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Concept
Occam (programming language)
Summary
occam is a programming language which is concurrent and builds on the communicating sequential processes (CSP) process algebra, and shares many of its features. It is named after philosopher William of Ockham after whom Occam's razor is named.
occam is an imperative procedural language (such as Pascal). It was developed by David May and others at Inmos (trademark INMOS), advised by Tony Hoare, as the native programming language for their transputer microprocessors, but implementations for other platforms are available. The most widely known version is occam 2; its programming manual was written by Steven Ericsson-Zenith and others at Inmos.
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 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 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
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.
Official source
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