Editing Occam (programming language) (section)

==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 (programming language)|Python]].

Communication between processes work through named ''[[Channel (programming)|channels]]''. One process outputs data to a channel via <code>!</code> while another one inputs data with <code>?</code>. Input and output cannot proceed until the other end is ready to accept or offer data. (In the ''not proceeding'' case it is often said that the process ''[[Blocking (computing)|blocks]]'' on the channel. However, the program will neither spin nor poll; thus terms like ''wait'', ''hang'' or ''yield'' may also convey the behaviour; also in the context that it will not ''block'' other independent processes from running.) Examples (c is a variable):

  keyboard ? c

  screen ! c

<code>SEQ</code> 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

<code>PAR</code> begins a list of expressions that may be evaluated concurrently. Example:

  PAR
    p()
    q()

<code>ALT</code> specifies a list of ''[[Guard (computer science)|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 <code>out</code>.