Mastering Perl for Bioinformatics [Electronic resources] نسخه متنی

3.4 Arrow Notation (->)

Object-oriented Perl code uses arrow notation (->) to call
methods. Understanding how this works is
essential to understanding OO Perl. Before you start reading OO Perl
code, let's look more closely at its main features
and how arrow notation is used to call methods.[2]

[2] The
arrow (->) also appears in Perl when dealing with complex data
structures, as you saw in Chapter 2, where
it's used for references to subroutines.

The arrow notation is used on an object to call a method in a class.
Because the object has been blessed (i.e., marked
with the class name), Perl can tell from the object what class
it's in and so knows to find the method in that same
class. With arrow notation, Perl also passes to the method a new
argument that automatically appears first in its argument list. The
other arguments are shifted over; the first argument is now the
second, and so on. The automatic passing of a new first
argument to the method is the key to understanding OO Perl
code.

The method name appears to the right of the arrow. Perl then uses
what's immediately to the left of the arrow to
identify the class in which to find the method. It also passes
information about what's on the left of the arrow to
the method, where it appears as the first argument of the method. The
left side of the arrow may be in one of two forms:

The name of the class. Here's an example:

TRNA->new(  );

Here Perl sees that the left side is the name of the
TRNA class; therefore it calls the
new subroutine in the TRNA
package. It also automatically passes the name
TRNA to that subroutine as its first argument,
shifting any other arguments that may have been explicitly given
(you'll see how this feature is used in later
examples). You need to save the new object (a
blessed reference to a hash) using the assignment
operator = as follows:

$trna_object = TRNA->new(  );

An object. Here's an example:

$trna_object->findloops(  );

Perl sees that on the left side of the arrow
$trna_object is an object of the
TRNA class; it therefore calls the
findloops method in the TRNA
class. (It can see that $trna_object is a
TRNA object because the object was
blessed into the TRNA class
when it was created by the new method, as
I'll explain later.) Perl also passes a reference to
the $trna_object object into the
findloops method as the first argument to the
method, shifting any other arguments that may have been explicitly
given.

Why does Perl do it this way? The short answer to that question is
that once you understand how it works, your code will become simpler
and more usable. You will need to type class names less frequently,
and you can use methods written for one class in another class
(inheritance).

The two new tricks that Perl performs here are:

Using arrow notation to find the correct method in the correct class

Passing the method a new first argument

It can find the correct class because objects are
blessed with their class name.