Occam (programming language)

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The correct title of this article is occam (programming language). The initial letter is shown capitalized due to technical restrictions.
Not to be confused with the OCaml programming language.
occam
Paradigm: concurrent
Appeared in: 1983
Developer: INMOS
Dialects: occam-pi
Influenced by: Communicating Sequential Processes

occam is a concurrent programming language that builds on the Communicating Sequential Processes (CSP) formalism,[1] and shares many of its features. It is named after William of Ockham of Occam's Razor fame. occam is, in a way, a practical executable implementation of CSP.

occam is an imperative procedural language (such as Pascal). It was developed by INMOS as the native programming language for their line of transputer microprocessors, but implementations for other platforms exist as well.

Contents

[edit] 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, also known as the off-side rule, is also found in other languages).

Communication between processes work through named channels. One process outputs data to a channel via ! while another one inputs data with ?. Input and output will block until the other end is ready to accept or offer data. Examples (c is a variable):

 keyboard ? c

 screen ! c

SEQ 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

PAR begins a list of expressions that may be evaluated concurrently. Example:

 PAR
   p()
   q()

ALT specifies a list of 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 out.

[edit] Language revisions

[edit] occam 1

occam 1[2] (released 1983) was a preliminary version of the language. 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.

[edit] occam 2

occam 2[3] 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 capable of expressing useful programs, whereas occam 1 was more suited to examining algorithms and exploring the new language.

[edit] occam 2.1

occam 2.1[1] is a second enhancement to occam, defined in 1994 by INMOS. It was the last of the series of language developments contributed by INMOS. Despite its "incremental" version number, it was a significant upgrade in the usefulness of the language, introducing:

  • 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 INMOS occam 2.1 Reference Manual.

[edit] occam-pi

occam-pi[4] is the common name for the occam variant implemented by later versions of KRoC, the Kent Retargetable occam Compiler. The addition of the word "pi" to the occam name is an allusion to the fact that KRoC occam includes several ideas inspired by the pi-calculus. It contains a significant number of extensions to the occam 2.1 compiler, for example:

  • nested protocols
  • run-time process creation
  • mobile channels, data, and processes
  • recursion
  • protocol inheritance
  • array constructors
  • extended rendezvous

[edit] See also

[edit] References

  1. ^ a b INMOS (1995-05-12). occam 2.1 Reference Manual. SGS-THOMSON Microelectronics Ltd. INMOS document 72 occ 45 03
  2. ^ INMOS (1984). occam Programming Manual. Prentice-Hall. ISBN 0-13-629296-8.
  3. ^ Ericsson-Zenith (1988). occam 2 Reference Manual. Prentice-Hall. ISBN 0-13-629312-3.
  4. ^ Fred Barnes and Peter Welch (2006-01-14). occam-pi: blending the best of CSP and the pi-calculus. Retrieved on November 24, 2006.

[edit] Further reading

  • Kerridge, John; (ed.) (1993). Transputer and Occam Research: New Directions. IOS Press. ISBN 0-8247-0711-7.
  • Roscoe, A W; C A R Hoare (1986). The Laws of Occam Programming. Programming Research Group, Oxford University.

[edit] External links

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

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