turn on memoization for a specified function
Major Section:  EVENTS

This documentation topic relates to an experimental extension of ACL2 under development by Bob Boyer and Warren Hunt. See hons-and-memoization for a general discussion of memoization and the related features of hash consing and applicative hash tables.

(memoize 'foo)                      ; remember the values of calls
                                    ;   of foo
(memoize 'foo :condition t)         ; same as above
(memoize 'foo :condition '(test x)) ; memoize for args satisfying
                                    ;   the given condition
(memoize 'foo :condition-fn 'test)  ; memoize for args satisfying
                                    ;   a call of the given function
(memoize 'foo :inline nil)          ; do not inline the definition
                                    ;   of foo
(memoize 'foo :recursive nil)       ; as above, i.e. :inline nil
(memoize 'foo :aokp t)              ; attachments OK for stored results
(memoize 'foo :ideal-okp t)         ; memoize even if foo is in :logic mode
                                    ;   but has not been guard-verified

Some Related Topics

General Form: (memoize fn ; memoizes fn and returns fn :condition condition ; optional (default t) :condition-fn condition-fn ; optional :hints hints ; optional, for verifying the ; guards of condition-fn :otf-flg otf-flg ; optional, for verifying the ; guards of condition-fn :inline inline ; optional (default t) :recursive inline ; optional (default t) :commutative t/lemma-name ; optional (default nil) :forget t/nil ; optional (default nil) :memo-table-init-size size ; optional (default *mht-default-size*) :aokp t/nil ; optional (default nil) :ideal-okp t/:warn/nil ; optional (default nil) :verbose t/nil ; optional (default t) )
where fn evaluates to a user-defined function symbol; condition is either t (the default), 't, nil, or 'nil, or else evaluates to an expression whose free variables are among the formal parameters of fn; and condition-fn is either nil (the default) or else evaluates to a legal function symbol. Further restrictions and options are discussed below. Note that all arguments are evaluated (but for the special handling of value t for :commutative, the argument must literally be t; see below).

Generally fn must evaluate to a defined function symbol. However, this value can be the name of a macro that is associated with such a function symbol; see macro-aliases-table. That associated function symbol is the one called ``memoized'' in the discussion below, but we make no more mention of this subtlety.

In the most common case, memoize takes a single argument, which evaluates to a function symbol. We call this function symbol the ``memoized function'' because ``memos'' are saved and re-used, in the following sense. When a call of the memoized function is evaluated, the result is ``memoized'' by associating the call's arguments with that result, in a suitable table. But first an attempt is made to avoid such evaluation, by doing a lookup in that table on the given arguments for the result, as stored for a previous call on those arguments. If such a result is found, then it is returned without further computation. This paragraph also applies if :condition is supplied but is t or 't.

If keyword argument :condition-fn is supplied, but :condition is not, then the result of evaluating :condition-fn must be a defined function symbol whose formal parameter list and guard are the same as for the function being memoized. If fn is in :logic mode, then guards must have been verified for :condition-fn. Such a ``condition function'' will be run whenever the memoized function is called, on the same parameters, and the lookup or table store described above are only performed if the result from the condition function call is non-nil.

Suppose however that :condition is supplied. If the value supplied is t or 't, then the lookup and table store described above are always done. If the value is nil or 'nil, then this lookup and table store are never done, although statistics may be gathered; see profile. Now consider other values for :condition. An attempt will be made to define a condition function whose guard and formal parameters list are the same as those of the memoized function, and whose body is the result, r, of evaluating the given condition. The name of that condition function is the result of evaluating :condition-fn if supplied, else is the result of concatenating the string "-MEMOIZE-CONDITION" to the end of the name of the memoized function. The condition function will be defined with guard verification turned off, but that definition will be followed immediately by a verify-guards event; and this is where the optional :hints and :otf-flg are attached. At evaluation time the condition function is used as described in the preceding paragraph; so in effect, the condition (r, above) is evaluated, with its variables bound to the corresponding actuals of the memoized function call, and the memoized function attempts a lookup or table store if and only if the result of that evaluation is non-nil.

Note that fn can be either a :logic mode function or a :program mode function. However, only the corresponding raw Lisp function is actually memoized, so guard violations can defeat memoization, and :logic mode functions without their guards verified will only be memoized when called by :program mode functions. (See guards-and-evaluation for more information about guards and evaluation in ACL2.) If fn is a :logic mode function and :condition is supplied and not t or nil, then the condition must be a guard-verified function.

Calls of this macro generate events of the form (table memoize-table fn ((:condition-fn fn) (:inline i) ...)). When successful, the returned value is of the form (mv nil function-symbol state).

Suppose that a function is already memoized. Then it is illegal to memoize that function. Moreover, if the function was memoized with an associated condition (i.e., was memoized with keyword :condition or :condition-fn having value other than t or nil), then it is also illegal to convert the function from :program to :logic mode (see verify-termination). To turn off memoization, see unmemoize.

Memoize is illegal for a function if its arguments include state or if it returns any stobjs. Also, memoize never allows attachments to be used (see defattach); if an attachment is used during evaluation, then the evaluation result will not be stored.

We conclude with by documenting keyword parameters not discussed above.

Keyword parameter :recursive is a synonym for :inline. Each must be given the same Boolean value, and both are t by default. When either is supplied the value nil, then memoize does not use the definitional body of fn in the body of the new, memoized definition of fn. Instead, memoize lays down a call to the symbol-function for fn that was in effect prior to memoization. Use value nil for :inline or :recursive to avoid memoizing recursive calls to fn directly from within fn.

If :trace has a non-nil value, then memoize also traces in a traditional Lisp style. If :trace has value notinline or notinline, then a corresponding declaration is added at the beginning of the new definition of fn.

A non-nil value for :commutative can be supplied if fn is a binary function in :logic mode. If the memoize event is successful, then subsequently: whenever each argument to fn is either a rational number or a hons, then when the evaluation of fn on those arguments is memoized, the evaluation of fn on the swap of those arguments is, in essence, also memoized. If :commutative is supplied and is not nil or t, then it should be the name of a previously-proved theorem whose formula states the commutativity of fn, i.e., is the formula (equal (fn x y) (fn y x)) for a pair {x,y} of distinct variables. If :commutative is t -- but not merely an expression that evaluates to t -- then an attempt to prove such a lemma will be made on-the-fly. The name of the lemma is the symbol in the same package as fn, obtained by adding the suffix "-COMMUTATIVE" to the symbol-name of fn. If the proof attempt fails, then you may want first to prove the lemma yourself with appropriate hints and perhaps supporting lemmas, and then supply the name of that lemma as the value of :commutative.

If :commutative is supplied, and a non-commutative condition is provided by :condition or :condition-fn, then although the results will be correct, the extra memoization afforded by :commutative is unspecified.

If :memo-table-init-size is supplied, then it should be a positive integer specifying the initial size of an associated hash table.

Argument :aokp is relevant only when attachments are used; see defattach for background on attachments. When :aokp is nil, the default, computed values are not stored when an attachment was used, or even when an attachment may have been used because a function was called that had been memoized using :aokp t. Otherwise, computed values are always stored, but saved values are not used except when attachments are allowed. To summarize:

  aokp=nil (default): ``Pure'', i.e., values do not depend on attachments
  - Fetch: always legal
  - Store: only store resulting value when attachments were not used

  aokp=t: ``Impure'', i.e., values may depend on attachments
  - Fetch: only legal when attachments are allowed (e.g., not during proofs)
  - Store: always legal

If :ideal-okp is supplied and not nil, then it is permitted to memoize an ``ideal-mode'' function: one in :logic mode whose guards have not been verified. In general, it is ill-advised to memoize an ideal-mode function, because its calls are typically evaluated ``in the logic'' without calling a memoized ``raw Lisp'' version of the function. However, if the function is called by a :program mode function, evaluation can transfer to raw Lisp before reaching the call of the memoized function, in which case memoization will take place. For such situations you can provide value :warn or t for keyword parameter :ideal-okp. Both of these values allow memoization of ideal-mode functions, but if :warn is supplied then a warning will take place. Note that you may set the key :memoize-ideal-okp of the acl2-defaults-table to value t or :warn to change the default, but if parameter :ideal-okp is supplied, the acl2-defaults-table value is ignored.

If :verbose is supplied, it should either be nil, which will inhibit proof, event, and summary output (see with-output), or else t (the default), which does not inhibit output. If the output baffles you, try

:trans1 (memoize ...)
to see the single-step macroexpansion of your memoize call.

The default for :forget is nil. If :forget is supplied, and not nil, then it must be t, which causes all memoization done for a top-level call of fn to be forgotten when that top-level call exits.