Major Section: MISCELLANEOUS
General Form of an Extended :Meta theorem:
(implies (and (pseudo-termp x) ; this hyp is optional
(alistp a) ; this hyp is optional
(ev (hyp-fn x mfc state) a) ; this hyp is optional
; meta-extract hyps may also be included (see below)
)
(equiv (ev x a)
(ev (fn x mfc state) a)))
where the restrictions are as described in the documentation for
meta where state is literally the symbol STATE, and x,
a, mfc, and state are distinct variable symbols. A :meta
theorem of the above form installs fn as a metatheoretic simplifier with
hypothesis function hyp-fn, exactly as for vanilla metafunctions. The
only difference is that when the metafunctions are applied, some contextual
information is passed in via the mfc argument and the ACL2 state is
made available.See meta for a discussion of vanilla flavored metafunctions. This
documentation assumes you are familiar with the simpler situation, in
particular, how to define a vanilla flavored metafunction, fn, and its
associated hypothesis metafunction, hyp-fn, and how to state and prove
metatheorems installing such functions. Defining extended metafunctions
requires that you also be familiar with many ACL2 implementation details.
This documentation is sketchy on these details; see the ACL2 source code or
email the acl2-help list if you need more help.
Additional hypotheses are supported, called ``meta-extract hypotheses'', that allow metafunctions to depend on the validity of certain terms extracted from the context or the logical world. These hypotheses provide an even more advanced form of metatheorem so we explain them elsewhere; see meta-extract.
The metafunction context, mfc, is a list containing many different data
structures used by various internal ACL2 functions. We do not document the
form of mfc. Your extended metafunction should just take mfc as its
second formal and pass it into the functions mentioned below. The ACL2
state is well-documented (see state). Below we present expressions
below that can be useful in defining extended metafunctions. Some of these
expressions involve keyword arguments, :forcep and :ttree, which are
optional and in most cases are fine to omit, and which we explain after we
present the useful expressions.
(mfc-clause mfc): returns the current goal, in clausal form. A clause is
a list of ACL2 terms, implicitly denoting the disjunction of the listed
terms. The clause returned by mfc-clause is the clausal form of the
translation (see trans) of the goal or subgoal on which the rewriter is
working. When a metafunction calls mfc-clause, the term being rewritten
by the metafunction either occurs somewhere in this clause or, perhaps more
commonly, is being rewritten on behalf of some lemma to which the rewriter
has backchained while trying to rewrite a term in the clause.
(mfc-ancestors mfc): returns an alist whose keys are the negations of the
backchaining hypotheses being pursued. In particular,
(null (mfc-ancestors mfc)) will be true exactly when rewriting is on part
of the current goal. Exception: An element of this alist whose key is of the
form (:binding-hyp hyp unify-subst) indicates that hyp has been
encountered as a hypothesis of the form (equal var term) or
(equiv var (double-rewrite-term)), in each case binding variable var
to the result of rewriting term under unify-subst.
(mfc-rdepth mfc): returns the remaining stack depth for calls of the
rewriter (by default, *default-rewrite-stack-limit* at the top level;
see rewrite-stack-limit). When this is 0, no further calls of the rewriter
can be made without error.
(mfc-type-alist mfc): returns the type-alist governing the occurrence of
the term, x, being rewritten by the metafunction. A type-alist is an
association list, each element of which is of the form (term ts . ttree).
Such an element means that the term term has the type-set ts.
The ttree component is probably irrelevant here. All the terms in the
type-alist are in translated form (see trans). The ts are numbers
denoting finite Boolean combinations of ACL2's primitive types
(see type-set). The type-alist includes not only information gleaned from
the conditions governing the term being rewritten but also that gleaned from
forward chaining from the (negations of the) other literals in the clause
returned by mfc-clause.
(mfc-unify-subst mfc): returns the unifying substitution when evaluating
a syntaxp or bind-free hypothesis, else returns nil.
(mfc-world mfc): returns the ACL2 logical world.
(mfc-ts term mfc state :forcep forcep :ttreep ttreep): returns the
type-set of term in the current context; see type-set.
(mfc-rw term obj equiv-info mfc state): returns the result of rewriting
term in the current context, mfc, with objective obj and the
equivalence relation described by equiv-info. Obj should be t,
nil, or ?, and describes your objective: try to show that term is
true, false, or anything. Equiv-info is either nil, t, a
function symbol fn naming a known equivalence relation, or a list of
congruence rules. Nil means return a term that is equal to term.
T means return a term that is propositionally equivalent (i.e., in the
iff sense) to term, while fn means return a term
fn-equivalent to term. The final case, which is intended only for
advanced users, allows the specification of generated equivalence relations,
as supplied to the geneqv argument of rewrite. Generally, if you
wish to establish that term is true in the current context, use the idiom
(equal (mfc-rw term t t mfc state) *t*)The constant
*t* is set to the internal form of the constant term t,
i.e., 't.(mfc-rw+ term alist obj equiv-info mfc state): if alist is nil
then this is equivalent to (mfc-rw term obj equiv-info mfc state).
However, the former takes an argument, alist, that binds variables to
terms, and returns the result of rewriting term under that alist,
where this rewriting is as described for mfc-rw above. The function
mfc-rw+ can be more efficient than mfc-rw, because the terms in the
binding alist have generally already been rewritten, and it can be
inefficient to rewrite them again. For example, consider a rewrite rule of
the following form.
(implies (and ...
(syntaxp (... (mfc-rw `(bar ,x) ...) ...))
...)
(equal (... x ...) ...))
Here, x is bound in the conclusion to the result of rewriting some term,
say, tm. Then the above call of mfc-rw will rewrite tm, which is
probably unnecessary. So a preferable form of the rule above may be as
follows, so that tm is not further rewritten by mfc-rw+.
(implies (and ...
(syntaxp (... (mfc-rw+ '(bar v) `((v . ,x)) ...) ...))
...)
(equal (... x ...) ...))
However, you may find that the additional rewriting done by mfc-rw is
useful in some cases.(mfc-ap term mfc state): returns t or nil according to whether
linear arithmetic can determine that term is false. To the cognoscenti:
returns the contradiction flag produced by linearizing term and adding it
to the linear-pot-lst.
(mfc-relieve-hyp hyp alist rune target bkptr mfc state): returns c[t or
nil according to whether the indicated hypothesis term, hyp, can be
relieved (proved) under the giving variable bindings, alist. Here,
hyp is the hypothesis of the indicated rune at (one-based) position
bkptr, and target is an instantiated term to which rune is being
applied. Note that no indication is returned for whether any assumptions
have been generated by force or case-split. (If you need such a
feature, feel free to request it of the ACL2 implementors.)
We explain the :forcep and :ttreep keyword arguments provided in some
expressions, as promised above. Their defaults are :same and nil,
respectively. A value of :same for :forcep means that forcing will
be allowed if and only if it is allowed in the current rewriting environment;
see force. A value of t or nil for :forcep overrides this
default and allows or disallows forcing, respectively. By default these
functions return a single value, val, but if :ttreep is t then
they return (mv val ttree), where ttree is the tag-tree (see ttree)
returned by the indicated operation, with an input tag-tree of nil).
During the execution of a metafunction by the theorem prover, the expressions
above compute the results specified. Typically, you should imagine that
there are no axioms about the mfc- function symbols: treat them as
uninterpreted function symbols. There is an advanced feature, meta-extract
hypotheses, that can avoid this logical weakness in some cases when proving
:meta rules; see meta-extract. But we assume for the rest of the
present documentation topic that you do not use meta-extract hypotheses.
Thus, in the proof of the correctness of a metafunction, no information is
available about the results of these functions: you should
use these functions for heuristic purposes only.
For example, your metafunction may use these functions to decide whether to
perform a given transformation, but the transformation must be sound
regardless of the value that your metafunction returns. We illustrate this
below. It is sometimes possible to use the hypothesis metafunction,
hyp-fn, to generate a sufficient hypothesis to justify the
transformation. The generated hypothesis might have already been ``proved''
internally by your use of mfc-ts or mfc-rw, but the system will have
to prove it ``officially'' by relieving it. We illustrate this below also.
We conclude with a script that defines, verifies, and uses several extended metafunctions. This script is based on one provided by Robert Krug, who was instrumental in the development of this style of metafunction and whose help we gratefully acknowledge.
; Here is an example. I will define (foo i j) simply to be (+ i j).
; But I will keep its definition disabled so the theorem prover
; doesn't know that. Then I will define an extended metafunction
; that reduces (foo i (- i)) to 0 provided i has integer type and the
; expression (< 10 i) occurs as a hypothesis in the clause.
; Note that (foo i (- i)) is 0 in any case.
(defun foo (i j) (+ i j))
(defevaluator eva eva-lst ((foo i j)
(unary-- i)
; I won't need these two cases until the last example below, but I
; include them now.
(if x y z)
(integerp x)))
(set-state-ok t)
(defun metafn (x mfc state)
(cond
((and (consp x)
(equal (car x) 'foo)
(equal (caddr x) (list 'unary-- (cadr x))))
; So x is of the form (foo i (- i)). Now I want to check some other
; conditions.
(cond ((and (ts-subsetp (mfc-ts (cadr x) mfc state)
*ts-integer*)
(member-equal `(NOT (< '10 ,(cadr x)))
(mfc-clause mfc)))
(quote (quote 0)))
(t x)))
(t x)))
(defthm metafn-correct
(equal (eva x a) (eva (metafn x mfc state) a))
:rule-classes ((:meta :trigger-fns (foo))))
(in-theory (disable foo))
; The following will fail because the metafunction won't fire.
; We don't know enough about i.
(thm (equal (foo i (- i)) 0))
; Same here.
(thm (implies (and (integerp i) (< 0 i)) (equal (foo i (- i)) 0)))
; But this will work.
(thm (implies (and (integerp i) (< 10 i))
(equal (foo i (- i)) 0)))
; This won't, because the metafunction looks for (< 10 i) literally,
; not just something that implies it.
(thm (implies (and (integerp i) (< 11 i))
(equal (foo i (- i)) 0)))
; Now I will undo the defun of metafn and repeat the exercise, but
; this time check the weaker condition that (< 10 i) is provable
; (by rewriting it) rather than explicitly present.
(ubt 'metafn)
(defun metafn (x mfc state)
(cond
((and (consp x)
(equal (car x) 'foo)
(equal (caddr x) (list 'unary-- (cadr x))))
(cond ((and (ts-subsetp (mfc-ts (cadr x) mfc state)
*ts-integer*)
(equal (mfc-rw `(< '10 ,(cadr x)) t t mfc state)
*t*))
; The mfc-rw above rewrites (< 10 i) with objective t and iffp t. The
; objective means the theorem prover will try to establish it. The
; iffp means the theorem prover can rewrite maintaining propositional
; equivalence instead of strict equality.
(quote (quote 0)))
(t x)))
(t x)))
(defthm metafn-correct
(equal (eva x a) (eva (metafn x mfc state) a))
:rule-classes ((:meta :trigger-fns (foo))))
(in-theory (disable foo))
; Now it will prove both:
(thm (implies (and (integerp i) (< 10 i))
(equal (foo i (- i)) 0)))
(thm (implies (and (integerp i) (< 11 i))
(equal (foo i (- i)) 0)))
; Now I undo the defun of metafn and change the problem entirely.
; This time I will rewrite (integerp (foo i j)) to t. Note that
; this is true if i and j are integers. I can check this
; internally, but have to generate a hyp-fn to make it official.
(ubt 'metafn)
(defun metafn (x mfc state)
(cond
((and (consp x)
(equal (car x) 'integerp)
(consp (cadr x))
(equal (car (cadr x)) 'foo))
; So x is (integerp (foo i j)). Now check that i and j are
; ``probably'' integers.
(cond ((and (ts-subsetp (mfc-ts (cadr (cadr x)) mfc state)
*ts-integer*)
(ts-subsetp (mfc-ts (caddr (cadr x)) mfc state)
*ts-integer*))
*t*)
(t x)))
(t x)))
; To justify this transformation, I generate the appropriate hyps.
(defun hyp-fn (x mfc state)
(declare (ignore mfc state))
; The hyp-fn is run only if the metafn produces an answer different
; from its input. Thus, we know at execution time that x is of the
; form (integerp (foo i j)) and we know that metafn rewrote
; (integerp i) and (integerp j) both to t. So we just produce their
; conjunction. Note that we must produce a translated term; we
; cannot use the macro AND and must quote constants! Sometimes you
; must do tests in the hyp-fn to figure out which case the metafn
; produced, but not in this example.
`(if (integerp ,(cadr (cadr x)))
(integerp ,(caddr (cadr x)))
'nil))
(defthm metafn-correct
(implies (eva (hyp-fn x mfc state) a)
(equal (eva x a) (eva (metafn x mfc state) a)))
:rule-classes ((:meta :trigger-fns (integerp))))
(in-theory (disable foo))
; This will not be proved.
(thm (implies (and (rationalp x) (integerp i)) (integerp (foo i j))))
; But this will be.
(thm (implies (and (rationalp x)
(integerp i)
(integerp j))
(integerp (foo i j))))
