# Inheritance

## Linking classes and slots with is_a

The is_a metamodel slot can be used to define a backbone hierarchy for your class. All inheritable metamodel slots are propagated down the is_a hierarchy.

classes:
NamedThing:
slots:
- id
- name
- description
- image

Person:
is_a: NamedThing
mixins:
- HasAliases
slots:
- primary_email
- birth_date
- age_in_years
- gender
- has_employment_history
- has_familial_relationships
- has_medical_history


Here Person will inherit the four slots from NamedThing.

Note: Slots can also be organized in hierarchies using is_a.

## abstract classes and slots

A model class (or slot) may be tagged with its abstract slot set to the boolean value true, to define whether it is abstract. This has comparable meaning to that in the computing science Object Oriented Paradigm: another class (or slot) can use the abstract class (or slot) as part of its inheritance hierarchy, but the abstract class itself cannot be directly instantiated.

classes:
NamedThing:
abstract: true
slots:
- id
- name
- description
- image


This forces data providers to always provide a more specific class than the generic “NamedThing”

Some generators may also choose to utilize abstract tags, e.g to mask generation of abstract classes.

Note: Slots can also be declared abstract.

## mixin classes and slots

• The mixin boolean slot to declare a class as a mixin

• The mixins multiavlued slot which specifies a range of mixin parents.

Mixin parents operate similarly to is_a parents, but they do not have the constraint if forming a tree.

Mixins can be extremely helpful in a number of ways: 1) to generalize a set of attributes that can apply to classes in different parts of the class hierarchy, 2) reduce duplication of shared attributes between classes that do not inherit from one another and 3) to prevent the sometimes confusing nature of multiple inheritance noted in the diamond problem.

classes:
NamedThing:
slots:
- id
- name
- description
- image

Person:
is_a: NamedThing
mixins:
- HasAliases
slots:
- primary_email
- birth_date
- age_in_years
- gender

Note that is_a has the characteristics of homeomorphicity: is_a SHOULD only connect either (1) two mixins (2) two non-mixin elements