Major Section: PROGRAMMING
Note: If you are interested in programming with state, see programming-with-state after reading the information below.
The ACL2 state object is used extensively in programming the ACL2 system, and has been used in other ACL2 programs as well. However, most users, especially those interested in specification and verification (as opposed to programming per se), need not be aware of the role of the state object in ACL2, and will not write functions that use it explicitly. We say more about this point at the end of this documentation topic.
The ACL2 state object is an example of a single-threaded object or
stobj. ACL2 allows the user to define new single-threaded objects.
Generally, ACL2 may need to access the ACL2 state but should not
(cannot) change it except via a certain set of approved functions
defthm. If you need a state-like object
to which you have complete rights, you may want a stobj.
Key to the idea of our
state is the notion of single-threadedness.
For an explanation, see stobj. The upshot of it is that
is a variable symbol with severe restrictions on its use, so that it
can be passed into only certain functions in certain slots, and must be
returned by those functions that ``modify'' it. Henceforth, we do not
discuss single-threaded objects in general (which the user can introduce
defabsstobj) but one in particular, namely
The global table is perhaps the most visible portion of the state
object. Using the interface functions
assign, a user
may bind global variables to the results of function evaluations
(much as an Nqthm user exploits the Nqthm utility
See @, and see assign.
ACL2 supports several facilities of a truly von Neumannesque state
machine character, including file io and global variables.
Logically speaking, the state is a true list of the 14 components
described below. There is a ``current'' state object at the
top-level of the ACL2 command loop. This object is understood to be
the value of what would otherwise be the free variable
appearing in top-level input. When any command returns a state
object as one of its values, that object becomes the new current
state. But ACL2 provides von Neumann style speed for state
operations by maintaining only one physical (as opposed to logical)
state object. Operations on the state are in fact destructive.
This implementation does not violate the applicative semantics
because we enforce certain draconian syntactic rules regarding the
use of state objects. For example, one cannot ``hold on'' to an old
state, access the components of a state arbitrarily, or ``modify'' a
state object without passing it on to subsequent state-sensitive
Every routine that uses the state facilities (e.g. does io, or calls
a routine that does io), must be passed a ``state object.'' And a
routine must return a state object if the routine modifies the state
in any way. Rigid syntactic rules governing the use of state
objects are enforced by the function
translate, through which all
ACL2 user input first passes. State objects can only be ``held'' in
the formal parameter
state, never in any other formal parameter and
never in any structure (excepting a multiple-value return list
field which is always a state object). State objects can only be
accessed with the primitives we specifically permit. Thus, for
example, one cannot ask, in code to be executed, for the length of
state or the
state. In the statement and proof of theorems,
there are no syntactic rules prohibiting arbitrary treatment of
Logically speaking, a state object is a true list whose members are as follows:
Open-input-channels, an alist with keys that are symbols in package
"ACL2-INPUT-CHANNEL". The value (
cdr) of each pair has the form
((:header type file-name open-time) . elements), where
typeis one of
elementsis a list of things of the corresponding
type, i.e. characters, integers of type
(mod 255), or lisp objects in our theory.
File-nameis a string.
Open-timeis an integer. See io.
Open-output-channels, an alist with keys that are symbols in package
"ACL2-OUTPUT-CHANNEL". The value of a pair has the form
((:header type file-name open-time) . current-contents). See io.
Global-table, an alist associating symbols (to be used as ``global variables'') with values. See @, and see assign.
T-stack, a list of arbitrary objects accessed and changed by the functions
32-bit-integer-stack, a list of arbitrary 32-bit-integers accessed and changed by the functions
Big-clock-entry, an integer, that is used logically to bound the amount of effort spent to evaluate a quoted form.
Idates, a list of dates and times, used to implement the function
print-current-idate, which prints the date and time.
Acl2-oracle, a list of objects, used for example to implement the functions that let ACL2 report how much time was used, but inaccessible to the user. Also see with-prover-time-limit.
File-clock, an integer that is increased on every file opening and closing, and on each call of
sys-call, and is used to maintain the consistency of the
Readable-files, an alist whose keys have the form
(string type time), where
stringis a file name and
timeis an integer. The value associated with such a key is a list of characters, bytes, or objects, according to
timefield is used in the following way: when it comes time to open a file for input, we will only look for a file of the specified name and
typewhose time field is that of
file-clock. This permits us to have a ``probe-file'' aspect to
open-file: one can ask for a file, find it does not exist, but come back later and find that it does now exist.
Written-files, an alist whose keys have the form
(string type time1 time2), where
stringis a file name,
typeis one of
time2correspond to the
file-clocktime at which the channel for the file was opened and closed. This field is write-only; the only operation that affects this field is
conses a new entry on the front.
Read-files, a list of the form
(string type time1 time2), where
stringis a file name and
time2were the times at which the file was opened for reading and closed. This field is write only.
Writeable-files, an alist whose keys have the form
(string type time). To open a file for output, we require that the name, type, and time be on this list.
List-all-package-names-lst, a list of
true-listps. Roughly speaking, the
carof this list is the list of all package names known to this Common Lisp right now and the
cdrof this list is the value of this
statevariable after you look at its
car. The function,
list-all-package-names, which takes the state as an argument, returns the
cdrs the list (returning a new state too). This essentially gives ACL2 access to what is provided by CLTL's
Defpkguses this feature to ensure that the about-to-be-created package is new in this lisp. Thus, for example, in
akclit is impossible to create the package
defpkgbecause it is on the list, while in Lucid that package name is not initially on the list.
User-stobj-alist, an alist which associates user-defined single-threaded objects (see stobj) with their values.
We recommend avoiding the use of the state object when writing ACL2 code intended to be used as a formal model of some system, for several reasons. First, the state object is complicated and contains many components that are oriented toward implementation and are likely to be irrelevant to the model in question. Second, there is currently not much support for reasoning about ACL2 functions that manipulate the state object, beyond their logical definitions. Third, the documentation about state is not as complete as one might wish.
User-defined single-threaded objects offer the speed of
giving the user complete access to all the fields. See stobj.
Again, if you are interested in programming with state see programming-with-state.