ACL2 property lists and the ACL2 logical data base

A ``world'' is a list of triples, each of the form (sym prop . val), implementing the ACL2 notion of property lists. ACL2 permits the simultaneous existence of many property list worlds. ``The world'' is often used as a shorthand for ``the ACL2 logical world'' which is the particular property list world used within the ACL2 system to maintain the data base of rules.

Common Lisp provides the notion of ``property lists'' by which one can attach ``properties'' and their corresponding ``values'' to symbols. For example, one can arrange for the 'color property of the symbol 'box-14 to be 'purple and the 'color property of the symbol 'triangle-7 to be 'yellow. Access to property lists is given via the Common Lisp function get. Thus, (get 'box-14 'color) might return 'purple. Property lists can be changed via the special form setf. Thus, (setf (get 'box-14 'color) 'blue) changes the Common Lisp property list configuration so that (get 'box-14 'color) returns 'blue. It should be obvious that ACL2 cannot provide this facility, because Common Lisp's get ``function'' is not a function of its argument, but instead a function of some implicit state object representing the property list settings for all symbols.

ACL2 provides the functions getprop and putprop which allow one to mimic the Common Lisp property list facility. However, ACL2's getprop takes as one of its arguments a list that is a direct encoding of what was above called the ``state object representing the property list settings for all symbols.'' Because ACL2 already has a notion of ``state'' that is quite distinct from that used here, we call this property list object a ``world.'' A world is just a true list of triples. Each triple is of the form (sym prop . val). This world can be thought of as a slightly elaborated form of association list and getprop is a slightly elaborated form of assoc that takes two keys. When getprop is called on a symbol, s, property p, and world, w, it scans w for the first triple whose sym is s and prop is p and returns the corresponding val. Getprop has two additional arguments, one of which that controls what it returns if no such sym and prop exist in w, and other other of which allows an extremely efficient implementation. To set some property's value for some symbol, ACL2 provides putprop. (putprop sym prop val w) merely returns a new world, w', in which (sym prop . val) has been consed onto the front of w, thus ``overwriting'' the prop value of sym in w to val and leaving all other properties in w unchanged.

One aspect of ACL2's property list arrangment is that it is possible to have many different property list worlds. For example, 'box-14 can have 'color 'purple in one world and can have 'color 'yes in another, and these two worlds can exist simultaneously because getprop is explicitly provided the world from which the property value is to be extracted.

The efficiency alluded to above stems from the fact that Common Lisp provides property lists. Using Common Lisp's provisions behind the scenes, ACL2 can ``install'' the properties of a given world into the Common Lisp property list state so as to make retrieval via getprop very fast in the special case that the world provided to getprop has been installed. To permit multiple installed worlds, each of which is permitted to be changed via putprop, ACL2 requires that worlds be named and these names are used to distinquish installed versions of the various worlds. At the moment we do not further document getprop and putprop.

However, the ACL2 system uses a property list world, named 'current-acl2-world, in which to store the succession of user commands and their effects on the logic. This world is often referred to in our documentation as ``the world'' though it should be stressed that the user is permitted to have worlds and ACL2's is in no way distinguished except that the user is not permitted to modify it except via event commands. The ACL2 world is part of the ACL2 state and may be obtained via (w state).

Warning: The ACL2 world is very large. Its length as of this writing (Version 2.5) is over 40,000 and it grows with each release. Furthermore, some of the values stored in it are pointers to old versions of itself. Printing (w state) is something you should avoid because you likely will not have the patience to await its completion. For these practical reasons, the only thing you should do with (w state) is provide it to getprop, as in the form

  (getprop sym prop default 'current-acl2-world (w state))
to inspect properties within it, or to pass it to ACL2 primitives, such as theory functions, where it is expected.

Some ACL2 command forms, such as theory expressions (see theories) and the values to be stored in tables (see table), are permitted to use the variable symbol world freely with the understanding that when these forms are evaluated that variable is bound to (w state). Theoretically, this gives those forms complete knowledge of the current logical configuration of ACL2. However, at the moment, few world scanning functions have been documented for the ACL2 user. Instead, supposedly convenient macro forms have been created and documented. For example, (current-theory :here), which is the theory expression which returns the currently enabled theory, actually macroexpands to (current-theory-fn :here world). When evaluated with world bound to (w state), current-theory-fn scans the current ACL2 world and computes the set of runes currently enabled in it.