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Objects As Session-Typed Processes Objects as Session-Typed Processes Stephanie Balzer and Frank Pfenning Computer Science Department Carnegie Mellon University Abstract an extensive survey of the object-oriented literature, Arm- A key idea in object-oriented programming is that objects strong [8] identifies 39 concepts generally associated with encapsulate state and interact with each other by message object-oriented programming, such as object, encapsulation, exchange. This perspective suggests a model of computation inheritance, message passing, information hiding, dynamic that is inherently concurrent (to facilitate simultaneous mes- dispatch, reuse, modularization, etc., out of which she dis- sage exchange) and that accounts for the effect of message tills the “quarks” of object-orientation, which are: object and exchange on an object’s state (to express valid sequences of class, encapsulation and abstraction, method and message state transitions). In this paper we show that such a model passing, dynamic dispatch and inheritance. of computation arises naturally from session-based commu- These findings are consistent with the concepts sup- nication. We introduce an object-oriented programming lan- ported by the protagonists of object-oriented languages: guage that has processes as its only objects and employs lin- Simula [19, 20] introduced the notions of an object and ear session types to express the protocols of message ex- a class and substantiated the idea to encapsulate the op- change and to reason about concurrency and state. Based on erations and the data on which they operate in an object. various examples we show that our language supports the Smalltalk [30] put forward the message-passing aspect of typical patterns of object-oriented programming (e.g., en- object-orientation by viewing computation as message ex- capsulation, dynamic dispatch, and subtyping) while guar- change, whereby messages are dispatched dynamically ac- anteeing session fidelity in a concurrent setting. In addition, cording to the actual receiver object. The Actor model im- we show that our language facilitates new forms of expres- plemented this idea in a distributed and concurrent context sion (e.g., type-directed reuse, internal choice), which are where message-passing is the sole means of exchange be- not available in current object-oriented languages. We have tween actors [6, 34]. implemented our language in a prototype compiler. Viewing computation as the exchange of messages be- tween stateful objects seems to be a key idea in object- Categories and Subject Descriptors D.1 [Programming oriented programming. Alan Kay phrased this view in a Techniques]: Concurrent Programming—Parallel program- publicly available email exchange on the meaning of object- ming; D.1 [Programming Techniques]: Object-oriented oriented programming as follows: “OOP to me means only Programming messaging, local retention and protection and hiding of Keywords object, session types, linear types, process, pro- state-process, and extreme late-binding of all things. It can tocol be done in Smalltalk and in LISP. There are possibly other systems in which this is possible, but I’m not aware of them.” 1. Introduction ([42]). This view suggests computation to be inherently con- current, as an object may simultaneously exchange mes- Since its inception in the 1960s [19, 20] and 1970s [30], sages with several objects, and the expression of the proto- object-oriented programming has become a ubiquitous pro- cols that govern message exchange. gramming model. A multitude of object-oriented languages Concurrency support in mainstream object-oriented lan- have emerged since, each with their own characteristics. In guages is the subject of an active area of research. Main- stream object-oriented languages typically support concur- Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed rency by purely operational synchronization idioms (e.g., syn- for profit or commercial advantage and that copies bear this notice and the full citation chronized statements/methods), but leave it to the program- on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, mer to ensure thread-safety of a class and absence of data to post on servers or to redistribute to lists, requires prior specific permission and/or a races. To alleviate the burden put on programmers, type sys- fee. Request permissions from [email protected]. tem extensions have been suggested that, for example, use 5th International Workshop on Programming based on Actors, Agents, and Decentral- ized Control (AGERE!) 2015, October 26, 2015, Pittsburgh, PA, USA. ownership types [18] to guarantee absence of data races and Copyright c 2015 ACM 978-1-nnnn-nnnn-n/yy/mm. $15.00. deadlocks [13] or the notion of an interval to make the or- http://dx.doi.org/10.1145/nnnnnnn.nnnnnnn dering between threads explicit for the prevention of data Session Type Behavior races [44]. Of particular concern are also high-level data !τ; σ Value output: send value v of type τ, continue as races, whose prevention is possible by means of a static process of type σ. analysis [59]. Moreover, a wide range of tools have been de- ?τ; σ Value input: receive value v of type τ, continue as process of type σ. veloped for static [49] and dynamic [25] data race detection. !σ1; σ2 Channel output: send channel of type σ1, continue Considerable effort has been devoted to protocol expres- as process of type σ2. sion in object-oriented languages. Work on typestate [11, 22] ?σ1; σ2 Channel input: receive channel of type σ1, continue allows programmers to annotate methods with the receiver as process of type σ2. !fl1 : σ1;:::; ln : σn g Internal choice: send label li , continue as process of object’s typestate in pre- and post-conditions. To check ad- type σi . herence to the protocols expressed in this way, static pro- ?fl1 : σ1;:::; ln : σn g External choice: receive label li , continue as process gram analysis techniques are employed [12, 22] that use of type σi . some means to control aliasing, such as fractional permis- Terminate process. sions [14]. Another line of research adopts session types [36, µt. σ Recursive session. 37] for protocol expression in a concurrent context and in- Table 1. Linear session types. corporates session types into existing object-oriented lan- guages [23, 39, 40] based on a linear treatment of channels. In this paper, we take a different approach to supporting Structure: Section 2 provides a short introduction to lin- concurrency and expressing protocols. Rather than extend- ear session types. Section 3 introduces CLOO and the basic ing existing object-oriented languages with concepts nec- correspondence between session-typed communication and essary to address the challenges, we derive a new model object-oriented programming. Section 4 discusses the prop- of object-oriented programming that internalizes the semi- erties of session fidelity, data race freedom, and deadlock nal idea of object-orientation that computation is concurrent freedom. Section 5 elaborates on the new forms of expres- message exchange between stateful objects. Our model takes sion available in CLOO. Section 6 provides a discussion of linear session types as its foundation. This choice is moti- encapsulation and aliasing and an outlook on future work. vated by the observation that object-oriented programming Section 7 summarizes related work, and Section 8 concludes arises naturally from session-typed communication and fa- the paper. cilitates program reasoning, because linear session types guarantee session fidelity and freedom of data races and 2. Background deadlocks. Session types [36, 37] prescribe the interaction behavior We present our concurrent object-oriented language CLOO of processes that communicate along channels, connecting (Concurrent Linear Object-Orientation). In CLOO, pro- an offering process with its client process. A session type cesses are the only objects, and objects interact with each thus governs the protocol of message exchange. Table 1 other by sending messages along channels. Message ex- provides an overview of the kinds of protocols that can be change is asynchronous, and an object is identified with the expressed using session types. For example, the session type channel along which it exchanges messages with its clients. !int;?int; requires sending an integer, then receiving an Objects (and their channels) are typed with session types, integer, and then terminating the session. Besides integers which define the protocol of message exchange. Protocol and other basic data values, processes can also send channels compliance is enforced by CLOO’s type system, relying on along channels (as in the π-calculus [46]) and can offer a linear treatment of channels. external and internal choices. An external choice ?fl1 : An important concern in the development of CLOO was σ1;:::; ln : σn g means to receive one of the labels li and to facilitate program reasoning while maintaining a gen- then behave as prescribed by σi . Conversely, an internal uine object-oriented programming style. Based on various choice !fl1 : σ1;:::; ln : σn g will send one of the labels li programming examples, we show that CLOO supports the and continue as σi . Session types can be recursive, allowing typical object-oriented concepts, such as encapsulation, dy- for protocol repetition. namic dispatch, and subtyping. In addition, we show that To guarantee that the interaction between a client process CLOO enables new forms of program expression, such as and an offering process indeed follows the protocol defined type-directed code reuse and internal choice, concepts not by the session type, aliasing of channels must be controlled. existing in current object-oriented languages. Whilst type- We treat channels linearly [29], making channels owned by directed reuse supports program extensibility in a similar their client process, with ownership transfer being possible way as inheritance, it does not suffer from the modular- by passing a channel to another process.
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