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Michael G Schwern
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Hash::StoredIterator - Functions for accessing a hashes internal iterator.


In perl all hashes have an internal iterator. This iterator is used by the each() function, as well as by keys() and values(). Because these all share use of the same iterator, they tend to interact badly with each other when nested.

Hash::StoredIterator gives you access to get, set, and init the iterator inside a hash. This allows you to store the current iterator, use each/keys/values/etc, and then restore the iterator, this helps you to ensure you do not interact badly with other users of the iterator.

Along with low-level get/set/init functions, there are also 2 variations of each() which let you act upon each key/value pair in a safer way than vanilla each()

This module can also export new implementations of keys() and values() which stash and restore the iterator so that they are safe to use within each().


    use Hash::StoredIterator qw{

    my %hash = map { $_ => uc( $_ )} 'a' .. 'z';

    my @keys = hkeys %hash;
    my @values = hvalues %hash;

Each section below is functionally identical.

    my $iterator = iterator %hash;
    while( my ( $k, $v ) = $i->() ) {
        print "$k: $value\n";

    hmap { print "$a: $b\n" } %hash;

    hamp { print "$_: $b\n" } %hash;

    hmap {
        my ( $key, $val ) = @_;
        print "$key: $val\n";
    } %hash;

It is safe to nest calls to hmap(), iterator(), hkeys(), and hvalues()

    hmap {
        my ( $key, $val ) = @_;
        print "$key: $val\n";
        my @keys = hkeys( %hash );
    } %hash;

hmap() and iterator() will also properly handle calls to CORE::each, CORE::keys, and Core::values nested within them.

    hmap {
        my ( $key, $val ) = @_;
        print "$key: $val\n";

        # No infinite loop!
        my @keys = keys %hash;
    } %hash;

Low Level:

    hash_init_iterator( \%hash );
    my $iter = hash_get_iterator( \%hash );
    # NOTE: Never manually specify an $iter value, ALWAYS use a value from
    # hash_get_iterator.
    hash_set_iterator( \%hash, $iter );


my $i = iterator %hash

Get an iterator that can be used to retrieve key/value pairs.

    my $i = iterator %hash;
    while( my ($k, $v) = $i->() ) {

The iterator is a coderef, so you call it like this: $i-()>. You can also use the sub anywhere you would use any other coderef.

hmap( \&callback, %hash )
hmap { ... } %hash

Iterate each key/pair calling $callback-( $key, $value )> for each set. In addition $a and $_ are set to the key, and $b is set to the value. This is done primarily for convenience of matching against the key, and short callbacks that will be cluttered by parsing @_ noise.

Note: See caveats.

my @keys = hkeys( %hash )

Same as the builtin keys(), except it stores and restores the iterator.

Note: Overriding the builtin keys(), even locally, causes strange interactions with other builtins. When trying to export hkeys as keys, a call to sort keys %hash would cause undef to be passed into keys() as the first and only argument.

my @values = hvalues( %hash )

Same as the builtin values(), except it stores and restores the iterator.

Note: Overriding the builtin values(), even locally, causes strange interactions with other builtins. When trying to export hvalues as values, a call to sort values %hash would cause undef to be passed into values() as the first and only argument.

my $i = hash_get_iterator( \%hash )

Get the current iterator value.

hash_set_iterator( \%hash, $i )

Set the iterator value.

Note: Only ever set this to the value retrieved by hash_get_iterator(), setting the iterator in any other way is untested, and may result in undefined behavior.

hash_init_iterator( \%hash )

Initialize or reset the hash iterator.


These have been deprecated because they were terrible names. eich was also deprecated because it was unnatural to use.


use iterator() instead


use hmap instead


Modification of hash during iteration

Just like with the builtin each() modifying the hash between calls to each is not recommended and can result in undefined behavior. The builtin each() does allow for deleting the iterations key, however that is NOT supported by this library.

sort() edge case

For some reason [sort hkeys %hash] and [sort hkeys(%hash)] both result in a list that has all the keys and values (and strangely not in sorted order). However [sort(hkeys(%hash))] works fine.


Chad Granum exodist7@gmail.com


Copyright (C) 2013 Chad Granum

Hash-StoredIterator is free software; Standard perl licence.

Hash-StoredIterator is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the license for more details.