NAME

Perl::MetaAssoc - A meta-model for Perl Classes

SYNOPSIS

  use Perl::MetaAssoc;

  # represent a one to many relationship between the
  # "Property" and "Class" MetaClasses.  The association is
  # called .properties on the Class end, and .class on the
  # property end.

  my $Property_mc = Perl::MetaClass::new("Property");
  my $Class_mc    = Perl::MetaClass::new("Class");

  $Class_mc.clsAssocs
       (properties => Perl::MetaAssoc::new
           (
             assocOrdered => false,
             assocRange => [0, Inf],
             assocCompanion => "class",
             assocIsComposite => 1,
             assocPair => Perl::MetaAssoc::new
                             ( assocRange => [1, 1],
                               assocClass => $Property_mc )
           )
       );

DESCRIPTION

A Perl::MetaAssoc represents a fairly abstract concept. Firstly, it is a class of the meta-model, which is not Class.meta() objects at all. That distinction is explained at Perl::MetaClass.

This class is used to describe relationships between two classes in the Class model. For instance, a Class can have any number of defined methods, and each method can only belong to one class. These two comments are frequently viewed as being seperate relationships, but in fact they are simply different ways of looking at the same relationship; that a method belongs to exactly one class (or package, or whatever).

Each side of the relationship is identical to the other; so, in this metamodel, relationships are represented with two objects - one for each side.

There is a lot of implicit 'magic' assumed to be happening in the example. This magic stems from rules such as the following;

   ∀ MetaClass A, MetaAssoc C : A.clsAssoc ∋ C ↔ C.assocClass = A

This is read as "For every MetaClass A and MetaAssoc C, such that A.clsAssoc contains C, it is automatically implied that C.assocClass is A".

We could have written this explicitly:

  my $properties_class_ma = Perl::MetaAssoc->new();
  $Class_mc->clsAssocs("properties" => $properties_class_ma);
  $properties_class_ma->assocClass($Class_mc);

However, this would be needless duplication of expressing the relationship; we want this all to happen automatically.

This rule is almost identical:

  ∀ MetaAssoc C₁, C₂ : C₁.assocPair = C₂ ↔ C₂.assocPair = C₁

Just by stating that the assocPair for one of the MetaAssoc objects is the other, we don't need to specify the reverse relation.

Similarly, as we have stated;

  ∀ MetaAssoc C₁, C₂, MetaClass M₁, M₂
     :   C₁.catPair = C₂  ∧ C₁.assocCompanion
       ∧ C₁.catClass = M₁ ∧ C₂.catClass = M₂
     → (   ∃ M₁.clsAssocs{C₂.catCompanion}
         ∧ ∃ M₂.clsAssocs{C₁.catCompanion}
         ∧ M₁.clsAssocs{C₂.catCompanion}[1] = C₁
         ∧ M₂.clsAssocs{C₁.catCompanion}[1] = C₂
         ∧ M₁.clsAssocs{C₂.catCompanion}[0] = M₂.clsAssocs{C₁.catCompanion}[0]
         )

Then simply by specifying that the properties MetaAssoc object's companion is called "class", this will mean that its pair MetaAssoc object's name within the "Property" clsAssocs collection is "class". It also implies that the pair MetaAssoc will have a companion property of "properties".

Finally, it is implied that the other side of the association cannot be composite;

  ∀ MetaAssoc C₁, C₂ : C₁.assocPair = C₂ ∧ C₁.assocIsComposite
     → ¬(C₂.catIsComposite)

AUTHORS

Sam Vilain

Stevan Little <stevan@iinteractive.com>