Provided by: libhash-multivalue-perl_0.16-3_all bug

NAME

       Hash::MultiValue - Store multiple values per key

SYNOPSIS

         use Hash::MultiValue;

         my $hash = Hash::MultiValue->new(
             foo => 'a',
             foo => 'b',
             bar => 'baz',
         );

         # $hash is an object, but can be used as a hashref and DWIMs!
         my $foo = $hash->{foo};         # 'b' (the last entry)
         my $foo = $hash->get('foo');    # 'b' (always, regardless of context)
         my @foo = $hash->get_all('foo'); # ('a', 'b')

         keys %$hash; # ('foo', 'bar')    not guaranteed to be ordered
         $hash->keys; # ('foo', 'foo', 'bar') guaranteed to be ordered

DESCRIPTION

       Hash::MultiValue is an object (and a plain hash reference) that may contain multiple
       values per key, inspired by MultiDict of WebOb.

RATIONALE

       In a typical web application, the request parameters (a.k.a CGI parameters) can be single
       value or multi values. Using CGI.pm style "param" is one way to deal with this problem
       (and it is good, as long as you're aware of its list context gotcha), but there's another
       approach to convert parameters into a hash reference, like Catalyst's
       "$c->req->parameters" does, and it sucks.

       Why? Because the value could be just a scalar if there is one value and an array ref if
       there are multiple, depending on user input rather than how you code it. So your code
       should always be like this to be defensive:

         my $p = $c->req->parameters;
         my @maybe_multi = ref $p->{m} eq 'ARRAY' ? @{$p->{m}} : ($p->{m});
         my $must_single = ref $p->{m} eq 'ARRAY' ? $p->{m}->[0] : $p->{m};

       Otherwise you'll get a random runtime exception of Can't use string as an ARRAY ref or get
       stringified array ARRAY(0xXXXXXXXXX) as a string, depending on user input and that is
       miserable and insecure.

       This module provides a solution to this by making it behave like a single value hash
       reference, but also has an API to get multiple values on demand, explicitly.

HOW THIS WORKS

       The object returned by "new" is a blessed hash reference that contains the last entry of
       the same key if there are multiple values, but it also keeps the original pair state in
       the object tracker (a.k.a inside out objects) and allows you to access the original pairs
       and multiple values via the method calls, such as "get_all" or "flatten".

       This module does not use "tie" or overload and is quite fast.

       Yes, there is Tie::Hash::MultiValue and this module tries to solve exactly the same
       problem, but using a different implementation.

UPDATING CONTENTS

       When you update the content of the hash, DO NOT UPDATE using the hash reference interface:
       this won't write through to the tracking object.

         my $hash = Hash::MultiValue->new(...);

         # WRONG
         $hash->{foo} = 'bar';
         delete $hash->{foo};

         # Correct
         $hash->add(foo => 'bar');
         $hash->remove('foo');

       See below for the list of updating methods.

METHODS

       new
             $hash = Hash::MultiValue->new(@pairs);

           Creates a new object that can be treated as a plain hash reference as well.

       get
             $value = $hash->get($key);
             $value = $hash->{$key};

           Returns a single value for the given $key. If there are multiple values, the last one
           (not first one) is returned. See below for why.

           Note that this always returns the single element as a scalar, regardless of its
           context, unlike CGI.pm's "param" method etc.

       get_one
             $value = $hash->get_one($key);

           Returns a single value for the given $key. This method croaks if there is no value or
           multiple values associated with the key, so you should wrap it with eval or modules
           like Try::Tiny.

       get_all
             @values = $hash->get_all($key);

           Returns a list of values for the given $key. This method always returns a list
           regardless of its context. If there is no value attached, the result will be an empty
           list.

       keys
             @keys = $hash->keys;

           Returns a list of all keys, including duplicates (see the example in the "SYNOPSIS").

           If you want only unique keys, use "keys %$hash", as normal.

       values
             @values = $hash->values;

           Returns a list of all values, in the same order as "$hash->keys".

       set
             $hash->set($key [, $value ... ]);

           Changes the stored value(s) of the given $key. This removes or adds pairs as necessary
           to store the new list but otherwise preserves order of existing pairs. "$hash->{$key}"
           is updated to point to the last value.

       add
             $hash->add($key, $value [, $value ... ]);

           Appends a new value to the given $key. This updates the value of "$hash->{$key}" as
           well so it always points to the last value.

       remove
             $hash->remove($key);

           Removes a key and associated values for the given $key.

       clear
             $hash->clear;

           Clears the hash to be an empty hash reference.

       flatten
             @pairs = $hash->flatten;

           Gets pairs of keys and values. This should be exactly the same pairs which are given
           to "new" method unless you updated the data.

       each
             $hash->each($code);

             # e.g.
             $hash->each(sub { print "$_[0] = $_[1]\n" });

           Calls $code once for each "($key, $value)" pair.  This is a more convenient
           alternative to calling "flatten" and then iterating over it two items at a time.

           Inside $code, $_ contains the current iteration through the loop, starting at 0.  For
           example:

             $hash = Hash::MultiValue->new(a => 1, b => 2, c => 3, a => 4);

             $hash->each(sub { print "$_: $_[0] = $_[1]\n" });
             # 0: a = 1
             # 1: b = 2
             # 2: c = 3
             # 3: a = 4

           Be careful not to change @_ inside your coderef!  It will update the tracking object
           but not the plain hash.  In the future, this limitation may be removed.

       clone
             $new = $hash->clone;

           Creates a new Hash::MultiValue object that represents the same data, but obviously not
           sharing the reference. It's identical to:

             $new = Hash::MultiValue->new($hash->flatten);

       as_hashref
             $copy = $hash->as_hashref;

           Creates a new plain (unblessed) hash reference where a value is a single scalar. It's
           identical to:

             $copy = +{%$hash};

       as_hashref_mixed, mixed
             $mixed = $hash->as_hashref_mixed;
             $mixed = $hash->mixed;

           Creates a new plain (unblessed) hash reference where the value is a single scalar, or
           an array ref when there are multiple values for a same key. Handy to create a hash
           reference that is often used in web application frameworks request objects such as
           Catalyst. Ths method does exactly the opposite of "from_mixed".

       as_hashref_multi, multi
             $multi = $hash->as_hashref_multi;
             $multi = $hash->multi;

           Creates a new plain (unblessed) hash reference where values are all array references,
           regardless of there are single or multiple values for a same key.

       from_mixed
             $hash = Hash::MultiValue->from_mixed({
                 foo => [ 'a', 'b' ],
                 bar => 'c',
             });

           Creates a new object out of a hash reference where the value is single or an array ref
           depending on the number of elements. Handy to convert from those request objects used
           in web frameworks such as Catalyst.  This method does exactly the opposite of
           "as_hashref_mixed".

WHY LAST NOT FIRST?

       You might wonder why this module uses the last value of the same key instead of first.
       There's no strong reasoning on this decision since one is as arbitrary as the other, but
       this is more consistent to what Perl does:

         sub x {
             return ('a', 'b', 'c');
         }

         my $x = x(); # $x = 'c'

         my %a = ( a => 1 );
         my %b = ( a => 2 );

         my %m = (%a, %b); # $m{a} = 2

       When perl gets a list in a scalar context it gets the last entry. Also if you merge hashes
       having a same key, the last one wins.

NOTES ON ref

       If you pass this MultiValue hash object to some upstream functions that you can't control
       and does things like:

         if (ref $args eq 'HASH') {
             ...
         }

       because this is a blessed hash reference it doesn't match and would fail. To avoid that
       you should call "as_hashref" to get a finalized (= non-blessed) hash reference.

       You can also use UNIVERSAL::ref to make it work magically:

         use UNIVERSAL::ref;    # before loading Hash::MultiValue
         use Hash::MultiValue;

       and then all "ref" calls to Hash::MultiValue objects will return HASH.

THREAD SAFETY

       Prior to version 0.09, this module wasn't safe in a threaded environment, including win32
       fork() emulation. Versions newer than 0.09 is considered thread safe.

AUTHOR

       Tatsuhiko Miyagawa <miyagawa@bulknews.net>

       Aristotle Pagaltzis

       Hans Dieter Pearcey

       Thanks to Michael Peters for the suggestion to use inside-out objects instead of tie.

LICENSE

       This library is free software; you can redistribute it and/or modify it under the same
       terms as Perl itself.

SEE ALSO

       •   <http://pythonpaste.org/webob/#multidict>

       •   Tie::Hash::MultiValue