Tie::Memoize - add data to hash when needed

- NAME
- SYNOPSIS
- DESCRIPTION
- Inheriting from Tie::Memoize
- EXAMPLE
- BUGS
- AUTHOR

# 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.

Tie::Memoize - add data to hash when needed