The is keyword tells a declaration that it has certain properties. The behaviour of each of those properties is specified as a distinct class (hereafter called a "property class"). A property class has certain special methods (hereafter called "property methods") that determine what effect the property has on the referent to which it is applied.

So, for example, a built-in property class like class dim has property methods that modify the dimensionality of the underlying Array referent that an is dim property is applied to.

Whereas a built-in property class like class constant has property methods that modify the writeability of the referent that the property is applied to.

Similarly, a user-defined class such as class Persistent might have user-defined property methods that modify the persistence behaviour of any referent to which the is Persistent property is applied.

If a user-defined class (say, class Foo) doesn't have explicit property methods, it automatically has implicit property methods (which it inherits from the Class meta-class). Those inherited methods cause class Foo to modify -- in one of two ways -- any referent to which the is Foo property is applied.

If is Foo is applied to a class (say Bar), class Foo's property methods add Foo to the list of class Bar's ancestors. Thus:

    class Bar is Foo {...}
[download]

is how we do inheritance.

On the other hand, if is Foo is applied to a non-type (say $bar), Foo's property methods replace the implementation type of $bar (i.e. Scalar) with the implementation type Foo. Thus:

    my $bar is Foo;
[download]

is how we do tying.

So this one property mechanism gives us a wide range of useful language features (i.e. type qualification, referent modification, inheritance, tying) from a single underlying behaviour (namely, apply certain methods of this property class to the referent of this declaration).

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.