1Memoize(3pm)           Perl Programmers Reference Guide           Memoize(3pm)
2
3
4

NAME

6       Memoize - Make functions faster by trading space for time
7

SYNOPSIS

9               # This is the documentation for Memoize 1.01
10               use Memoize;
11               memoize('slow_function');
12               slow_function(arguments);    # Is faster than it was before
13
14       This is normally all you need to know.  However, many options are
15       available:
16
17               memoize(function, options...);
18
19       Options include:
20
21               NORMALIZER => function
22               INSTALL => new_name
23
24               SCALAR_CACHE => 'MEMORY'
25               SCALAR_CACHE => ['HASH', \%cache_hash ]
26               SCALAR_CACHE => 'FAULT'
27               SCALAR_CACHE => 'MERGE'
28
29               LIST_CACHE => 'MEMORY'
30               LIST_CACHE => ['HASH', \%cache_hash ]
31               LIST_CACHE => 'FAULT'
32               LIST_CACHE => 'MERGE'
33

DESCRIPTION

35       `Memoizing' a function makes it faster by trading space for time.  It
36       does this by caching the return values of the function in a table.  If
37       you call the function again with the same arguments, "memoize" jumps in
38       and gives you the value out of the table, instead of letting the
39       function compute the value all over again.
40
41       Here is an extreme example.  Consider the Fibonacci sequence, defined
42       by the following function:
43
44               # Compute Fibonacci numbers
45               sub fib {
46                 my $n = shift;
47                 return $n if $n < 2;
48                 fib($n-1) + fib($n-2);
49               }
50
51       This function is very slow.  Why?  To compute fib(14), it first wants
52       to compute fib(13) and fib(12), and add the results.  But to compute
53       fib(13), it first has to compute fib(12) and fib(11), and then it comes
54       back and computes fib(12) all over again even though the answer is the
55       same.  And both of the times that it wants to compute fib(12), it has
56       to compute fib(11) from scratch, and then it has to do it again each
57       time it wants to compute fib(13).  This function does so much
58       recomputing of old results that it takes a really long time to
59       run---fib(14) makes 1,200 extra recursive calls to itself, to compute
60       and recompute things that it already computed.
61
62       This function is a good candidate for memoization.  If you memoize the
63       `fib' function above, it will compute fib(14) exactly once, the first
64       time it needs to, and then save the result in a table.  Then if you ask
65       for fib(14) again, it gives you the result out of the table.  While
66       computing fib(14), instead of computing fib(12) twice, it does it once;
67       the second time it needs the value it gets it from the table.  It
68       doesn't compute fib(11) four times; it computes it once, getting it
69       from the table the next three times.  Instead of making 1,200 recursive
70       calls to `fib', it makes 15.  This makes the function about 150 times
71       faster.
72
73       You could do the memoization yourself, by rewriting the function, like
74       this:
75
76               # Compute Fibonacci numbers, memoized version
77               { my @fib;
78                 sub fib {
79                   my $n = shift;
80                   return $fib[$n] if defined $fib[$n];
81                   return $fib[$n] = $n if $n < 2;
82                   $fib[$n] = fib($n-1) + fib($n-2);
83                 }
84               }
85
86       Or you could use this module, like this:
87
88               use Memoize;
89               memoize('fib');
90
91               # Rest of the fib function just like the original version.
92
93       This makes it easy to turn memoizing on and off.
94
95       Here's an even simpler example: I wrote a simple ray tracer; the
96       program would look in a certain direction, figure out what it was
97       looking at, and then convert the `color' value (typically a string like
98       `red') of that object to a red, green, and blue pixel value, like this:
99
100           for ($direction = 0; $direction < 300; $direction++) {
101             # Figure out which object is in direction $direction
102             $color = $object->{color};
103             ($r, $g, $b) = @{&ColorToRGB($color)};
104             ...
105           }
106
107       Since there are relatively few objects in a picture, there are only a
108       few colors, which get looked up over and over again.  Memoizing
109       "ColorToRGB" sped up the program by several percent.
110

DETAILS

112       This module exports exactly one function, "memoize".  The rest of the
113       functions in this package are None of Your Business.
114
115       You should say
116
117               memoize(function)
118
119       where "function" is the name of the function you want to memoize, or a
120       reference to it.  "memoize" returns a reference to the new, memoized
121       version of the function, or "undef" on a non-fatal error.  At present,
122       there are no non-fatal errors, but there might be some in the future.
123
124       If "function" was the name of a function, then "memoize" hides the old
125       version and installs the new memoized version under the old name, so
126       that "&function(...)" actually invokes the memoized version.
127

OPTIONS

129       There are some optional options you can pass to "memoize" to change the
130       way it behaves a little.  To supply options, invoke "memoize" like
131       this:
132
133               memoize(function, NORMALIZER => function,
134                                 INSTALL => newname,
135                                 SCALAR_CACHE => option,
136                                 LIST_CACHE => option
137                                );
138
139       Each of these options is optional; you can include some, all, or none
140       of them.
141
142   INSTALL
143       If you supply a function name with "INSTALL", memoize will install the
144       new, memoized version of the function under the name you give.  For
145       example,
146
147               memoize('fib', INSTALL => 'fastfib')
148
149       installs the memoized version of "fib" as "fastfib"; without the
150       "INSTALL" option it would have replaced the old "fib" with the memoized
151       version.
152
153       To prevent "memoize" from installing the memoized version anywhere, use
154       "INSTALL => undef".
155
156   NORMALIZER
157       Suppose your function looks like this:
158
159               # Typical call: f('aha!', A => 11, B => 12);
160               sub f {
161                 my $a = shift;
162                 my %hash = @_;
163                 $hash{B} ||= 2;  # B defaults to 2
164                 $hash{C} ||= 7;  # C defaults to 7
165
166                 # Do something with $a, %hash
167               }
168
169       Now, the following calls to your function are all completely
170       equivalent:
171
172               f(OUCH);
173               f(OUCH, B => 2);
174               f(OUCH, C => 7);
175               f(OUCH, B => 2, C => 7);
176               f(OUCH, C => 7, B => 2);
177               (etc.)
178
179       However, unless you tell "Memoize" that these calls are equivalent, it
180       will not know that, and it will compute the values for these
181       invocations of your function separately, and store them separately.
182
183       To prevent this, supply a "NORMALIZER" function that turns the program
184       arguments into a string in a way that equivalent arguments turn into
185       the same string.  A "NORMALIZER" function for "f" above might look like
186       this:
187
188               sub normalize_f {
189                 my $a = shift;
190                 my %hash = @_;
191                 $hash{B} ||= 2;
192                 $hash{C} ||= 7;
193
194                 join(',', $a, map ($_ => $hash{$_}) sort keys %hash);
195               }
196
197       Each of the argument lists above comes out of the "normalize_f"
198       function looking exactly the same, like this:
199
200               OUCH,B,2,C,7
201
202       You would tell "Memoize" to use this normalizer this way:
203
204               memoize('f', NORMALIZER => 'normalize_f');
205
206       "memoize" knows that if the normalized version of the arguments is the
207       same for two argument lists, then it can safely look up the value that
208       it computed for one argument list and return it as the result of
209       calling the function with the other argument list, even if the argument
210       lists look different.
211
212       The default normalizer just concatenates the arguments with character
213       28 in between.  (In ASCII, this is called FS or control-\.)  This
214       always works correctly for functions with only one string argument, and
215       also when the arguments never contain character 28.  However, it can
216       confuse certain argument lists:
217
218               normalizer("a\034", "b")
219               normalizer("a", "\034b")
220               normalizer("a\034\034b")
221
222       for example.
223
224       Since hash keys are strings, the default normalizer will not
225       distinguish between "undef" and the empty string.  It also won't work
226       when the function's arguments are references.  For example, consider a
227       function "g" which gets two arguments: A number, and a reference to an
228       array of numbers:
229
230               g(13, [1,2,3,4,5,6,7]);
231
232       The default normalizer will turn this into something like
233       "13\034ARRAY(0x436c1f)".  That would be all right, except that a
234       subsequent array of numbers might be stored at a different location
235       even though it contains the same data.  If this happens, "Memoize" will
236       think that the arguments are different, even though they are
237       equivalent.  In this case, a normalizer like this is appropriate:
238
239               sub normalize { join ' ', $_[0], @{$_[1]} }
240
241       For the example above, this produces the key "13 1 2 3 4 5 6 7".
242
243       Another use for normalizers is when the function depends on data other
244       than those in its arguments.  Suppose you have a function which returns
245       a value which depends on the current hour of the day:
246
247               sub on_duty {
248                 my ($problem_type) = @_;
249                 my $hour = (localtime)[2];
250                 open my $fh, "$DIR/$problem_type" or die...;
251                 my $line;
252                 while ($hour-- > 0)
253                   $line = <$fh>;
254                 }
255                 return $line;
256               }
257
258       At 10:23, this function generates the 10th line of a data file; at 3:45
259       PM it generates the 15th line instead.  By default, "Memoize" will only
260       see the $problem_type argument.  To fix this, include the current hour
261       in the normalizer:
262
263               sub normalize { join ' ', (localtime)[2], @_ }
264
265       The calling context of the function (scalar or list context) is
266       propagated to the normalizer.  This means that if the memoized function
267       will treat its arguments differently in list context than it would in
268       scalar context, you can have the normalizer function select its
269       behavior based on the results of "wantarray".  Even if called in a list
270       context, a normalizer should still return a single string.
271
272   "SCALAR_CACHE", "LIST_CACHE"
273       Normally, "Memoize" caches your function's return values into an
274       ordinary Perl hash variable.  However, you might like to have the
275       values cached on the disk, so that they persist from one run of your
276       program to the next, or you might like to associate some other
277       interesting semantics with the cached values.
278
279       There's a slight complication under the hood of "Memoize": There are
280       actually two caches, one for scalar values and one for list values.
281       When your function is called in scalar context, its return value is
282       cached in one hash, and when your function is called in list context,
283       its value is cached in the other hash.  You can control the caching
284       behavior of both contexts independently with these options.
285
286       The argument to "LIST_CACHE" or "SCALAR_CACHE" must either be one of
287       the following four strings:
288
289               MEMORY
290               FAULT
291               MERGE
292               HASH
293
294       or else it must be a reference to a list whose first element is one of
295       these four strings, such as "[HASH, arguments...]".
296
297       "MEMORY"
298           "MEMORY" means that return values from the function will be cached
299           in an ordinary Perl hash variable.  The hash variable will not
300           persist after the program exits.  This is the default.
301
302       "HASH"
303           "HASH" allows you to specify that a particular hash that you supply
304           will be used as the cache.  You can tie this hash beforehand to
305           give it any behavior you want.
306
307           A tied hash can have any semantics at all.  It is typically tied to
308           an on-disk database, so that cached values are stored in the
309           database and retrieved from it again when needed, and the disk file
310           typically persists after your program has exited.  See "perltie"
311           for more complete details about "tie".
312
313           A typical example is:
314
315                   use DB_File;
316                   tie my %cache => 'DB_File', $filename, O_RDWR|O_CREAT, 0666;
317                   memoize 'function', SCALAR_CACHE => [HASH => \%cache];
318
319           This has the effect of storing the cache in a "DB_File" database
320           whose name is in $filename.  The cache will persist after the
321           program has exited.  Next time the program runs, it will find the
322           cache already populated from the previous run of the program.  Or
323           you can forcibly populate the cache by constructing a batch program
324           that runs in the background and populates the cache file.  Then
325           when you come to run your real program the memoized function will
326           be fast because all its results have been precomputed.
327
328       "TIE"
329           This option is no longer supported.  It is still documented only to
330           aid in the debugging of old programs that use it.  Old programs
331           should be converted to use the "HASH" option instead.
332
333                   memoize ... [TIE, PACKAGE, ARGS...]
334
335           is merely a shortcut for
336
337                   require PACKAGE;
338                   { my %cache;
339                     tie %cache, PACKAGE, ARGS...;
340                   }
341                   memoize ... [HASH => \%cache];
342
343       "FAULT"
344           "FAULT" means that you never expect to call the function in scalar
345           (or list) context, and that if "Memoize" detects such a call, it
346           should abort the program.  The error message is one of
347
348                   `foo' function called in forbidden list context at line ...
349                   `foo' function called in forbidden scalar context at line ...
350
351       "MERGE"
352           "MERGE" normally means the function does not distinguish between
353           list and sclar context, and that return values in both contexts
354           should be stored together.  "LIST_CACHE => MERGE" means that list
355           context return values should be stored in the same hash that is
356           used for scalar context returns, and "SCALAR_CACHE => MERGE" means
357           the same, mutatis mutandis.  It is an error to specify "MERGE" for
358           both, but it probably does something useful.
359
360           Consider this function:
361
362                   sub pi { 3; }
363
364           Normally, the following code will result in two calls to "pi":
365
366               $x = pi();
367               ($y) = pi();
368               $z = pi();
369
370           The first call caches the value 3 in the scalar cache; the second
371           caches the list "(3)" in the list cache.  The third call doesn't
372           call the real "pi" function; it gets the value from the scalar
373           cache.
374
375           Obviously, the second call to "pi" is a waste of time, and storing
376           its return value is a waste of space.  Specifying "LIST_CACHE =>
377           MERGE" will make "memoize" use the same cache for scalar and list
378           context return values, so that the second call uses the scalar
379           cache that was populated by the first call.  "pi" ends up being
380           called only once, and both subsequent calls return 3 from the
381           cache, regardless of the calling context.
382
383           Another use for "MERGE" is when you want both kinds of return
384           values stored in the same disk file; this saves you from having to
385           deal with two disk files instead of one.  You can use a normalizer
386           function to keep the two sets of return values separate.  For
387           example:
388
389                   tie my %cache => 'MLDBM', 'DB_File', $filename, ...;
390
391                   memoize 'myfunc',
392                     NORMALIZER => 'n',
393                     SCALAR_CACHE => [HASH => \%cache],
394                     LIST_CACHE => MERGE,
395                   ;
396
397                   sub n {
398                     my $context = wantarray() ? 'L' : 'S';
399                     # ... now compute the hash key from the arguments ...
400                     $hashkey = "$context:$hashkey";
401                   }
402
403           This normalizer function will store scalar context return values in
404           the disk file under keys that begin with "S:", and list context
405           return values under keys that begin with "L:".
406

OTHER FACILITIES

408   "unmemoize"
409       There's an "unmemoize" function that you can import if you want to.
410       Why would you want to?  Here's an example: Suppose you have your cache
411       tied to a DBM file, and you want to make sure that the cache is written
412       out to disk if someone interrupts the program.  If the program exits
413       normally, this will happen anyway, but if someone types control-C or
414       something then the program will terminate immediately without
415       synchronizing the database.  So what you can do instead is
416
417           $SIG{INT} = sub { unmemoize 'function' };
418
419       "unmemoize" accepts a reference to, or the name of a previously
420       memoized function, and undoes whatever it did to provide the memoized
421       version in the first place, including making the name refer to the
422       unmemoized version if appropriate.  It returns a reference to the
423       unmemoized version of the function.
424
425       If you ask it to unmemoize a function that was never memoized, it
426       croaks.
427
428   "flush_cache"
429       "flush_cache(function)" will flush out the caches, discarding all the
430       cached data.  The argument may be a function name or a reference to a
431       function.  For finer control over when data is discarded or expired,
432       see the documentation for "Memoize::Expire", included in this package.
433
434       Note that if the cache is a tied hash, "flush_cache" will attempt to
435       invoke the "CLEAR" method on the hash.  If there is no "CLEAR" method,
436       this will cause a run-time error.
437
438       An alternative approach to cache flushing is to use the "HASH" option
439       (see above) to request that "Memoize" use a particular hash variable as
440       its cache.  Then you can examine or modify the hash at any time in any
441       way you desire.  You may flush the cache by using "%hash = ()".
442

CAVEATS

444       Memoization is not a cure-all:
445
446       ·   Do not memoize a function whose behavior depends on program state
447           other than its own arguments, such as global variables, the time of
448           day, or file input.  These functions will not produce correct
449           results when memoized.  For a particularly easy example:
450
451                   sub f {
452                     time;
453                   }
454
455           This function takes no arguments, and as far as "Memoize" is
456           concerned, it always returns the same result.  "Memoize" is wrong,
457           of course, and the memoized version of this function will call
458           "time" once to get the current time, and it will return that same
459           time every time you call it after that.
460
461       ·   Do not memoize a function with side effects.
462
463                   sub f {
464                     my ($a, $b) = @_;
465                     my $s = $a + $b;
466                     print "$a + $b = $s.\n";
467                   }
468
469           This function accepts two arguments, adds them, and prints their
470           sum.  Its return value is the numuber of characters it printed, but
471           you probably didn't care about that.  But "Memoize" doesn't
472           understand that.  If you memoize this function, you will get the
473           result you expect the first time you ask it to print the sum of 2
474           and 3, but subsequent calls will return 1 (the return value of
475           "print") without actually printing anything.
476
477       ·   Do not memoize a function that returns a data structure that is
478           modified by its caller.
479
480           Consider these functions:  "getusers" returns a list of users
481           somehow, and then "main" throws away the first user on the list and
482           prints the rest:
483
484                   sub main {
485                     my $userlist = getusers();
486                     shift @$userlist;
487                     foreach $u (@$userlist) {
488                       print "User $u\n";
489                     }
490                   }
491
492                   sub getusers {
493                     my @users;
494                     # Do something to get a list of users;
495                     \@users;  # Return reference to list.
496                   }
497
498           If you memoize "getusers" here, it will work right exactly once.
499           The reference to the users list will be stored in the memo table.
500           "main" will discard the first element from the referenced list.
501           The next time you invoke "main", "Memoize" will not call
502           "getusers"; it will just return the same reference to the same list
503           it got last time.  But this time the list has already had its head
504           removed; "main" will erroneously remove another element from it.
505           The list will get shorter and shorter every time you call "main".
506
507           Similarly, this:
508
509                   $u1 = getusers();
510                   $u2 = getusers();
511                   pop @$u1;
512
513           will modify $u2 as well as $u1, because both variables are
514           references to the same array.  Had "getusers" not been memoized,
515           $u1 and $u2 would have referred to different arrays.
516
517       ·   Do not memoize a very simple function.
518
519           Recently someone mentioned to me that the Memoize module made his
520           program run slower instead of faster.  It turned out that he was
521           memoizing the following function:
522
523               sub square {
524                 $_[0] * $_[0];
525               }
526
527           I pointed out that "Memoize" uses a hash, and that looking up a
528           number in the hash is necessarily going to take a lot longer than a
529           single multiplication.  There really is no way to speed up the
530           "square" function.
531
532           Memoization is not magical.
533

PERSISTENT CACHE SUPPORT

535       You can tie the cache tables to any sort of tied hash that you want to,
536       as long as it supports "TIEHASH", "FETCH", "STORE", and "EXISTS".  For
537       example,
538
539               tie my %cache => 'GDBM_File', $filename, O_RDWR|O_CREAT, 0666;
540               memoize 'function', SCALAR_CACHE => [HASH => \%cache];
541
542       works just fine.  For some storage methods, you need a little glue.
543
544       "SDBM_File" doesn't supply an "EXISTS" method, so included in this
545       package is a glue module called "Memoize::SDBM_File" which does provide
546       one.  Use this instead of plain "SDBM_File" to store your cache table
547       on disk in an "SDBM_File" database:
548
549               tie my %cache => 'Memoize::SDBM_File', $filename, O_RDWR|O_CREAT, 0666;
550               memoize 'function', SCALAR_CACHE => [HASH => \%cache];
551
552       "NDBM_File" has the same problem and the same solution.  (Use
553       "Memoize::NDBM_File instead of plain NDBM_File.")
554
555       "Storable" isn't a tied hash class at all.  You can use it to store a
556       hash to disk and retrieve it again, but you can't modify the hash while
557       it's on the disk.  So if you want to store your cache table in a
558       "Storable" database, use "Memoize::Storable", which puts a hashlike
559       front-end onto "Storable".  The hash table is actually kept in memory,
560       and is loaded from your "Storable" file at the time you memoize the
561       function, and stored back at the time you unmemoize the function (or
562       when your program exits):
563
564               tie my %cache => 'Memoize::Storable', $filename;
565               memoize 'function', SCALAR_CACHE => [HASH => \%cache];
566
567               tie my %cache => 'Memoize::Storable', $filename, 'nstore';
568               memoize 'function', SCALAR_CACHE => [HASH => \%cache];
569
570       Include the `nstore' option to have the "Storable" database written in
571       `network order'.  (See Storable for more details about this.)
572
573       The "flush_cache()" function will raise a run-time error unless the
574       tied package provides a "CLEAR" method.
575

EXPIRATION SUPPORT

577       See Memoize::Expire, which is a plug-in module that adds expiration
578       functionality to Memoize.  If you don't like the kinds of policies that
579       Memoize::Expire implements, it is easy to write your own plug-in module
580       to implement whatever policy you desire.  Memoize comes with several
581       examples.  An expiration manager that implements a LRU policy is
582       available on CPAN as Memoize::ExpireLRU.
583

BUGS

585       The test suite is much better, but always needs improvement.
586
587       There is some problem with the way "goto &f" works under threaded Perl,
588       perhaps because of the lexical scoping of @_.  This is a bug in Perl,
589       and until it is resolved, memoized functions will see a slightly
590       different "caller()" and will perform a little more slowly on threaded
591       perls than unthreaded perls.
592
593       Some versions of "DB_File" won't let you store data under a key of
594       length 0.  That means that if you have a function "f" which you
595       memoized and the cache is in a "DB_File" database, then the value of
596       "f()" ("f" called with no arguments) will not be memoized.  If this is
597       a big problem, you can supply a normalizer function that prepends "x"
598       to every key.
599

MAILING LIST

601       To join a very low-traffic mailing list for announcements about
602       "Memoize", send an empty note to "mjd-perl-memoize-request@plover.com".
603

AUTHOR

605       Mark-Jason Dominus ("mjd-perl-memoize+@plover.com"), Plover Systems co.
606
607       See the "Memoize.pm" Page at http://www.plover.com/~mjd/perl/Memoize/
608       for news and upgrades.  Near this page, at
609       http://www.plover.com/~mjd/perl/MiniMemoize/ there is an article about
610       memoization and about the internals of Memoize that appeared in The
611       Perl Journal, issue #13.  (This article is also included in the Memoize
612       distribution as `article.html'.)
613
614       The author's book Higher Order Perl (2005, ISBN 1558607013, published
615       by Morgan Kaufmann) discusses memoization (and many other fascinating
616       topics) in tremendous detail. It will also be available on-line for
617       free.  For more information, visit http://perl.plover.com/book/ .
618
619       To join a mailing list for announcements about "Memoize", send an empty
620       message to "mjd-perl-memoize-request@plover.com".  This mailing list is
621       for announcements only and has extremely low traffic---about two
622       messages per year.
623
625       Copyright 1998, 1999, 2000, 2001  by Mark Jason Dominus
626
627       This library is free software; you may redistribute it and/or modify it
628       under the same terms as Perl itself.
629

THANK YOU

631       Many thanks to Jonathan Roy for bug reports and suggestions, to Michael
632       Schwern for other bug reports and patches, to Mike Cariaso for helping
633       me to figure out the Right Thing to Do About Expiration, to Joshua
634       Gerth, Joshua Chamas, Jonathan Roy (again), Mark D. Anderson, and
635       Andrew Johnson for more suggestions about expiration, to Brent Powers
636       for the Memoize::ExpireLRU module, to Ariel Scolnicov for delightful
637       messages about the Fibonacci function, to Dion Almaer for thought-
638       provoking suggestions about the default normalizer, to Walt Mankowski
639       and Kurt Starsinic for much help investigating problems under threaded
640       Perl, to Alex Dudkevich for reporting the bug in prototyped functions
641       and for checking my patch, to Tony Bass for many helpful suggestions,
642       to Jonathan Roy (again) for finding a use for "unmemoize()", to
643       Philippe Verdret for enlightening discussion of "Hook::PrePostCall", to
644       Nat Torkington for advice I ignored, to Chris Nandor for portability
645       advice, to Randal Schwartz for suggesting the '"flush_cache" function,
646       and to Jenda Krynicky for being a light in the world.
647
648       Special thanks to Jarkko Hietaniemi, the 5.8.0 pumpking, for including
649       this module in the core and for his patient and helpful guidance during
650       the integration process.
651
652
653
654perl v5.12.4                      2011-06-07                      Memoize(3pm)
Impressum