Major Section: MISCELLANEOUS

Examples: (assign print-doc-start-column nil) (assign print-doc-start-column 17)

This state global variable controls the column in which the
``one-liner'' of a formatted documentation string is printed.
Generally, when `:`

`doc`

is used to print a documentation string, the
name of the documented concept is printed and then `:`

`doc`

tabs over to
`print-doc-start-column`

and prints the one-liner. If the name
extends past the desired column, `:`

`doc`

outputs a carriage return and
then tabs over to the column. If `print-doc-start-column`

is `nil`

,
`:`

`doc`

just starts the one-liner two spaces from the end of the name,
on the same line. The initial value of `print-doc-start-column`

is
15.

`ld`

Major Section: MISCELLANEOUS

The prompt printed by ACL2 conveys information about various
``modes.'' See default-print-prompt and see ld-prompt for
details.

`e0-ord-<`

is well-founded on `e0-ordinalp`

s
Major Section: MISCELLANEOUS

The soundness of ACL2 rests in part on the well-foundedness of
`e0-ord-<`

on `e0-ordinalp`

s. This can be taken as obvious if one is
willing to grant that those concepts are simply encodings of the
standard mathematical notions of the ordinals below `epsilon-0`

and
its natural ordering relation. But it is possible to prove that
`e0-ord-<`

is well-founded on `e0-ordinalp`

s without having to assert
any connection to the ordinals and that is what we do here.

We first observe three facts about `e0-ord-<`

on ordinals that have
been proved by ACL2 using only structural induction on lists.

(defthm transitivity (implies (and (e0-ordinalp x) (e0-ordinalp y) (e0-ordinalp z) (e0-ord-< x y) (e0-ord-< y z)) (e0-ord-< x z)) :rule-classes nil)These three properties establish that(defthm non-circularity (implies (and (e0-ordinalp x) (e0-ordinalp y) (e0-ord-< x y)) (not (e0-ord-< y x))) :rule-classes nil)

(defthm trichotomy (implies (and (e0-ordinalp x) (e0-ordinalp y)) (or (equal x y) (e0-ord-< x y) (e0-ord-< y x))) :rule-classes nil)

`e0-ord-<`

orders the
`e0-ordinalp`

s. To put such a statement in the most standard
mathematical nomenclature, we can define the function:
(defun e0-ord-<= (x y) (or (equal x y) (e0-ord-< x y)))and then establish that

`e0-ord-<=`

is a relation that is a simple,
complete (i.e., total) order on ordinals by the following three
lemmas, which have been proved:
(defthm antisymmetry (implies (and (e0-ordinalp x) (e0-ordinalp y) (e0-ord-<= x y) (e0-ord-<= y x)) (equal x y)) :rule-classes nil :hints (("Goal" :use non-circularity)))Crucially important to the proof of the well-foundedness of(defthm e0-ord-<=-transitivity (implies (and (e0-ordinalp x) (e0-ordinalp y) (e0-ordinalp z) (e0-ord-<= x y) (e0-ord-<= y z)) (e0-ord-<= x z)) :rule-classes nil :hints (("Goal" :use transitivity)))

(defthm trichotomy-of-e0-ord-< (implies (and (e0-ordinalp x) (e0-ordinalp y)) (or (e0-ord-<= x y) (e0-ord-<= y x))) :rule-classes nil :hints (("Goal" :use trichotomy)))

`e0-ord-<`

on `e0-ordinalp`

s is the concept of ordinal-depth,
abbreviated `od`

:
(defun od (l) (if (atom l) 0 (1+ (od (car l)))))If the

`od`

of an `e0-ordinalp`

`x`

is smaller than that of an
`e0-ordinalp`

`y`

, then `x`

is `e0-ord-<`

`y`

:
(defthm od-implies-ordlessp (implies (and (e0-ordinalp x) (e0-ordinalp y) (< (od x) (od y))) (e0-ord-< x y)))Remark. A consequence of this lemma is the fact that if

`s = s(1)`

,
`s(2)`

, ... is an infinite, `e0-ord-<`

descending sequence, then
`od(s(1))`

, `od(s(2))`

, ... is a ``weakly'' descending sequence of
non-negative integers: `od(s(i))`

is greater than or equal to
`od(s(i+1))`

.
*Lemma Main.* For each non-negative integer `n`

, `e0-ord-<`

well-orders
the set of `e0-ordinalp`

s with `od`

less than or equal to `n`

.

Base Case. n = 0. The e0-ordinalps with 0 od are the non-negative integers. On the non-negative integers, e0-ord-< is the same as <.Theorem.Induction Step. n > 0. We assume that e0-ord-< well-orders the e0-ordinalps with od less than n.

If e0-ord-< does not well-order the e0-ordinalps with od less than or equal to n, we may let O be the e0-ord-<-least e0-ordinalp which is the car of the first member of an infinite, e0-ord-< descending sequence of e0-ordinalps of od less than or equal to n. The od of O is n-1.

Let k be the least integer > 0 such that for some infinite, e0-ord-< descending sequence s of e0-ordinalps with od less than or equal to n, the first element of s begins with k occurrences of O but not k+1 occurrences of O.

Having fixed O and k, let s = s(1), s(2), ... be an infinite, e0-ord-< descending sequence of e0-ordinalps with od less than or equal to n such that O occurs exactly k times at the beginning of s(1).

O occurs exactly k times at the beginning of each s(i). For suppose that s(j) is the first member of s with exactly m occurrences of O at the beginning, m /= k. If m = 0, then the first member of s(j) must be e0-ord-< O, contradicting the minimality of O. If 0 < m < k, then the fact that the sequence beginning at s(j) is infinitely descending contradicts the minimality of k. If m > k, then s(j) is greater than its predecessor, which has only k occurrences of O at the beginning; but this contradicts the fact that s is descending.

Let t = t(1), t(2), ... be the sequence of e0-ordinalps that is obtained by letting t(i) be the result of removing O from the front of s(i) exactly k times. t is infinitely descending. Furthermore, t(1) begins with an e0-ordinalp O' that is e0-ord-< O, and hence has od at most N-1 by the lemma od-implies-ordlessp. But this contradicts the minimality of O. Q.E.D.

`e0-ord-<`

well-orders the `e0-ordinalp`

s. Proof. Every
infinite,` e0-ord-<`

descending sequence of `e0-ordinalp`

s has the
property that each member has `od`

less than or equal to the `od`

, `n`

, of
the first member of the sequence. This contradicts Lemma Main.
Q.E.D.
Major Section: MISCELLANEOUS

Example Forms: (pseudo-termp '(car (cons x 'nil))) ; has value t (pseudo-termp '(car x y z)) ; also has value t! (pseudo-termp '(delta (h x))) ; has value t (pseudo-termp '(delta (h x) . 7)) ; has value nil (not a true-listp) (pseudo-termp '((lambda (x) (car x)) b)) ; has value t (pseudo-termp '(if x y 123)) ; has value nil (123 is not quoted) (pseudo-termp '(if x y '123)) ; has value tIf

`x`

is the quotation of a term, then `(pseudo-termp x)`

is `t`

.
However, if `x`

is not the quotation of a term it is not necessarily
the case that `(pseudo-termp x)`

is `nil`

.
See term for a discussion of the various meanings of the word
``term'' in ACL2. In its most strict sense, a term is either a
legal variable symbol, a quoted constant, or the application of an
`n`

-ary function symbol or closed `lambda`

-expression to `n`

terms. By
``legal variable symbol'' we exclude constant symbols, such as `t`

,
`nil`

, and `*ts-rational*`

. By ``quoted constants'' we include `'t`

(aka
`(quote t)`

), `'nil`

, `'31`

, etc., and exclude constant names such as `t`

,
`nil`

and `*ts-rational*`

, unquoted constants such as `31`

or `1/2`

, and
ill-formed `quote`

expressions such as `(quote 3 4)`

. By ``closed
lambda expression'' we exclude expressions, such as
`(lambda (x) (cons x y))`

, containing free variables in their bodies.
Terms typed by the user are translated into strict terms for
internal use in ACL2.

The predicate `termp`

checks this strict sense of ``term'' with
respect to a given ACL2 logical world; See world. Many ACL2
functions, such as the rewriter, require certain of their arguments
to satisfy `termp`

. However, as of this writing, `termp`

is in `:`

`program`

mode and thus cannot be used effectively in conjectures to be
proved. Furthermore, if regarded simply from the perspective of an
effective guard for a term-processing function, `termp`

checks many
irrelevant things. (Does it really matter that the variable symbols
encountered never start and end with an asterisk?) For these
reasons, we have introduced the notion of a ``pseudo-term'' and
embodied it in the predicate `pseudo-termp`

, which is easier to
check, does not require the logical world as input, has `:`

`logic`

mode, and is often perfectly suitable as a guard on term-processing
functions.

A `pseudo-termp`

is either a symbol, a true list of length 2
beginning with the word `quote`

, the application of an `n`

-ary
pseudo-`lambda`

expression to a true list of `n`

pseudo-terms, or
the application of a symbol to a true list of `n`

`pseudo-termp`

s.
By an ```n`

-ary pseudo-`lambda`

expression'' we mean an expression
of the form `(lambda (v1 ... vn) pterm)`

, where the `vi`

are
symbols (but not necessarily distinct legal variable symbols) and
`pterm`

is a `pseudo-termp`

.

Metafunctions may use `pseudo-termp`

as a guard.

Major Section: MISCELLANEOUS

Example and General Form: ACL2 !>:redefThis command sets

`ld-redefinition-action`

to `'(:query . :overwrite)`

.
As explained elsewhere (see ld-redefinition-action), this
allows redefinition of functions and other events without undoing.
A query will be made every time a redefinition is commanded; the
user must explicitly acknowledge that the redefinition is
intentional. It is possible to set `ld-redefinition-action`

so that
the redefinition of non-system functions occurs quietly.
See ld-redefinition-action.

Major Section: MISCELLANEOUS

Example and General Form: ACL2 !>:redef! ACL2 p!>This command sets

`ld-redefinition-action`

to `'(:warn! . :overwrite)`

and sets the default defun-mode to `:`

`program`

.
This is the ACL2 system hacker's redefinition command. Note that
even system functions can be redefined with a mere warning. Be
careful!

Major Section: MISCELLANEOUS

Example and General Forms: (redefined-names state)

This function collects names that have been redefined in the current ACL2
state. `:`

`Program`

mode functions that were reclassified to
`:`

`logic`

functions are not collected, since such reclassification
cannot imperil soundness because it is allowed only when the new and old
definitions are identical.

Thus, if `(redefined-names state)`

returns `nil`

then no unsafe
definitions have been made, regardless of `ld-redefinition-action`

.
See ld-redefinition-action.

Major Section: MISCELLANEOUS

Sometimes an event will announce that it is ``redundant''. When this happens, no change to the logical world has occurred. This happens when the logical name being defined is already defined and has exactly the same definition, from the logical point of view. This feature permits two independent books, each of which defines some name, to be included sequentially provided they use exactly the same definition.

When are two logical-name definitions considered exactly the same? It depends upon the kind of name being defined.

A `deflabel`

event is never redundant. This means that if you have a
`deflabel`

in a book and that book has been included (without error),
then references to that label denote the point in history at which
the book introduced the label. See the note about shifting logical
names, below.

A `defun`

or `mutual-recursion`

(or `defuns`

) event is redundant if for
each function to be introduced, there has already been introduced a
function with the same name, the same formals, and syntactically
identical `guard`

, type declarations, and `body`

(before
macroexpansion).

A `verify-guards`

event is redundant if the function has already had
its guards verified.

A `defaxiom`

or `defthm`

event is redundant if there is already an axiom
or theorem of the given name and both the formula (after
macroexpansion) and the rule-classes are syntactically identical.
Note that a `defaxiom`

can make a subsequent `defthm`

redundant, and a
`defthm`

can make a subsequent `defaxiom`

redundant as well.

A `defconst`

is redundant if the name has been defined to have the
same value.

A `defmacro`

is redundant if there is already a macro defined with the
same name and syntactically identical arguments, guard, and body.

A `defpkg`

is redundant if a package of the same name with exactly the
same imports has been defined.

A `deftheory`

is never redundant. The ``natural'' notion of
equivalent `deftheory`

s is that the names and values of the two theory
expressions are the same. But since most theory expressions are
sensitive to the context in which they occur, it seems unlikely to
us that two `deftheory`

s coming from two sequentially included books
will ever have the same values. So we prohibit redundant theory
definitions. If you try to define the same theory name twice, you
will get a ``name in use'' error.

An `in-theory`

event is never redundant because it doesn't define any
name.

`Table`

and `defdoc`

events are never redundant because they don't
define any name.

An `encapsulate`

event is redundant if and only if a syntactically
identical `encapsulate`

has already been executed under the same
`default-defun-mode`

.

An `include-book`

is redundant if the book has already been included.

*Note About Shifting Logical Names:*

Suppose a book defines a function `fn`

and later uses `fn`

as a logical
name in a theory expression. Consider the value of that theory
expression in two different sessions. In session A, the book is
included in a world in which `fn`

is not already defined, i.e., in a
world in which the book's definition of `fn`

is not redundant. In
session B, the book is included in a world in which `fn`

is already
identically defined. In session B, the book's definition of `fn`

is
redundant. When `fn`

is used as a logical name in a theory
expression, it denotes the point in history at which `fn`

was
introduced. Observe that those points are different in the two
sessions. Hence, it is likely that theory expressions involving `fn`

will have different values in session A than in session B.

This may adversely affect the user of your book. For example,
suppose your book creates a theory via `deftheory`

that is advertised
just to contain the names generated by the book. But suppose you
compute the theory as the very last event in the book using:

(set-difference-theories (universal-theory :here) (universal-theory fn))where

`fn`

is the very first event in the book and happens to be a
`defun`

event. This expression returns the advertised set if `fn`

is
not already defined when the book is included. But if `fn`

were
previously (identically) defined, the theory is larger than
advertised.
The moral of this is simple: when building books that other people
will use, it is best to describe your theories in terms of logical
names that will not shift around when the books are included. The
best such names are those created by `deflabel`

.

Major Section: MISCELLANEOUS

Examples: ACL2 !>:Q >(make-lib "file") ... >(note-lib "file") >(LP) ACL2 !>

To save the current ACL2 logical world to a file, exit ACL2 with `:`

`q`

and invoke `(make-lib "file")`

in Common Lisp. This creates a file
`"file.lib"`

and a file `"file.lisp"`

. The latter will be compiled.
It generally takes half an hour to save an ACL2 logical world and
creates a 20Mb file. All things considered it is probably better to
just save your core image.

To restore such a saved ACL2 world, invoke `(note-lib "file")`

from
Common Lisp, and then enter ACL2 with `(lp)`

. We do not save the `io`

system, the stacks, or the global table, hence bindings of your
globals will not be restored.

This save/restore mechanism is a temporary expedient. We know of
faster mechanisms, mechanisms that consume less disk space, and
mechanisms that provide more functionality. We don't know of good
compromises between these various desirable features.

Major Section: MISCELLANEOUS

Examples: (hd (x) t) (printer (x state) (mv t t state)) (printer (x state) (mv er-flg val state))whereGeneral Form: (fn formals result)

`fn`

is the constrained function symbol, `formals`

is a suitable
list of formal parameters for it, and `result`

is either a symbol
denoting that the function returns one result (which is a state
object or not depending on whether the symbol is `state`

) or else
result is an `mv`

expression, `(mv s1 ... sn)`

, where `n>1`

, each `si`

is a
symbol, and at most one of the `si`

is the symbol `state`

. The latter
form of result indicates that the function returns `n`

results and
indicates which of them (if any) is a state object. The non-`state`

`si`

are just place holders and may all be identical, e.g., `t`

, though
we often use symbols that suggest the type of the corresponding
value. It is illegal for `state`

to be used in result if `state`

does
not appear in `formals`

.
`:`

`definition`

and `:`

`rewrite`

rules used in preprocessing
Major Section: MISCELLANEOUS

Example of simple rewrite rule: (equal (car (cons x y)) x)Examples of simple definition: (defun file-clock-p (x) (integerp x)) (defun naturalp (x) (and (integerp x) (>= x 0)))

The theorem prover output sometimes refers to ``simple'' definitions
and rewrite rules. These rules can be used by the preprocessor,
which is one of the theorem prover's ``processes'' understood by the
`:do-not`

hint; see hints.

The preprocessor expands certain definitions and uses certain
rewrite rules that it considers to be ``fast''. There are two ways
to qualify as fast. One is to be an ``abbreviation'', where a
rewrite rule with no hypotheses or loop stopper is an
``abbreviation'' if the right side contains no more variable
occurrences than the left side, and the right side does not call the
functions `if`

, `not`

or `implies`

. Definitions and rewrite rules can
both be abbreviations; the criterion for definitions is similar,
except that the definition must not be recursive. The other way to
qualify applies only to a non-recursive definition, and applies when
its body is a disjunction or conjunction, according to a perhaps
subtle criterion that is intended to avoid case splits.

Major Section: MISCELLANEOUS

Occasionally the ACL2 theorem prover reports that the current goal simplifies to itself or to a set including itself. Such simplifications are said to be ``specious'' and are ignored in the sense that the theorem prover acts as though no simplification were possible and tries the next available proof technique. Specious simplifications are almost always caused by forcing.

The simplification of a formula proceeds primarily by the local
application of `:`

`rewrite`

, `:`

`type-prescription`

, and other rules to its
various subterms. If no rewrite rules apply, the formula cannot be
simplified and is passed to the next ACL2 proof technique, which is
generally the elimination of destructors. The experienced ACL2 user
pays special attention to such ``maximally simplified'' formulas;
the presence of unexpected terms in them indicates the need for
additional rules or the presence of some conflict that prevents
existing rules from working harmoniously together.

However, consider the following interesting possibility: local
rewrite rules apply but, when applied, reproduce the goal as one of
its own subgoals. How can rewrite rules apply and reproduce the
goal? Of course, one way is for one rule application to undo the
effect of another, as when commutativity is applied twice in
succession to the same term. Another kind of example is when rules
conflict and undermine each other. For example, under suitable
hypotheses, `(length x)`

might be rewritten to `(+ 1 (length (cdr x)))`

by the `:`

`definition`

of `length`

and then a `:`

`rewrite`

rule might be used
to ``fold'' that back to `(length x)`

. Generally speaking the
presence of such ``looping'' rewrite rules causes ACL2's simplifier
either to stop gracefully because of heuristics such as that
described in the documentation for `loop-stopper`

or to cause a
stack overflow because of indefinite recursion.

A more insidious kind of loop can be imagined: two rewrites in different parts of the formula undo each other's effects ``at a distance,'' that is, without ever being applied to one another's output. For example, perhaps the first hypothesis of the formula is simplified to the second, but then the second is simplified to the first, so that the end result is a formula propositionally equivalent to the original one but with the two hypotheses commuted. This is thought to be impossible unless forcing or case-splitting occurs, but if those features are exploited (see force and see case-split) it can be made to happen relatively easily.

Here is a simple example. Declare `foo`

to be a function of one
argument returning one result:

(defstub foo (x) t)Add the following

`:`

`type-prescription`

rule about `foo`

:
(defaxiom forcer (implies (force (not (true-listp x))) (equal (foo x) t)) :rule-classes :type-prescription)Note that we could define a

`foo`

with this property; `defstub`

and
`defaxiom`

are only used here to get to the gist of the problem
immediately. Consider the proof attempt for the following formula.
(thm (implies (and (consp x) ; hyp 1 (true-listp (cdr x)) ; hyp 2 (true-listp x)) ; hyp 3 (foo x))) ; conclWhen we simplify this goal,

`hyp 1`

cannot be simplified. `Hyp 2`

simplifies to `t`

, because `x`

is known to be a non-`nil`

true list so its
`cdr`

is a true list by type reasoning; because true hypotheses are
dropped, `hyp 2`

simply disappears. `Hyp 3`

simplifies to
`(true-listp (cdr x))`

by opening up the `:`

`definition`

of
`true-listp`

. Note that `hyp 3`

has simplified to the old `hyp 2`

.
So at this point, the ``current (intermediate) goal'' is
(implies (and (consp x) ; rewritten hyp 1 (true-listp (cdr x))) ; rewritten hyp 3 (foo x)) ; unrewritten concland we are working on

`(foo x)`

. But the `:`

`type-prescription`

rule
above tells us that `(foo x)`

is `t`

if the hypothesis of the rule is
true. Thus, in the case that the hypothesis of the rule is true, we
are done. It remains to prove the current intermediate goal under
the assumption that the hypothesis of the rule is false. This is
done by adding the negation of the `:`

`type-prescription`

rule's
hypothesis to the current intermediate goal. This is what `force`

does in this situation. The negation of the hypothesis is
`(true-listp x)`

. Adding it to the current goal produces the subgoal
(implies (and (consp x) ; rewritten hyp 1 (true-listp (cdr x)) ; rewritten hyp 3 (true-listp x)) ; FORCEd hyp (foo x)). ; unrewritten conclObserve that this is just our original goal. Despite all the rewriting, no progress was made! In more common cases, the original goal may simplify to a set of subgoals, one of which includes the original goal.

If ACL2 were to adopt the new set of subgoals, it would loop indefinitely. Therefore, it checks whether the input goal is a member of the output subgoals. If so, it announces that the simplification is ``specious'' and pretends that no simplification occurred.

``Maximally simplified'' formulas that produce specious simplifications are maximally simplified in a very technical sense: were ACL2 to apply every applicable rule to them, no progress would be made. Since ACL2 can only apply every applicable rule, it cannot make further progress with the formula. But the informed user can perhaps identify some rule that should not be applied and make it inapplicable by disabling it, allowing the simplifier to apply all the others and thus make progress.

When specious simplifications are a problem it might be helpful to disable all forcing (including case-splits) and resubmit the formula to observe whether forcing is involved in the loop or not. See force. The commands

ACL2 !>:disable-forcing and ACL2 !>:enable-forcingdisable and enable the pragmatic effects of both

`force`

and `case-split`

. If the loop is broken when forcing is
disabled, then it is very likely some forced hypothesis of
some rule is ``undoing'' a prior simplification. The most common
cause of this is when we force a hypothesis that is actually
false but whose falsity is somehow temporarily hidden (more below).
To find the offending rule, compare the specious simplification with
its non-specious counterpart and look for rules that were speciously
applied that are not applied in the non-specious case. Most likely
you will find at least one such rule and it will have a `force`

d
hypothesis. By disabling that rule, at least for the subgoal in
question, you may allow the simplifier to make progress on the
subgoal.
To illustrate what we mean by the claim that specious
simplifications often arise because the system forces a false
hypothesis, reconsider the example above. At the time we used the
`:`

`type-prescription`

rule, the known assumptions were `(consp x)`

and
`(true-listp (cdr x))`

. Observe that this tells us that `x`

is a true
list. But the hypothesis forced to be true was
`(not (true-listp x))`

. Why was the falsity of this hypothesis
missed? The most immediate reason is that the encoding of the two
assumptions above does not produce a context (``type-alist'') in
which `x`

is recorded to be a true-list. When we look up
`(not (true-listp x))`

in that context, we are not told that it is
false. More broadly, the problem stems from the fact that when we
force a hypothesis we do not bring to bear on it all of the
resources of the theorem prover. Thus it could be -- as here --
that the hypothesis could be proved false in the current context but
is not obviously so. No matter how sophisticated we made the
forcing mechanism, the unavoidable incompleteness of the theorem
prover would still permit the occasional specious simplification.
While that does not excuse us from trying to avoid specious
simplifications when we can -- and we may well strengthen the type
mechanism to deal with the problem illustrated here -- specious
simplifications will probably remain a problem deserving of the
user's attention.

Major Section: MISCELLANEOUS

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 *global table* is perhaps the most visible portion of the state
object. Using the interface functions `@`

and `assign`

, a user
will bind global variables to the results of function evaluations
(much as an Nqthm user exploits the Nqthm utility `r-loop`

).
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 as
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 `state`

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
functions.

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-values 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 `car`

of `state`

. In the statement and proof of theorems,
there are no syntactic rules prohibiting arbitrary treatment of
state objects.

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`type`

is one of`:character`

,`:byte`

, or`:object`

and`elements`

is a list of things of the corresponding`type`

, i.e. characters, integers of type`(mod 255)`

, or lisp objects in our theory.`File-name`

is a string.`Open-time`

is 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`aref-t-stack`

and`aset-t-stack`

.

`32-bit-integer-stack`

, a list of arbitrary 32-bit-integers accessed and changed by the functions`aref-32-bit-integer-stack`

and`aset-32-bit-integer-stack`

.

`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.

`Run-times`

, a list of integers, used to implement the functions that let ACL2 report how much time was used, but inaccessible to the user.

`File-clock`

, an integer that is increased on every file opening and closing and used to maintain the consistency of the`io`

primitives.

`Readable-files`

, an alist whose keys have the form`(string type time)`

, where`string`

is a file name and`time`

is an integer. The value associated with such a key is a list of characters, bytes, or objects, according to`type`

. The`time`

field 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`type`

whose 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`string`

is a file name,`type`

is one of`:character`

,`:byte`

or`:object`

, and`time1`

and`time2`

are integers.`Time1`

and`time2`

correspond to the`file-clock`

time at which the channel for the file was opened and closed. This field is write-only; the only operation that affects this field is`close-output-channel`

, which`cons`

es a new entry on the front.

`Read-files`

, a list of the form`(string type time1 time2)`

, where`string`

is a file name and`time1`

and`time2`

were 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`car`

of this list is the list of all package names known to this Common Lisp right now and the`cdr`

of this list is the value of this`state`

variable after you look at its`car`

. The function,`list-all-package-names`

, which takes the state as an argument, returns the`car`

and`cdr`

s the list (returning a new state too). This essentially gives ACL2 access to what is provided by CLTL's`list-all-packages`

.`Defpkg`

uses this feature to insure that the about-to-be-created package is new in this lisp. Thus, for example, in`akcl`

it is impossible to create the package`"COMPILER"`

with`defpkg`

because it is on the list, while in Lucid that package name is not initially on the list.

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. Third, the documentation about state is not as complete as one might wish for serious programming that uses state.

If a user is building a model that includes a system state, it is
better to represent that state explicitly in the model rather than
use the ACL2 state object. ACL2 functions that manipulate
association lists (for example, see assoc) can be used in place
of `@`

and `assign`

to access and update the state component of
the model. As of this writing, the `"books"`

directory of the
ACL2 distribution contains a number of theorems already proved about
such functions.

A consequence of this recommendation is that ACL2 constructs like
`pprogn`

and `er-progn`

that depend on the state object will not
appear in user-built models.