1557419
500:dequeue
496:enqueue
469:buffers
461:threads
394:objects
386:methods
308:Planner
274:methods
250:effects
238:sharing
223:else if
44:improve
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689:using
679:Milnor
661:1975,
583:Scheme
570:Scheme
548:per se
411:boxing
332:status
320:sender
316:global
278:object
256:Simula
147:, see
82:, the
33:, but
1486:(PDF)
1399:(PDF)
1366:S2CID
1228:S2CID
1198:(PDF)
1144:S2CID
1013:S2CID
945:S2CID
897:S2CID
780:(PDF)
757:(PDF)
675:Milne
671:Hoare
625:(see
544:E.g.,
504:E.g.,
488:queue
465:locks
392:into
336:reply
1442:ISBN
1298:EECS
1282:EECS
1164:EECS
1113:EECS
1066:EECS
1050:EECS
978:EECS
918:ISBN
860:ISBN
815:help
677:and
605:and
577:and
552:eval
539:eval
535:eval
531:Lisp
527:eval
498:and
467:and
444:I.e.
420:Lisp
414:and
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402:etc.
398:Java
390:etc.
379:Eval
350:Logo
344:etc.
268:and
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227:then
219:then
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145:e.g.
135:and
129:etc.
1461:hdl
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1358:doi
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1161:MIT
1134:hdl
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