user-specified invariants on theories
Major Section:  EVENTS

(theory-invariant (not (and (member-equal '(:rewrite left-to-right)
                            (member-equal '(:rewrite right-to-left)

General Forms: (theory-invariant term) ; conjoin a new invariant term or (theory-invariant term key) ; change an existing invariant term

where term is a term that uses no variables other than theory and world, and key is an arbitrary ``name'' for this invariant. If key is omitted, an integer is generated and used. Theory-invariant is an event that adds to or modifies the table of user-supplied theory invariants that are checked each time a theory expression is evaluated.

The theory invariant mechanism is provided via a table (see table) named theory-invariant-table. In fact, the theory-invariant ``event'' is just a macro that expands into a use of the table event. More general access to the theory-invariant table is provided by table itself. For example, the table can be inspected or cleared (setting the invariant to t) with table.

Theory-invariant-table maps arbitrary keys to terms mentioning, at most, the variables theory and world. Every time an alleged theory expression is evaluated, e.g., in the in-theory event or :in-theory hint, each of the terms in theory-invariant-table is evaluated with theory bound to the runic theory (see theories) obtained from the theory expression and world bound to the current ACL2 world (see world). If the result is nil, a warning message is printed. Thus, the table can be thought of as a list of conjuncts. Each term in the table has a ``name,'' which is just the key under which the term is stored. When a theory violates the restrictions specified by some term, both the name and the term are printed. By calling theory-invariant with a new term but the same name, you can overwrite that conjunct of the theory invariant.

Theory invariants are particularly useful in the context of large rule sets intended for re-use. Such sets often contain conflicting rules, e.g., rules that are to be enabled when certain function symbols are disabled, rules that rewrite in opposite directions and thus loop if simultaneously enabled, groups of rules which should be enabled in concert, etc. The developer of such rule sets understands these restrictions and probably documents them. The theory invariant mechanism allows the developer to codify his restrictions so that when they are violated the user is warned.

Since theory invariants are arbitrary terms, macros may be used to express commonly used restrictions. Because theory invariants are a new idea in ACL2, we have only defined one such macro for illustrative purposes. Executing the event

(theory-invariant (incompatible (:rewrite left-to-right)
                                (:rewrite right-to-left)))
would subsequently cause a warning message any time the current theory contained both of the two runes shown. Of course, incompatible is just defined as a macro. Its definition may be inspected with :pe incompatible.

Note: If the table event is used directly to :put a term into the theory invariant table, be aware that the term must be in translated form. This is enforced by the value invariant for theory-invariant-table. But the upshot of this is that you will be unable to use macros in theory invariants stored directly with the :put table event.