Occam (from William of Ockham of Occam's Razor fame) is a parallel programming language that builds on Communicating Sequential Processes (CSP) and shares many of their features. It is, in a way, a practical executable implementation of CSP.

Occam is an imperative procedural language (such as Pascal). Occam was developed by Inmos for their line of Transputers, but implementations for other platforms exist as well.

Overview

Note that in the examples below 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 is also found in other languages, such as Python, Haskell, Icon, NGL, Miranda, and ABC).

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
   x := x + 1
   y := y * 2

ALT specifies a list of guarded commands. The guards are 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.

Occam 2

Occam 2 is an extension produced in 1987. Occam 2 adds floating-point, functions and a type system.

["occam 2 Reference Manual", INMOS, P-H 1988, ISBN 0-13-629312-3].

External links

• The Occam Archive
• Internet Parallel Computing Archive: Occam (Documentation and implementations)