Learning Perl Objects, References amp;amp; Modules [Electronic resources] نسخه متنی

4.6 Creating an Anonymous Hash

Similar to
creating an anonymous array, you can also create an anonymous hash.
Consider the crew roster from Chapter 3:

my %gilligan_info = (
name => 'Gilligan',
hat => 'White',
shirt => 'Red',
position => 'First Mate',
);
my %skipper_info = (
name => 'Skipper',
hat => 'Black',
shirt => 'Blue',
position => 'Captain',
);
my @crew = (\%gilligan_info, \%skipper_info);

The variables
%gilligan_info and
%skipper_info are just temporaries, needed to
create the hashes for the final data structure. You can construct the
reference directly with the anonymous hash
constructor, which is Yet Another Meaning for curly
braces, as you'll see. Replace this:

my $ref_to_gilligan_info;
{
my %gilligan_info = (
name => 'Gilligan',
hat => 'White',
shirt => 'Red',
position => 'First Mate',
);
$ref_to_gilligan_info = \%gilligan_info;
}

with the anonymous hash constructor:

my $ref_to_gilligan_info = {
name => 'Gilligan',
hat => 'White',
shirt => 'Red',
position => 'First Mate',
};

The value between the open and closing
curly braces is an eight-element list. The eight-element list becomes
a four-element anonymous hash (four key-value pairs). A reference to
this hash is taken and returned as a single scalar value, which is
placed into the scalar variable. Thus, you cam rewrite the roster
creation as:

my $ref_to_gilligan_info = {
name => 'Gilligan',
hat => 'White',
shirt => 'Red',
position => 'First Mate',
};
my $ref_to_skipper_info = {
name => 'Skipper',
hat => 'Black',
shirt => 'Blue',
position => 'Captain',
};
my @crew = ($ref_to_gilligan_info, $ref_to_skipper_info);

As before, you can now avoid the temporary variables and insert the
values directly into the top-level list:

my @crew = (
{
name => 'Gilligan',
hat => 'White',
shirt => 'Red',
position => 'First Mate',
},
{
name => 'Skipper',
hat => 'Black',
shirt => 'Blue',
position => 'Captain',
},
);

Note the use of
trailing commas on the lists when the element is not immediately next
to the closing brace, bracket, or parenthesis. This is a nice style
element to adopt because it allows for easy maintenance. Lines can be
added quickly, rearranged, or commented out without destroying the
integrity of the list.

Now @crew is identical to the value it had before,
but you no longer need to invent names for the intermediate data
structures. As before, the @crew variable contains
two elements, each of which is a reference to a hash containing
keyword-based information about a particular crew member.

The anonymous hash constructor always
evaluates its contents in a list context and then constructs a hash
from key/value pairs, just as if you had assigned that list to a
named hash. A reference to that hash is returned as a single value
that fits wherever a scalar fits.

Now, a word from our parser: because
blocks and anonymous hash constructors both use curly braces in
roughly the same places in the syntax tree, the compiler has to make
ad hoc determinations about which of the two you mean. If the
compiler ever decides incorrectly, you might need to provide a hint
to get what you want. To show the compiler that you want an anonymous
hash constructor, put a plus sign before the opening curly brace:
+{ ... }. To be sure to get a block of code, just
put a semicolon (representing an empty statement) at the beginning of
the block: {; ... }.