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Hash::MultiValue - Store multiple values per key
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
$hash->keys; # ('foo', 'foo', 'bar') guaranteed to be ordered
Hash::MultiValue is an object (and a plain hash reference) that may
contain multiple values per key, inspired by MultiDict of WebOb.
In a typical web application, the request parameters (a.k.a CGI
parameters) can be single value or multi values. Using style
"param" is one way to deal with this problem (and it is good, as long as
you're aware of its list context gotcha), but there's another approach
to convert parameters into a hash reference, like Catalyst's
"$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
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.
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.
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(...);
$hash->{foo} = 'bar';
delete $hash->{foo};
# Correct
$hash->add(foo => 'bar');
See below for the list of updating methods.
$hash = Hash::MultiValue->new(@pairs);
Creates a new object that can be treated as a plain hash reference
as well.
$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's "param" method etc.
$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.
@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 = $hash->keys;
Returns a list of all keys, including duplicates (see the example in
the "SYNOPSIS").
If you want only unique keys, use "keys %$hash", as normal.
@values = $hash->values;
Returns a list of all values, in the same order as "$hash->keys".
$hash->add($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.
Removes a key and associated values for the given $key.
Clears the hash to be an empty hash reference.
@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.
# 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.
$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);
$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.
$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
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.
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*.
Tatsuhiko Miyagawa <>
Aristotle Pagaltzis
Hans Dieter Pearcey
Thanks to Michael Peters for the suggestion to use inside-out objects
instead of tie.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
* <>
* Tie::Hash::MultiValue