Synchronous programming language: Difference between revisions

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{{lead too long|date=April 2012}}

A '''synchronous programming language''' is a [[computer]] [[computer programming|programming]] [[programming language|language]] optimized for programming '''reactive systems'''. Computer systems can be sorted in three main classes: (1) '''transformational systems''' that take some inputs, process them, deliver their outputs, and terminate their execution; a typical example is a compiler; (2) '''interactive systems''' that interact continuously with their environment, at their own speed; a typical example is the web; and (3) reactive systems that interact continuously with their environment, at a speed imposed by the environment; a typical example is the automatic flight control system of modern airplanes. Reactive systems must therefore react to stimuli from the environment within strict time bounds. For this reason they are often also called [[Real-time computing|real-time systems]], and are found often in [[Embeddedembedded system|embedded systems]]s.

'''Synchronous programming''' (also '''synchronous reactive programming''' or '''SRP''') is a computer programming [[Programming paradigm|paradigm]] supported by synchronous programming languages. The principle of SRP is to make the same abstraction for programming languages as the synchronous abstraction in digital circuits. Synchronous circuits are indeed designed at a high-level of abstraction where the timing characteristics of the electronic transistors are neglected. Each gate of the circuit (or, and, ...) is therefore assumed to compute its result instantaneously, each wire it assumed to transmit it signal instantaneously. A synchronous circuit is clocked and at each tick of its clock, it computes instantaneously its output values and the new values of its memory cells (latches) from the its input values and the current values of its memory cells. In other words, the circuit behaves as if as if the electrons were flowing infinitely fast. The first synchronous programming languages were invented in France in the 80s: [[Esterel]], [[Lustre_Lustre (programming_languageprogramming language)|Lustre]] and [[SIGNAL_SIGNAL (programming_languageprogramming language)|Signal]]. Since then, many other synchronous languages have emerged.

The synchronous abstraction makes reasoning about time in a synchronous program a lot easier, thanks to the notion of '''logical ticks''': a synchronous program reacts to its environment in a sequence of ticks, and computations within a tick are assumed to be instantaneous, i.e., as if the processor executing them was infinitely fast. The statement “'''a||b'''” is therefore abstracted as the package “'''ab'''” where “'''a'''” and “'''b'''” are simultaneous. To take a concrete exempleexample, the Esterel statement “'''every 60 second emit minute'''” specifies that the signal “'''minute'''” is exactly synchronous with the 60-th occurrence of the signal “'''second'''”. At a more fundamental level, the synchronous abstraction eliminates the non-determinism resulting from the interleaving of concurrent behaviors. This allows '''deterministic''' semantics, therefore making synchronous programs amenable to formal analysis, verification and certified code generation, and usable as formal specification formalisms.

In contrast, in the asynchronous model of computation, on a sequential processor, the statement “'''a||b'''” can be either implemented as "'''a;b'''" or as "'''b;a'''". This is known as the '''interleaving-based non determinism'''. The drawback is that it intrinsically forbids deterministic semantics (e.g., race conditions), which makes formal reasoning such as analysis and verification more complex. Nonetheless, asynchronous formalisms are very useful to model, design and verify distributed systems, because they are intrinsically asynchronous.

Also in contrast are systems with processes that basically ''interact synchronously''. An example would be systems built based on the [[Communicating_sequential_processesCommunicating sequential processes|Communicating sequential processes (CSP)]] model, which also allows nondeterministic choice.

==Implementations==

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==Synchronous languages==

*[[Argos programming language|Argos]]

*[[Atom_Atom (programming_languageprogramming language)|Atom]] (a DSL in [[Haskell_Haskell (programming_languageprogramming language)|Haskell]] for hard realtime embedded programming)

*[[Averest]]

*[[ChucK]] (a synchronous reactive programming language for audio)

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*[[Secure Operations Language|SOL]]

*[[SyncCharts]]

== References ==

* Nicolas Halbwachs. "Synchronous programming of reactive systems". Kluwer Academic Publishers, 1993. http://www-verimag.imag.fr/~halbwach/newbook.pdf

{{Reflist}}

== External links ==

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*[http://www.irisa.fr/espresso/Polychrony/ The SIGNAL programming language].

*[http://www.ece.purdue.edu/~hankd/CARP/XPC/paper.html Unification of Synchronous and Asynchronous Models for Parallel Programming Languages] {{Dead link|date=February 2012}} -- Proposes [[parallel languages]] based on [[C (programming language)|C]], lets programmers specify and manage parallelism on a broad range of computer architectures.

== References ==

* Nicolas Halbwachs. "Synchronous programming of reactive systems". Kluwer Academic Publishers, 1993. http://www-verimag.imag.fr/~halbwach/newbook.pdf

{{Reflist}}

==See also==

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[[Category:Synchronous programming languages| Synchronous programming language]]

[[Category:Programming language classification]]

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[[de:Synchrone Programmiersprache]]