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.
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
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,
s declarations and
(before macroexpansion), and an appropriate
mode (see the discussion of
modes'' below). Exceptions: (1) If either definition is
:non-executable (see xargs), then the two events must be
identical. (2) It is permissible for one definition to have a
t and the other to have no explicit guard (hence, the
guard is implicitly
verify-guards event is redundant if the function has already had
its guards verified.
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.
defconst is redundant if the name is already defined either with a
defconst event or one that defines it to have the
defstobj is redundant if there is already a
defstobj event with
the same name that has exactly the same field descriptors (see defstobj), in
the same order, and with the same
:renaming value if
supplied for either event.
defmacro is redundant if there is already a macro defined with the
same name and syntactically identical arguments, guard, and body.
defpkg is redundant if a package of the same name with exactly the
same imports has been defined.
deftheory is never redundant. The ``natural'' notion of
deftheory forms 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
deftheorys 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.
in-theory event is never redundant because it doesn't define any
push-untouchable event is redundant if every name supplied is
already a member of the untouchable symbols.
defdoc events are never redundant because they don't
define any name.
encapsulate event is redundant if and only if a syntactically
encapsulate has already been executed under the same
include-book is redundant if the book has already been included.
Note About Appropriate Modes:
Suppose a function is being redefined and that the formals, guards, types,
stobjs, and bodies are identical. When are the modes (
logic) ``appropriate?'' Identical modes are appropriate.
But what if the old mode was
:program and the new mode is
This is appropriate, provided the definition meets the requirements of the
logical definitional principle. That is, you may redefine ``redundantly''
:program mode function as a
:logic mode function provide the
measure conjectures can be proved. This is what
does. Now consider the reverse style of redefinition. Suppose the
function was defined in
:logic mode and is being identically redefined
:program mode. This is inappropriate. We do not permit the
downgrading of a function from
:logic mode to
:program mode, since
that might produce a logical world in which there were theorems about a
:program mode function, violating one of ACL2's basic assumptions.
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 used as a logical name in a theory
expression, it denotes the point in history at which
introduced. Observe that those points are different in the two
sessions. Hence, it is likely that theory expressions involving
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
fnis the very first event in the book and happens to be a
defunevent. This expression returns the advertised set if
fnis not already defined when the book is included. But if
fnwere 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
Note About Unfortunate Redundancies:
Notice that our syntactic criterion for redundancy of
does not allow redefinition to take effect unless there is a syntactic change
in the definition. The following example shows how an attempt to redefine a
function can fail to make any change.
(set-ld-redefinition-action '(:warn . :overwrite) state) (defmacro mac (x) x) (defun foo (x) (mac x)) (defmacro mac (x) (list 'car x)) (defun foo (x) (mac x)) ; redundant, unfortunately; foo does not change (thm (equal (foo 3) 3)) ; succeeds, showing that redef of foo didn't happenThe call of macro
macwas expanded away when the first definition of
foowas processed, so the new definition of
macis not seen in
foois redefined; yet our attempt at redefinition failed! An easy workaround is first to supply a different definition of
foo, just before the last definition of
fooabove. Then that final definition will no longer be redundant.
The phenomenon illustrated above can occur even without macros. Here is a more complex example, based on one supplied by Grant Passmore.
(defun n3 () 0) (defun n4 () 1) (defun n5 () (> (n3) (n4))) ; body is normalized to nil (thm (equal (n5) nil)) ; succeeds, trivially (set-ld-redefinition-action '(:warn . :overwrite) state) (defun n3 () 2)If now we execute
(thm (equal (n5) nil)), it still succeeds even though we expect
(> (n3) (n4))=
(> 2 1)=
t. That is because the body of
n5was normalized to
nil. (Such normalization can be avoided; see the brief discussion of
:normalizein the documentation for
defun.) So, given this unfortunate situation, one might expect at this point simply to redefine
n5using the same definition as before, in order to pick up the new definition of
n3. Such ``redefinition'' would, however, be redundant, for the same reason as in the previous example: no syntactic change was made to the definition. The same workaround applies as before: redefine
n5to be something different, and then redefine
n5again to be as desired.
A related phenomenon can occur for
encapsulate. As explained above, an
encapsulate event is redundant if it is identical to one already in the
database. Consider then the following contrived example.
(encapsulate () (defun foo (x) x)) (set-ld-redefinition-action '(:warn . :overwrite) state) (defun foo (x) (cons x x)) (encapsulate () (defun foo (x) x)) ; redundant!The last
encapsulateevent is redundant because it meets the criterion for redundancy: it is identical to the earlier
encapsulateevent. A workaround can be to add something trivial to the
encapsulate, for example:
(encapsulate () (deflabel try2) ; ``Increment'' to try3 next time, and so on. (defun foo (x) x))
The examples above are suggestive but by no means exhaustive. Consider the following example.
(defstub f1 () => *) (set-ld-redefinition-action '(:warn . :overwrite) state) (defun f1 () 3) (defstub f1 () => *) ; redundant -- has no effectThe reason that the final
defstubis redundant is that
defstubis a macro that expands to a call of
encapsulate; so this is very similar to the immediately preceding example.