defun
Major Section: MISCELLANEOUS
Common Lisp's defun
function does not easily allow one to pass extra
arguments such as ``hints''. ACL2 therefore supports a peculiar new
declaration (see declare) designed explicitly for passing
additional arguments to defun
via a keyword-like syntax.
The following declaration is nonsensical but does illustrate all of
the xargs
keywords (which are the members of the list
*xargs-keywords*
).
(declare (xargs :guard (symbolp x) :guard-hints (("Goal" :in-theory (theory batch1))) :guard-debug t :hints (("Goal" :in-theory (theory batch1))) :measure (- i j) :ruler-extenders :basic :mode :logic :non-executable t :normalize nil :otf-flg t :stobjs ($s) :verify-guards t :well-founded-relation my-wfr))where the keywords and their respective values are as shown below. Note that once ``inside'' the xargs form, the ``extra arguments'' toGeneral Form: (xargs :key1 val1 ... :keyn valn)
defun
are passed exactly as though they were keyword arguments.
:
GUARD
Value
is a term involving only the formals of the function being
defined. The actual guard used for the definition is the
conjunction of all the guards and types (see declare) declared.
:GUARD-HINTS
Value
: hints (see hints), to be used during the guard
verification proofs as opposed to the termination proofs of the
defun
.
:GUARD-DEBUG
Value
: nil
by default, else directs ACL2 to decorate each guard
proof obligation with hypotheses indicating its sources. See guard-debug.
:
HINTS
Value: hints (see hints), to be used during the termination
proofs as opposed to the guard verification proofs of the defun
.
:MEASURE
Value
is a term involving only the formals of the function being
defined. This term is indicates what is getting smaller in the
recursion. The well-founded relation with which successive measures
are compared is o<
. Also allowed is a special case,
(:? v1 ... vk)
, where (v1 ... vk)
enumerates a subset of the
formal parameters such that some valid measure involves only those
formal parameters. However, this special case is only allowed for
definitions that are redundant (see redundant-events) or are
executed when skipping proofs (see skip-proofs).
:RULER-EXTENDERS
For recursive definitions (possibly mutually recursive), value
controls
termination analysis and the resulting stored induction scheme.
See ruler-extenders for a discussion of legal values and their effects.
:MODE
Value
is :
program
or :
logic
, indicating the defun
mode of the
function introduced. See defun-mode. If unspecified, the
defun
mode defaults to the default defun
mode of the current world.
To convert a function from :
program
mode to :
logic
mode,
see verify-termination.
:NON-EXECUTABLE
Value
is t
or nil
(the default). Rather than using this keyword
argument directly, which requires :
logic
mode and that the
definitional body has a certain form, we suggest using the macro
defun-nx
; see defun-nx.
:NORMALIZE
Value is a flag telling defun
whether to propagate if
tests
upward. Since the default is to do so, the value supplied is only of
interest when it is nil
.
(See defun).
:
OTF-FLG
Value is a flag indicating ``onward through the fog''
(see otf-flg).
:STOBJS
Value
is either a single stobj
name or a true list of stobj names
(see stobj and see defstobj). Every stobj name among the formals of the
function must be listed, if the corresponding actual is to be treated as a
stobj. That is, if a function uses a stobj name as a formal parameter but
the name is not declared among the :stobjs
then the corresponding
argument is treated as ordinary. The only exception to this rule is
state
: whether you include it or not, state
is always treated as a
single-threaded object. This declaration has two effects. One is to enforce
the syntactic restrictions on single-threaded objects. The other is to
strengthen the guard
of the function being defined so that it includes
conjuncts specifying that each declared single-threaded object argument
satisfies the recognizer for the corresponding single-threaded object.
:
VERIFY-GUARDS
Value
is t
or nil
, indicating whether or not guards are to be
verified upon completion of the termination proof. This flag should
only be t
if the :mode
is unspecified but the default defun
mode is
:
logic
, or else the :mode
is :
logic
.
:
WELL-FOUNDED-RELATION
Value
is a function symbol that is known to be a well-founded
relation in the sense that a rule of class :
well-founded-relation
has been proved about it. See well-founded-relation.