EMBEDDED-EVENT-FORM

forms that may be embedded in other events 
Major Section:  MISCELLANEOUS


Examples:
(defun hd (x) (if (consp x) (car x) 0))
(local (defthm lemma23 ...))
(progn (defun fn1 ...)
       (local (defun fn2 ...))
       ...)


General Form:
An embedded event form is a term, x, such that:

x is a call of an event function other than DEFPKG (see events for a listing of the event functions);

x is of the form (LOCAL x1) where x1 is an embedded event form;

x is of the form (SKIP-PROOFS x1) where x1 is an embedded event form;

x is of the form (MAKE-EVENT &), where & is any term whose expansion is an embedded event (see make-event);

x is of the form (WITH-OUTPUT ... x1) where x1 is an embedded event form;

x is of the form (VALUE-TRIPLE &), where & is any term;

x is a call of ENCAPSULATE, PROGN, PROGN!, or INCLUDE-BOOK;

x macroexpands to one of the forms above; or

[intended only for the implementation] x is (RECORD-EXPANSION x1 x2), where x1 and x2 are embedded event forms.

An exception: an embedded event form may not set the acl2-defaults-table when in the context of local. Thus for example, the form 
(local (table acl2-defaults-table :defun-mode :program))
is not an embedded event form, nor is the form (local (program)), since the latter sets the acl2-defaults-table implicitly. An example at the end of the discussion below illustrates why there is this restriction.

Only embedded event forms are allowed in a book after its initial in-package form. See books. However, you may find that make-event allows you to get the effect you want for a form that is not an embedded event form. For example, you can put the following into a book, which assigns the value 17 to state global variable x: 

(make-event (er-progn (assign x 17)
                      (value '(value-triple nil)))
            :check-expansion t)

When an embedded event is executed while ld-skip-proofsp is 'include-book, those parts of it inside local forms are ignored. Thus, 

   (progn (defun f1 () 1)
          (local (defun f2 () 2))
          (defun f3 () 3))
will define f1, f2, and f3 when ld-skip-proofsp is nil but will define only f1 and f3 when ld-skip-proofsp is 'include-book.

Discussion:

Encapsulate, progn, and include-book place restrictions on the kinds of forms that may be processed. These restrictions ensure that the non-local events are indeed admissible provided that the sequence of local and non-local events is admissible when proofs are done, i.e., when ld-skip-proofs is nil. But progn! places no such restrictions, hence is potentially dangerous and should be avoided unless you understand the ramifications; so it is illegal unless there is an active trust tag (see defttag).

Local permits the hiding of an event or group of events in the sense that local events are processed when we are trying to establish the admissibility of a sequence of events embedded in encapsulate forms or in books, but are ignored when we are constructing the world produced by assuming that sequence. Thus, for example, a particularly ugly and inefficient :rewrite rule might be made local to an encapsulate that ``exports'' a desirable theorem whose proof requires the ugly lemma.

To see why we can't allow just anything in as an embedded event, consider allowing the form 

(if (ld-skip-proofsp state)
    (defun foo () 2)
    (defun foo () 1))
followed by 
(defthm foo-is-1 (equal (foo) 1)).
When we process the events with ld-skip-proofsp, nil the second defun is executed and the defthm succeeds. But when we process the events with ld-skip-proofsp 'include-book, the second defun is executed, so that foo no longer has the same definition it did when we proved foo-is-1. Thus, an invalid formula is assumed when we process the defthm while skipping proofs. Thus, the first form above is not a legal embedded event form.

If you encounter a situation where these restrictions seem to prevent you from doing what you want to do, then you may find make-event to be helpful. See make-event.

Defpkg is not allowed because it affects how things are read after it is executed. But all the forms embedded in an event are read before any are executed. That is, 

(encapsulate nil
             (defpkg "MY-PKG" nil)
             (defun foo () 'my-pkg::bar))
makes no sense since my-pkg::bar must have been read before the defpkg for "MY-PKG" was executed.

Finally, let us elaborate on the restriction mentioned earlier related to the acl2-defaults-table. Consider the following form. 

(encapsulate
 ()
 (local (program))
 (defun foo (x)
   (if (equal 0 x)
       0
     (1+ (foo (- x))))))
See local-incompatibility for a discussion of how encapsulate processes event forms. Briefly, on the first pass through the events the definition of foo will be accepted in defun mode :program, and hence accepted. But on the second pass the form (local (program)) is skipped because it is marked as local, and hence foo is accepted in defun mode :logic. Yet, no proof has been performed in order to admit foo, and in fact, it is not hard to prove a contradiction from this definition!