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 ? cSEQ introduces a list of expressions that are evaluated sequentially. This is not implicit as it is in most other programming languages. Example:
screen ! c
SEQ x := x + 1 y := x * xPAR begins a list of expressions that may be evaluated concurrently. Example:
PAR x := x + 1 y := y * 2ALT 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 ! count2This 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