Synopsis

:- 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!

Description

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.

Join Patterns

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 Prolog unification.

So if our Jolog system has the following outstanding messages:

• temperature(75)
• humidity(50)
• air(overcast, windy)

then all the following join patterns would match

• temperature(75)
• temperature(T) - binds T to 75
• air(overcast,windy), humidity(H) - binds H to 50

Guards

If a Jolog clause body contains the goal then/0, all goals occuring before 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 predicate 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. The channel 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 balance/1 and selected/1 messages are consumed and no longer available to other Jolog clauses.

If there is no then/0 goal, it's the same as if the guard had been true.

Processes

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

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 message.

Changes in this Version

See History.md file.

Installation

Using SWI-Prolog 6.3 or later:

?- pack_install(jolog).

This module uses semantic versioning.

Source code available and pull requests accepted at http://github.com/mndrix/jolog