Manpages - Tie_Memoize.3perl

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 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.

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.

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).

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

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