Occam (programming Language) - Overview

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.

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.