Major Section: EVENTS
Make-event is a utility for generating events. It provides a
capability not offered by Lisp macros (see defmacro), as it allows access to
state and logical world. In essence, the expression
(make-event form) replaces itself with the result of evaluating
ev, as though one had submitted
ev instead of the
call. But the evaluation of
form may involve
state and even modify
state, for example by attempting to admit some definitions and theorems.
Make-event protects the ACL2 logical world so that it is restored
form is evaluated, before
ev is submitted.
; Trivial example: evaluate (quote (defun foo (x) x)) to obtain ; (defun foo (x) x), which is then evaluated. (make-event (quote (defun foo (x) x)))
; Evaluate (generate-form state) to obtain (mv nil val state), and ; then evaluate val. (Generate-form is not specified here, but ; imagine for example that it explores the state and then generates ; some desired definition or theorem.) (make-event (generate-form state))
; As above, but make sure that if this form is in a book, then when ; we include the book, the evaluation of (generate-form state) ; should return the same value as it did when the book was ; certified. (make-event (generate-form state) :check-expansion t)
; As above (where the :check-expansion value can be included or ; not), where if there is an error during expansion, then the error ; message will explain that expansion was on behalf of the indicated ; object, typically specified as the first argument. (make-event (generate-form state) :on-behalf-of (generate-form state))
General Form: (make-event form :check-expansion chk :on-behalf-of obj)
t, or the intended ``expansion result'' from the evaluation of
form(as explained below); and if supplied,
objis an arbitrary ACL2 object, used only in reporting errors in expansion, i.e., in the evaluation of form.
We strongly recommend that you look at
summarizes and suggests browsing of some
.lisp files in that directory,
in order to understand
make-event, perhaps before continuing to read this
documentation. For example,
eval.lisp contains definitions of macros
must-fail that are useful for testing and are used
in many other books in that directory, especially
than the examples, the explanations here should suffice for most users. If
you want explanations of subtler details, see make-event-details.
Make-event is related to Lisp macroexpansion in the sense that its
argument is evaluated to obtain an expansion result, which is evaluated
again. Let us elaborate on each of these notions in turn: ``is evaluated,''
``expansion result'', and ``evaluated again.''
Note that the result of expansion can be an ordinary event, but it can instead be another call of
``is evaluated'' -- The argument can be any expression, which is evaluated as would be any expression submitted to ACL2's top level loop. Thus,
stobjs may appear in the form supplied to
make-event. Henceforth, we will refer to this evaluation as ``expansion.'' Expansion is actually done in a way that restores ACL2's built-in
stateglobal variables, including the logical world, to their pre-expansion values (with a few exceptions -- see make-event-details -- and where we note that changes to user-defined
stateglobal variables (see assign) are preserved). So, for example, events might be evaluated during expansion, but they will disappear from the logical world after expansion returns its result. Moreover, proofs are enabled by default at the start of expansion (see ld-skip-proofsp), because an anticipated use of
make-eventis to call the prover to decide which event to generate, and that would presumably be necessary even if proofs had been disabled.
``expansion result'' -- The above expansion may result in an ordinary (non-
stobj) value, which we call the ``expansion result.'' Or, expansion may result in a multiple value of the form
(mv erp val state stobj-1 ... stobj-k), where
kmay be 0; in fact the most common case is probably
(mv erp val state). In that case, if
nil, then there is no expansion result, and the original
make-eventevaluates to a soft error. If however
nil, then the resulting value is
valmust be an embedded event form (see embedded-event-form); otherwise, the original
make-eventevaluates to a soft error. Note that error messages from expansion are printed as described under ``Error Reporting'' below.
``evaluated again'' -- the expansion result is evaluated in place of the original
make-event, or even of a call of a macro that expands to a call of
make-event. Or, expansion itself can cause subsidiary calls of
make-event, for example if expansion uses
ldto evaluate some
make-eventforms. The state global variable
make-event-debugmay be set to a non-
nilvalue in order to see a trace of the expansion process, where the level shown (as in ``
3>'') indicates the depth of expansions in progress.
Expansion of a
make-event call will yield an event that replaces the
make-event call. In particular, if you put a
form in a book, then in essence it is replaced by its expansion result,
created during the proof pass of the
certify-book process. We now
elaborate on this idea of keeping the original expansion.
By default, a
make-event call in a certified book is replaced (by a
process hidden from the user, in an
:expansion-alist field of the book's
certificate) by the expansion result from evaluation of its first
argument. Thus, although the book is not textually altered during
certification, one may imagine a ``book expansion'' corresponding to the
original book in which all of the events for which expansion took place
(during the proof phase of certification) have been replaced by their
expansions. A subsequent
include-book will then include the book
expansion corresponding to the indicated book. When a book is compiled
certify-book, it is actually the corresponding book expansion,
stored as a temporary file, that is compiled instead. That temporary file is
deleted after compilation unless one first evaluates the form
(assign keep-tmp-files t). Note however that all of the original forms
must still be legal events (see embedded-event-form). So for example,
if the first event in a book is
(local (defmacro my-id (x) x)), followed
(my-id (make-event ...)), the final ``
include-book'' pass of
certify-book will fail because
my-id is not defined when the
my-id call is encountered.
The preceding paragraph begins with ``by default'' because if you specify
:check-expansion t, then subsequent evaluation of the same
call -- during the second pass of an
encapsulate or during event
processing on behalf of
include-book -- will do the expansion again
and check that the expansion result equals the original expansion result. In
the unusual case that you know the expected expansion result,
:check-expansion res. This will will cause a check that
every subsequent expansion result for the
make-event form is
including the original one. IMPORTANT NOTE: That expansion check is only
done when processing events, not during a preliminary load of a book's
compiled file. The following paragraph elaborates.
(Here are details on the point made just above, for those who use the
:check-expansion argument to perform side-effects on the state.
When you include a book, ACL2 generally loads a compiled file before
processing the events in the book; see book-compiled-file. While it is true
that a non-
:check-expansion argument causes
perform expansion of the
make-event form during event processing it does
not perform expansion when the compiled file (or expansion file; again,
see book-compiled-file) is loaded.)
ACL2 performs the following space-saving optimization for book certificates:
local event arising from
make-event expansion is replaced in that
(local (value-triple :ELIDED)).
Finally, we note that ACL2 extends the notion of ``make-event expansion'' to
the case that a call of
make-event is found in the course of
macroexpansion. We illustrate with the following example.
(encapsulate () (defmacro my-mac () '(make-event '(defun foo (x) x))) (my-mac)) :pe :hereThe above call of
peshows that the form
(DEFUN FOO (X) X):
(ENCAPSULATE NIL (DEFMACRO MY-MAC NIL '(MAKE-EVENT '(DEFUN FOO (X) X))) (RECORD-EXPANSION (MY-MAC) (DEFUN FOO (X) X)))
Suppose that expansion produces a soft error as described above. That is,
suppose that the argument of a
make-event call evaluates to a multiple
(mv erp val state ...) where
erp is not
a string, then that string is printed in the error message. If
cons pair whose
car is a string, then the error prints
#\0 bound to that
cons pair; see fmt. Any other
nil value of
erp causes a generic error message to be printed.
Restriction to the Top Level.
Every form enclosing a
make-event call must be an embedded event form
(see embedded-event-form). This restriction enables ACL2 to track
expansions produced by
make-event. For example:
; Legal: (progn (with-output :on summary (make-event '(defun foo (x) x))))Low-level remark, for system implementors. There is the one exception to this restriction: a single
; Illegal: (mv-let (erp val state) (make-event '(defun foo (x) x)) (mv erp val state))
state-global-let*form immediately under a
progn!call. For example:
(progn! (state-global-let* <bindings> (make-event ...)))However, the following form may be preferable (see progn!):
(progn! :state-global-bindings <bindings> (make-event ...))See remove-untouchable for an interesting use of this exception.
Examples illustrating how to access state
You can modify the ACL2 state by doing your state-changing computation during the expansion phase, before expansion returns the event that is submitted. Here are some examples.
First consider the following. Notice that expansion modifies state global
make-event expansion, and then expansion returns a
defun event to be evaluated.
(make-event (er-progn (assign my-global (length (w state))) (value '(defun foo (x) (cons x x)))))Then we get:
ACL2 !>(@ my-global) 72271 ACL2 !>:pe foo L 1:x(MAKE-EVENT (ER-PROGN # #)) >L (DEFUN FOO (X) (CONS X X)) ACL2 !>
Here's a slightly fancier example, where the computation affects the
defun. In a new session, execute:
(make-event (er-progn (assign my-global (length (w state))) (value `(defun foo (x) (cons x ,(@ my-global))))))Then:
ACL2 !>(@ my-global) 72271 ACL2 !>:pe foo L 1:x(MAKE-EVENT (ER-PROGN # #)) >L (DEFUN FOO (X) (CONS X 72271)) ACL2 !>Note that ACL2 table events may avoid the need to use state globals. For example, instead of the example above, consider this example in a new session.
(make-event (let ((world-len (length (w state)))) `(progn (table my-table :stored-world-length ,world-len) (defun foo (x) (cons x ,world-len)))))Then:
ACL2 !>(table my-table) ((:STORED-WORLD-LENGTH . 72271)) ACL2 !>:pe foo 1:x(MAKE-EVENT (LET # #)) >L (DEFUN FOO (X) (CONS X 72271)) ACL2 !>
By the way, most built-in state globals revert after expansion. But
your own global (like
my-global above) can be set during expansion, and
the new value will persist.