man Tie::Memoize () - add data to hash when needed

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 CWexists) result in calls to the functions; the modify-accesses are performed as on a normal hash.

The required arguments during CWtie are the hash, the package, and the reference to the CWFETCHing function. The optional arguments are an arbitrary scalar CW$data, the reference to the CWEXISTS function, and initial values of the hash and of the existence cache.

Both the CWFETCHing function and the CWEXISTS functions have the same signature: the arguments are CW$key, $data; CW$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 CWEXISTS function is different from the CWFETCHing function, it should return a TRUE value on success. The CWFETCHing 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 CW$directory as a hash. The modifications are that the keys fake_file1 and fake_file2 fetch values CW$content1 and CW$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 CW%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>.