NAME

       Tie::Memoize - add data to hash when needed

SYNOPSIS

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

DESCRIPTION

       This package allows a tied hash to autoload its values on the first access, and to use the cached value
       on the following 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 package, and the reference to the "FETCH"ing
       function.  The optional arguments are an arbitrary scalar $data, the reference 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 success.  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 elements

         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 particular, if TIEHASH is overwritten, it should call
       SUPER::TIEHASH.

EXAMPLE

         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 $content2, 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).

BUGS

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

AUTHOR

       Ilya Zakharevich <mailto:perl-module-hash-memoize@ilyaz.org>.