MirBSD manpage: Tie::Memoize(3p)

Tie::Memoize(3p)Perl Programmers Reference Guide Tie::Memoize(3p)


     Tie::Memoize - add data to hash when needed


       require Tie::Memoize;
       tie %hash, 'Tie::Memoize',
           \&fetch,                  # The rest is optional
           $DATA, \&exists,
           {%ini_value}, {%ini_existence};


     This package allows a tied hash to autoload its values on
     the first access, and to use the cached value on the follow-
     ing accesses.

     Only read-accesses (via fetching the value or "exists")
     result in calls to the functions; the modify-accesses are
     performed as on a normal hash.

     The required arguments during "tie" are the hash, the pack-
     age, and the reference to the "FETCH"ing function.  The
     optional arguments are an arbitrary scalar $data, the refer-
     ence to the "EXISTS" function, and initial values of the
     hash and of the existence cache.

     Both the "FETCH"ing function and the "EXISTS" functions have
     the same signature: the arguments are "$key, $data"; $data
     is the same value as given as argument during tie()ing.
     Both functions should return an empty list if the value does
     not exist.  If "EXISTS" function is different from the
     "FETCH"ing function, it should return a TRUE value on suc-
     cess.  The "FETCH"ing function should return the intended
     value if the key is valid.

Inheriting from Tie::Memoize
     The structure of the tied() data is an array reference with

       0:  cache of known values
       1:  cache of known existence of keys
       2:  FETCH  function
       3:  EXISTS function
       4:  $data

     The rest is for internal usage of this package.  In particu-
     lar, if TIEHASH is overwritten, it should call


perl v5.8.8                2005-02-05                           1

Tie::Memoize(3p)Perl Programmers Reference Guide Tie::Memoize(3p)

       sub slurp {
         my ($key, $dir) = shift;
         open my $h, '<', "$dir/$key" or return;
         local $/; <$h>                      # slurp it all
       sub exists { my ($key, $dir) = shift; return -f "$dir/$key" }

       tie %hash, 'Tie::Memoize', \&slurp, $directory, \&exists,
           { fake_file1 => $content1, fake_file2 => $content2 },
           { pretend_does_not_exists => 0, known_to_exist => 1 };

     This example treats the slightly modified contents of
     $directory as a hash.  The modifications are that the keys
     fake_file1 and fake_file2 fetch values $content1 and $con-
     tent2, and pretend_does_not_exists will never be accessed.
     Additionally, the existence of known_to_exist is never
     checked (so if it does not exists when its content is
     needed, the user of %hash may be confused).


     FIRSTKEY and NEXTKEY methods go through the keys which were
     already read, not all the possible keys of the hash.


     Ilya Zakharevich

perl v5.8.8                2005-02-05                           2

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