---
name: "Tie::Memoize"
section: "3perl"
platform: "linux"
package: "perl"
version: "5.44.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/Tie::Memoize/3perl"
---

# Tie::Memoize(3perl)

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 `"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\>.