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

# [NAME](#NAME)

Tie::Memoize - add data to hash when needed

# [SYNOPSIS](#SYNOPSIS)

```
  require Tie::Memoize;
  tie %hash, 'Tie::Memoize',
      \&fetch,                  # The rest is optional
      $DATA, \&exists,
      {%ini_value}, {%ini_existence};
```

# [DESCRIPTION](#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**](#Inheriting_from_)

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](#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](#BUGS)

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

# [AUTHOR](#AUTHOR)

Ilya Zakharevich &lt;mailto:perl-module-hash-memoize@ilyaz.org&gt;.

2026-10-21 perl v5.44.0

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Document: Tie::Memoize(3pm) from perl 5.44.0-r2 (Chainguard OS)
Source: https://github.com/Perl/perl5.git