Occam programming language

Occam is a parallel programming language developed by Inmos for their line of Transputers. Implementations for other platforms exist as well. (The language's name refers to William of Ockham, responsible for Occam's Razor.)

Occam builds on Communicating Sequential Processes (CSP) and shares many of the same features. It is, in a way, a practical implementation of CSP. Occam is a procedural language, similar to Pascal etc.

One feature (also found in Python) necessary to understand the examples below is that indentation and formatting is critical: expressions are terminated by the end of the line, lists of expressions need to be on the same level of indentation.

The most interesting language constructs are described in the following.

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.

Documentation and implementations are available from the Internet Parallel Computing Archive: Occam