Editing Occam (programming language)

{{Short description|Concurrent programming language}}
{{Distinguish|OCaml}}
{{lowercase title|occam (programming language)}}
{{Infobox programming language
| name = occam
| logo = 1983 1988 Trademark occam and occam 2 INMOS Limited.jpg
| paradigm = [[Imperative programming|Imperative]], [[Procedural programming|procedural]], [[Concurrent computing|concurrent]]
| designer = [[David May (computer scientist)|David May]]
| developer = [[Inmos]]
| released = {{Start date and age|1983}}
| latest release version = 2.1 (official), 2.5 (unofficial), 3 (not fully implemented)
| latest release date = {{Start date and age|1994}}
| typing = 
| implementations = 
| dialects = [[occam-π]] (pi)
| influenced by = [[Communicating sequential processes]]
| influenced = [[Ease (programming language)|Ease]], [[Go (programming language)|Go]], [[Python (programming language)|Python]]
}}

'''occam''' is a [[programming language]] which is [[Concurrent computing|concurrent]] and builds on the [[communicating sequential processes]] (CSP) process algebra,<ref name="oc21refman">{{cite book |author=<!--Must be person--> |author-link=Inmos |url=http://www.wotug.org/occam/documentation/oc21refman.pdf |title=occam 2.1 Reference Manual |publisher=SGS-Thomson Microelectronics Ltd |date=1995-05-12}} Inmos document 72 occ 45 03</ref> and shares many of its features. It is named after philosopher [[William of Ockham]] after whom [[Occam's razor]] is named.

Occam is an [[Imperative programming|imperative]] [[Procedural programming|procedural]] language (such as [[Pascal (programming language)|Pascal]]). It was developed by [[David May (computer scientist)|David May]] and others at [[Inmos]] (trademark INMOS), advised by [[Tony Hoare]], as the native programming language for their [[transputer]] [[microprocessor]]s, but implementations for other platforms are available. The most widely known version is occam 2; its programming manual was written by Steven Ericsson-Zenith and others at [[Inmos]].

==Overview==
In the following examples indentation and formatting are critical for parsing the code: expressions are terminated by the end of the line, lists of expressions need to be on the same level of indentation. This feature, named the [[off-side rule]], is also found in other languages such as [[Haskell]] and [[Python (programming language)|Python]].

Communication between processes work through named ''[[Channel (programming)|channels]]''. One process outputs data to a channel via <code>!</code> while another one inputs data with <code>?</code>. Input and output cannot proceed until the other end is ready to accept or offer data. (In the ''not proceeding'' case it is often said that the process ''[[Blocking (computing)|blocks]]'' on the channel. However, the program will neither spin nor poll; thus terms like ''wait'', ''hang'' or ''yield'' may also convey the behaviour; also in the context that it will not ''block'' other independent processes from running.) Examples (c is a variable):

  keyboard ? c

  screen ! c

<code>SEQ</code> introduces a list of expressions that are evaluated sequentially. This is not implicit as it is in most other programming languages. Example:

  SEQ
    x := x + 1
    y := x * x

<code>PAR</code> begins a list of expressions that may be evaluated concurrently. Example:

  PAR
    p()
    q()

<code>ALT</code> specifies a list of ''[[Guard (computer science)|guarded]]'' commands. The guards are a combination of a Boolean condition and an input expression, both optional. Each guard for which the condition is true and the input channel is ready is successful. One of the successful alternatives is selected for execution. Example:

  ALT
    count1 < 100 & c1 ? data
      SEQ
        count1 := count1 + 1
        merged ! data
    count2 < 100 & c2 ? data
      SEQ
        count2 := count2 + 1
        merged ! data
    status ? request
      SEQ
        out ! count1
        out ! count2

This will read data from channels c1 or c2 (whichever is ready) and pass it into a merged channel. If countN reaches 100, reads from the corresponding channel will be disabled. A request on the status channel is answered by outputting the counts to <code>out</code>.

==Language revisions==
[[File:1983 Programming manual occam by INMOS Limited.jpg|thumb|1983 "occam" by INMOS Limited]]

===occam 1===
''occam 1''<ref name="oc1refman">{{cite book |author=<!--Must be person--> |author-link=Inmos |title=occam Programming Manual |publisher=Prentice-Hall |year=1984 |isbn=0-13-629296-8}}</ref> (released 1983) was a preliminary version of the language which borrowed from [[David May (computer scientist)|David May]]'s work on EPL and Tony Hoare's CSP. This supported only the VAR data type, which was an integral type corresponding to the native word length of the target architecture, and arrays of only one dimension.

===occam 2===
''occam 2''<ref name="oc2refman">{{cite book |last=Ericsson-Zenith |first=Steven |title=occam 2 Reference Manual |publisher=Prentice-Hall |year=1988 |isbn=0-13-629312-3}}</ref> is an extension produced by Inmos Ltd in 1987 that adds [[floating-point]] support, functions, multi-dimensional arrays and more data types such as varying sizes of integers (INT16, INT32) and bytes.

With this revision, occam became a language able to express useful programs, whereas occam 1 was more suited to examining algorithms and exploring the new language (however, the occam 1 compiler was written in occam 1,<ref name="cook1">{{cite conference |url=https://books.google.com/books?id=nvnnZtJWAZkC&q=architectures+languages+and+techniques+barry+cook |title= Occam on Field-Programmable Gate Arrays |last1=Cook |first1=Barry M |last2=Peel |first2=RMA |date=1999-04-11 |conference=22nd World Occam and Transputer User Group Technical Meeting |editor-last=Cook |editor-first=Barry M. |book-title=Architectures, Languages and Techniques for Concurrent Systems |publisher=IOS Press |location=Keele, United Kingdom |isbn= 90-5199-480-X |page=219 |access-date=2016-11-28}}</ref> so there is an existence proof that reasonably sized, useful programs could be written in occam 1, despite its limits).

===occam 2.1===
''occam 2.1''<ref name="oc21refman"/> was the last of the series of occam language developments contributed by Inmos. Defined in 1994, it was influenced by an earlier proposal for an '''occam 3''' language (also referred to as "occam91" during its early development) created by Geoff Barrett at Inmos in the early 1990s. A revised Reference Manual describing occam 3 was distributed for community comment,<ref name="occam3">{{cite journal |last1=Barrett |first1=Geoff |last2=Ericsson-Zenith |first2=Steven |title=occam 3 Reference Manual |url=http://www.wotug.org/occam/documentation/oc3refman.pdf |date=1992-03-31 |publisher=[[Inmos]] |access-date=2008-03-24}}</ref> but the language was never fully implemented in a compiler.

occam 2.1 introduced several new features to occam 2, including:
*Named data types (DATA TYPE x IS y)
*Named records
*Packed records
*Relaxation of some of the type conversion rules
*New operators (e.g. BYTESIN)
*Channel retyping and channel arrays
*Ability to return fixed-length array from function.

For a full list of the changes see Appendix P of the [http://www.wotug.org/occam/documentation/oc21refman.pdf Inmos occam 2.1 Reference Manual].

===occam-π===
''[[occam-π]]''<ref name="kroc">{{cite web |url=http://www.cs.kent.ac.uk/projects/ofa/kroc/ |title=occam-pi: Blending the best of CSP and the pi-calculus |last1=Barnes |first1=Fred |last2=Welch |first2=Peter |date=2006-01-14 |access-date=2006-11-24}}</ref> is the common name for the occam variant implemented by later versions of the Kent Retargetable occam Compiler ([[KRoC]]). The addition of the symbol ''[[Pi (letter)|π]]'' (pi) to the occam name is an allusion to KRoC occam including several ideas inspired by the [[π-calculus]]. It contains several significant extensions to the occam 2.1 compiler, for example:
*[[Nesting (computing)|Nested]] [[Protocol (object-oriented programming)|protocols]]
*Run-time process creation
*Mobile channels, data, and processes
*[[Recursion (computer science)|Recursion]]
*Protocol [[Inheritance (object-oriented programming)|inheritance]]
*Array [[Constructor (object-oriented programming)|constructors]]
*Extended [[Barrier (computer science)|rendezvous]]

==See also==
* The XC programming language, which is based on occam but with C-style syntax.
* [[Concurrent computing#Languages supporting concurrent programming|Concurrent programming languages]]
* [[List of concurrent and parallel programming languages]]

==References==
{{Reflist}}

==Further reading==
*{{cite book |title=Communicating Process Architectures 2007 – WoTUG-30 |year=2007 |publisher=IOS Press |isbn=978-1-58603-767-3|pages=513 pages}}  [https://web.archive.org/web/20070927235308/http://www.booksonline.iospress.com/Content/View.aspx?piid=5962]
*{{cite book |title=Communicating Process Architectures 2006 – WoTUG-29 |year=2006 |publisher=IOS Press |isbn=978-1-58603-671-3 |pages=391 pages}}  [https://web.archive.org/web/20070927235321/http://www.booksonline.iospress.com/Content/View.aspx?piid=91]
*{{cite book |title=Communicating Process Architectures 2005 – WoTUG-28 |year=2005 |publisher=IOS Press |isbn=978-1-58603-561-7 |pages=405 pages}}  [https://web.archive.org/web/20070927235351/http://www.booksonline.iospress.com/Content/View.aspx?piid=56]
*{{cite book |editor-last=Kerridge |editor-first=Jon |title=Transputer and Occam Research: New Directions |year=1993 |publisher=IOS Press |isbn=0-8247-0711-7 |pages=[https://archive.org/details/insulintherapy00will/page/253 253 pages] |url-access=registration |url=https://archive.org/details/insulintherapy00will/page/253 }}
*{{cite book |last1=Roscoe |first1=Andrew William |author-link1=Bill Roscoe |last2=Hoare |first2=Charles Antony Richard |author-link2=Tony Hoare |title=The Laws of Occam Programming |year=1986 |publisher=Programming Research Group, Oxford University}}
*Egorov, A., Technical University – Sofia, (1983–2011) '''Записки по Компютърни архитектури'''

== External links ==

* [http://www.wotug.org/occam/ Information, compilers, editors and utilities at the WoTUG occam pages]
* [http://www.wotug.org/parallel/occam/ Compilers, documentation, examples, projects and utilities at the Internet Parallel Computing Archive] (no longer maintained)
* [http://www.transputer.net/obooks/obooks.asp Occam books on Transputer.net]
* [http://www.cs.kent.ac.uk/projects/ofa/kroc/ The occam-pi language].
* [http://projects.cs.kent.ac.uk/projects/tock/trac/ Tock occam compiler] – (translator from occam to C from Kent) a Haskell-based compiler for occam and related languages.

{{Authority control}}

[[Category:Concurrent programming languages]]
[[Category:Procedural programming languages]]
[[Category:Programming languages created in 1983]]