INSTALL

        memoize('fib', INSTALL => 'fastfib')

installs the memoized version of "fib" as "fastfib"; without the "INSTALL" option it would have replaced the old "fib" with the memoized version.

To prevent "memoize" from installing the memoized version anywhere, use "INSTALL => undef".

NORMALIZER

        # Typical call: f('aha!', A => 11, B => 12);
        sub f {
          my $a = shift;
          my %hash = @_;
          $hash{B} ||= 2;  # B defaults to 2
          $hash{C} ||= 7;  # C defaults to 7

          # Do something with $a, %hash
        }

Now, the following calls to your function are all completely equivalent:

        f(OUCH);
        f(OUCH, B => 2);
        f(OUCH, C => 7);
        f(OUCH, B => 2, C => 7);
        f(OUCH, C => 7, B => 2);
        (etc.)

However, unless you tell "Memoize" that these calls are equivalent, it will not know that, and it will compute the values for these invocations of your function separately, and store them separately.

To prevent this, supply a "NORMALIZER" function that turns the program arguments into a string in a way that equivalent arguments turn into the same string. A "NORMALIZER" function for "f" above might look like this:

        sub normalize_f {
          my $a = shift;
          my %hash = @_;
          $hash{B} ||= 2;
          $hash{C} ||= 7;

          join(',', $a, map ($_ => $hash{$_}) sort keys %hash);
        }

Each of the argument lists above comes out of the "normalize_f" function looking exactly the same, like this:

        OUCH,B,2,C,7

You would tell "Memoize" to use this normalizer this way:

        memoize('f', NORMALIZER => 'normalize_f');

"memoize" knows that if the normalized version of the arguments is the same for two argument lists, then it can safely look up the value that it computed for one argument list and return it as the result of calling the function with the other argument list, even if the argument lists look different.

The default normalizer just concatenates the arguments with character 28 in between. (In ASCII, this is called FS or control-\.) This always works correctly for functions with only one string argument, and also when the arguments never contain character 28. However, it can confuse certain argument lists:

        normalizer("a\034", "b")
        normalizer("a", "\034b")
        normalizer("a\034\034b")

for example.

Since hash keys are strings, the default normalizer will not distinguish between "undef" and the empty string. It also won't work when the function's arguments are references. For example, consider a function "g" which gets two arguments: A number, and a reference to an array of numbers:

        g(13, [1,2,3,4,5,6,7]);

The default normalizer will turn this into something like "13\034ARRAY(0x436c1f)". That would be all right, except that a subsequent array of numbers might be stored at a different location even though it contains the same data. If this happens, "Memoize" will think that the arguments are different, even though they are equivalent. In this case, a normalizer like this is appropriate:

        sub normalize { join ' ', $_[0], @{$_[1]} }

For the example above, this produces the key "13 1 2 3 4 5 6 7".

Another use for normalizers is when the function depends on data other than those in its arguments. Suppose you have a function which returns a value which depends on the current hour of the day:

        sub on_duty {
          my ($problem_type) = @_;
          my $hour = (localtime)[2];
          open my $fh, "$DIR/$problem_type" or die...;
          my $line;
          while ($hour-- > 0)
            $line = <$fh>;
          } 
          return $line;
        }

At 10:23, this function generates the 10th line of a data file; at 3:45 PM it generates the 15th line instead. By default, "Memoize" will only see the $problem_type argument. To fix this, include the current hour in the normalizer:

        sub normalize { join ' ', (localtime)[2], @_ }

The calling context of the function (scalar or list context) is propagated to the normalizer. This means that if the memoized function will treat its arguments differently in list context than it would in scalar context, you can have the normalizer function select its behavior based on the results of "wantarray". Even if called in a list context, a normalizer should still return a single string.

"SCALAR_CACHE", "LIST_CACHE"

There's a slight complication under the hood of "Memoize": There are actually two caches, one for scalar values and one for list values. When your function is called in scalar context, its return value is cached in one hash, and when your function is called in list context, its value is cached in the other hash. You can control the caching behavior of both contexts independently with these options.

The argument to "LIST_CACHE" or "SCALAR_CACHE" must either be one of the following four strings:

        MEMORY
        FAULT
        MERGE
        HASH

or else it must be a reference to an array whose first element is one of these four strings, such as "[HASH, arguments...]".

"MEMORY"

"MEMORY" means that return values from the function will be cached in an ordinary Perl hash variable. The hash variable will not persist after the program exits. This is the default.

"HASH"

"HASH" allows you to specify that a particular hash that you supply will be used as the cache. You can tie this hash beforehand to give it any behavior you want.

A tied hash can have any semantics at all. It is typically tied to an on-disk database, so that cached values are stored in the database and retrieved from it again when needed, and the disk file typically persists after your program has exited. See "perltie" for more complete details about "tie".

A typical example is:

        use DB_File;
        tie my %cache => 'DB_File', $filename, O_RDWR|O_CREAT, 0666;
        memoize 'function', SCALAR_CACHE => [HASH => \%cache];
    

This has the effect of storing the cache in a "DB_File" database whose name is in $filename. The cache will persist after the program has exited. Next time the program runs, it will find the cache already populated from the previous run of the program. Or you can forcibly populate the cache by constructing a batch program that runs in the background and populates the cache file. Then when you come to run your real program the memoized function will be fast because all its results have been precomputed.

Another reason to use "HASH" is to provide your own hash variable. You can then inspect or modify the contents of the hash to gain finer control over the cache management.

"TIE"

This option is no longer supported. It is still documented only to aid in the debugging of old programs that use it. Old programs should be converted to use the "HASH" option instead.

        memoize ... ['TIE', PACKAGE, ARGS...]
    

is merely a shortcut for

        require PACKAGE;
        { tie my %cache, PACKAGE, ARGS...;
          memoize ... [HASH => \%cache];
        }
    

"FAULT"

"FAULT" means that you never expect to call the function in scalar (or list) context, and that if "Memoize" detects such a call, it should abort the program. The error message is one of

        `foo' function called in forbidden list context at line ...
        `foo' function called in forbidden scalar context at line ...
    

"MERGE"

"MERGE" normally means that the memoized function does not distinguish between list and sclar context, and that return values in both contexts should be stored together. Both "LIST_CACHE => MERGE" and "SCALAR_CACHE => MERGE" mean the same thing.

Consider this function:

        sub complicated {
          # ... time-consuming calculation of $result
          return $result;
        }
    

The "complicated" function will return the same numeric $result regardless of whether it is called in list or in scalar context.

Normally, the following code will result in two calls to "complicated", even if "complicated" is memoized:

    $x = complicated(142);
    ($y) = complicated(142);
    $z = complicated(142);
    

The first call will cache the result, say 37, in the scalar cache; the second will cach the list "(37)" in the list cache. The third call doesn't call the real "complicated" function; it gets the value 37 from the scalar cache.

Obviously, the second call to "complicated" is a waste of time, and storing its return value is a waste of space. Specifying "LIST_CACHE => MERGE" will make "memoize" use the same cache for scalar and list context return values, so that the second call uses the scalar cache that was populated by the first call. "complicated" ends up being called only once, and both subsequent calls return 3 from the cache, regardless of the calling context.

List values in scalar context

Consider this function:

    sub iota { return reverse (1..$_[0]) }

This function normally returns a list. Suppose you memoize it and merge the caches:

    memoize 'iota', SCALAR_CACHE => 'MERGE';

    @i7 = iota(7);
    $i7 = iota(7);

Here the first call caches the list (1,2,3,4,5,6,7). The second call does not really make sense. "Memoize" cannot guess what behavior "iota" should have in scalar context without actually calling it in scalar context. Normally "Memoize" would call "iota" in scalar context and cache the result, but the "SCALAR_CACHE => 'MERGE'" option says not to do that, but to use the cache list-context value instead. But it cannot return a list of seven elements in a scalar context. In this case $i7 will receive the first element of the cached list value, namely 7.

Merged disk caches

Another use for "MERGE" is when you want both kinds of return values stored in the same disk file; this saves you from having to deal with two disk files instead of one. You can use a normalizer function to keep the two sets of return values separate. For example:

        tie my %cache => 'MLDBM', 'DB_File', $filename, ...;

        memoize 'myfunc',
          NORMALIZER => 'n',
          SCALAR_CACHE => [HASH => \%cache],
          LIST_CACHE => 'MERGE',
        ;

        sub n {
          my $context = wantarray() ? 'L' : 'S';
          # ... now compute the hash key from the arguments ...
          $hashkey = "$context:$hashkey";
        }

This normalizer function will store scalar context return values in the disk file under keys that begin with "S:", and list context return values under keys that begin with "L:".

"unmemoize"

    $SIG{INT} = sub { unmemoize 'function' };

"unmemoize" accepts a reference to, or the name of a previously memoized function, and undoes whatever it did to provide the memoized version in the first place, including making the name refer to the unmemoized version if appropriate. It returns a reference to the unmemoized version of the function.

If you ask it to unmemoize a function that was never memoized, it croaks.

"flush_cache"

Note that if the cache is a tied hash, "flush_cache" will attempt to invoke the "CLEAR" method on the hash. If there is no "CLEAR" method, this will cause a run-time error.

An alternative approach to cache flushing is to use the "HASH" option (see above) to request that "Memoize" use a particular hash variable as its cache. Then you can examine or modify the hash at any time in any way you desire. You may flush the cache by using "%hash = ()".