TABLE

Major Section:  EVENTS

Examples:
(table tests 1 '(...))                ; set contents of tests[1] to '(...)
(table tests 25)                      ; get contents of tests[25]
(table tests)                         ; return table tests as an alist
(table tests nil nil :clear)          ; clear table tests
(table tests nil '((foo . 7)) :clear) ; set table tests to (foo 7)
(table tests nil nil :guard)          ; fetch the table guard
(table tests nil nil :guard term)     ; set the table guard

General Form:
(table table-name key-term value-term op term)

Important Note: The table forms above are calls of a macro that expand to involve the special variable state. This will prevent you from accessing a table from within a hint or theory where where you do not have the state variable. However, the form

(table-alist 'tests world)

The ACL2 system provides ``tables'' by which the user can associate one object with another. Tables are in essence just conventional association lists -- lists of pairs -- but the ACL2 environment provides a means of storing these lists in the ``ACL2 world'' of the current state. The ACL2 user could accomplish the same ends by using ACL2 ``global variables;'' however, limitations on global variable names are imposed to ensure ACL2's soundness. By convention, no table is important to ACL2's soundness, even though some features of the system use tables, and the user is invited to make free use of tables. Because tables are stored in the ACL2 world they are restored by include-book and undone by :ubt. Many users of Nqthm requested a facility by which user data could be saved in Nqthm ``lib files'' and tables are ACL2's answer to that request.

Abstractly, each table is an association list mapping ``keys'' to ``values.'' In addition, each table has a ``:guard,'' which is a term that must be true of any key and value used. By setting the :guard on a table you may enforce an invariant on the objects in the table, e.g., that all keys are positive integers and all values are symbols. Each table has a ``name,'' which must be a symbol. Given a table name, there are six operations one might perform on the table.

:put -- associate a value with a key (possibly changing the value currently associated with that key).

:get -- retrieve the value associated with a key (or nil if no value has been associated with that key).

:alist -- return an alist showing all keys and non-nil values in the table.

:clear -- clear the table (so that every value is nil), or if val is supplied then set table to that value (which must be an alist).

:guard -- fetch or set the :guard of the table.

When the operations above suggest that the table or its :guard are modified, what is actually meant is that the current state is redefined so that in it, the affected table name has the appropriate properties. in such cases, the table form is an event.

Table forms are commonly typed by the user while interacting with the system. :Put and :get forms are especially common. Therefore, we have adopted a positional syntax that is intended to be convenient for most applications. Essentially, some operations admit a ``short form'' of invocation.

(table name key-term value-term :put)   ; long form
(table name key-term value-term)        ; short form

(table name key-term nil :get)          ; long form
(table name key-term)                   ; short form

(table name nil nil :alist)             ; long form
(table name)                            ; short form

(table name nil val :clear)

(table name nil nil :guard)

(table name nil nil :guard term)

Note that it is not allowed to change the :guard on a table once it has been explicitly set. Before the :guard is explicitly set, it is effectively just t. After it is set it can be changed only by undoing the event that set it. The purpose of this restriction is to prevent the user from changing the :guards on tables provided by other people or the system.

The intuition behind the :guard mechanism on tables is to enforce invariants on the keys and values in a table, so that the values, say, can be used without run-time checking. But if the :guard of a table is sensitive to the ACL2 world, it may be possible to cause some value in the table to cease satisfying the :guard without doing any operations on the table. Consider for example the :guard ``no value in this table is the name of an event.'' As described, that is enforced each time a value is stored. Thus, 'bang can be :put in the table provided there is no event named bang. But once it is in the table, there is nothing to prevent the user from defining bang as a function, causing the table to contain a value that could not be :put there anymore. Observe that not all state-sensitive :guards suffer this problem. The :guard ``every value is an event name'' remains invariant, courtesy of the fact that undoing back through an event name in the table would necessarily undo the :put of the name into the table.

Table was designed primarily for convenient top-level use. Tables are not especially efficient. Each table is represented by an alist stored on the property list of the table name. :Get is just a getprop and assoc-equal. :Put does a getprop to the get the table alist, a put-assoc-equal to record the new association, and a putprop to store the new table alist -- plus the overhead associated with :guards and undoable events. Note that there are never duplicate keys in the resulting alist; in particular, when the operation :clear is used to install new alist, duplicate keys are removed from that alist.

A table name may be any symbol whatsoever. Symbols already in use as function or theorem names, for example, may be used as table names. Symbols in use only as table names may be defined with defun, etc. Because there are no restrictions on the user's choice of table names, table names are not included among the logical names. Thus, :pe name will never display a table event (for a logical name other than :here). Either :pe name will display a ``normal'' event such as (defun name ...) or (defthm name ...) or else :pe name will cause an error indicating that name is not a logical name. This happens even if name is in use as a table name. Similarly, we do not permit table names to have documentation strings, since the same name might already have a documentation string. If you want to associate a documentation string with a table name that is being used no other way, define the name as a label and use the :doc feature of deflabel (see deflabel); also see defdoc.