Tie::Memoize

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



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 nor-
       mal 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 dif-
       ferent 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>.



perl v5.8.8			  2001-09-21		      Tie::Memoize(3)