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|Packs (add-ons) for SWI-Prolog|
|Title:||Concurrency via join calculus|
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|Author:||Michael Hendricks <email@example.com>|
|Maintainer:||Michael Hendricks <firstname.lastname@example.org>|
|Packager:||Michael Hendricks <email@example.com>|
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:- use_module(library(jolog)). % Prolog runs this predicate automatically main :- writeln('Starting Jolog'), % start Jolog, block until it's done start_jolog(user). % Jolog runs this pattern when it starts main &- writeln('In Jolog main'), then, ( send(hello) % in one parallel process & sleep(1), % in another parallel process send(world) ). % Once there are messages on both the 'hello' and 'world' channels world, hello &- writeln('Hello, World!').
which generates the following output with a small delay between the second and third lines.
Starting Jolog In Jolog main Hello, World!
WARNING: As suggested by the version number, this is early alpha software. APIs and behavior are likely to change.
Jolog is an implementation of join patterns for Prolog that's inspired by JoCaml. Join patterns provide a clean, powerful way of thinking about concurrent and parallel programming.
Jolog clauses are defined using the &-/2 operator. Use it just like the `:-/2` operator. The clause's head contains a join pattern. The body contains processes and optional guards.
A join pattern is typically a conjunction (using
,/2) of smaller
patterns. Each small pattern is a term. The term's name indicates a
channel. The term's arguments indicate the content of a message on
that channel. As you'd expect, patterns are matched via standard
So if our Jolog system has the following outstanding messages:
then all the following join patterns would match
temperature(T)- binds T to
air(overcast,windy), humidity(H)- binds H to
If a Jolog clause body contains the goal
then/0, all goals occuring
then/0 are considered guards. The guards typically examine
those bindings created by the head's join patterns. All guard goals
must succeed before the matched messages are consumed.
For example, imagine we're programming a vending machine. The
price(+Item, -Cost) tells us how much it costs to purchase
a particular item. The channel
balance/1 receives messages
indicating how much money has been deposited into the vending machine.
selected/1 receives a message when a customer presses an
item selection button on the vending machine. We only want to
dispense an item if the balance exceeds the selected item's price.
balance(Bal), selected(Item) &- price(Item, Cost), Bal >= Cost, then, ...
Before the elided code (
...) starts executing, the
selected/1 messages are consumed and no longer available to other
If there is no
then/0 goal, it's the same as if the guard had been
Processes are concurrent computations. They're executed after a join pattern matches and all associated guards succeed. They're inexpensive to create, so don't worry about making thousands of them if you need to.
A process can be any code that'd you pass to
call/1. The simplest
process is just a goal. For example, in the
world, hello pattern in
the Synopsis above, the process is
writeln('Hello, World!'). That
goal is executed concurrently and the enclosing Jolog clause doesn't
wait for it to complete.
It's often convenient to create multiple processes from a single Jolog
clause. That's done with the &/2 operator. Use this operator just
as you would use
;/2. For example the
main Jolog clause in the
Synopsis creates two parallel processes.
Additional examples are available.
Messages are consumed via join patterns (described above). They're
created by calling
send/1. You can send any term as a message, but
you'll get a warning if no Jolog clauses are listening for that
See History.md file.
Using SWI-Prolog 6.3 or later:
This module uses semantic versioning.
Source code available and pull requests accepted at http://github.com/mndrix/jolog
Pack contains 6 files holding a total of 16.8K bytes.