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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 FETCHing 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 FETCHing 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 FETCHing function, it should return a TRUE value on success. The FETCHing 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.


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