ALPACA stands for a language for programming arbitrary cellular automata.
While RUBE was being developed it became clear to the author that the "bully" approach to writing a complex cellular automaton would result in a program extremely difficult to understand and even worse to maintain.
ALPACA was developed in order to have a terse, precise and readable language in which to express the rules for any given cellular automaton. It is in ALPACA, then, that REDGREEN, a successor to RUBE, is written. Being described in ALPACA instead of C, the source code for REDGREEN is easily a hundred times clearer than the knotted mess that is RUBE.
Other cellular automata that have been successfully described in ALPACA include John Conway's relatively famous Game of Life automaton and the lesser-known WireWorld automaton.
You can download a copy of the ALPACA v0.90 distribution from this web server. It contains Perl 5 source code, documentation, example ALPACA descriptions, and example automaton forms to use for them.
The compiler has now been hand-coded in Perl 5 (instead of C automatically generated by CoCo/R, as was in v0.80) and produces a Perl program that accepts an automaton form (a start state) as input, in the form of an ASCII text file, and animates it.
In brief, a description of a Cellular Automaton (CA) in ALPACA consists of a number of state or class declarations, seperated with semicolons and ending with a period.
A state declaration defines a state, associates a visual element with it (currently an ASCII character, possibly someday a colour or graphic) and is followed by a list of transition rules seperated by commas.
A class declaration defines a class of states. Each state
can belong to many classes, and are listed in overload order.
Classes can have their own rules, and the is operator can
be used to check for any of the states of a class instead of
a single state.
A transition rule specifies a state to change to and a boolean expression to evaluate to see if that state should be changed to. The expression may make use of whether there are a certain minimum number of neighbours of a state or class, whether neighbours in certain positions hold a certain state or class, and constant true, false, and random boolean terms, manipulated by infix boolean operators.
For more detail see the EBNF description of the grammar, and examine the source for the REDGREEN automaton in detail.
As an example, here is John Conway's Game of Life automaton, expressed in ALPACA (it's short):
state Dead " " to Alive when 3 Alive and 5 Dead; state Alive "*" to Dead when 4 Alive or 7 Dead.