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Free Love is a Scala project that illustrates the concepts of subtype polymorphism, parametric polymorphism, ad-hoc polymorphism, variance, upper and lower bounds, and the Curiously Recurring Template Pattern (CRTP). It does so by implementing a model of a library, which includes various types of publications, and individuals who author them.
Polymorphism is a key principle in Object Oriented Programming (OOP) that allows objects of different types to be accessed through a single, uniform interface.
In this project, we use subtype polymorphism in the definition of publications and the people
involved in their creation.
For example, Book and Comic are subtypes of Publication.
Parametric polymorphism, achieved through the use of generics, is extensively used in this project.
For example, the Author class has a generic type parameter which is constrained to be a subtype
of Authorable.
Ad-hoc polymorphism is not used in this project, but it is exemplified and explained in the related PPT presentation (in Spanish).
It is also known as function overloading, and it is a form of polymorphism in which a function is applied to arguments of different types, but behaves differently depending on the type of the arguments to which it is applied.
Variance deals with the subtyping relationship between complex types (types that consist of other types, e.g., generics, collections). Scala supports variance annotations on type parameters, which helps in defining how subtyping between more complex types relates to subtyping between their components.
In the Library class, we use covariance (+T) to allow a Library of a certain publication
type to also hold its subtypes.
This makes our Library more flexible and easier to use.
In the context of generics, upper and lower bounds are constraints put on the type parameters. They restrict what types can be used as the actual type argument for a given class or trait.
In our Library class, we use an upper bound in the type parameter of the add methods
(+[U >: T <: Publication] and ++[U >: T <: Publication]) to ensure that we can only add
Publication types (or subtypes) to our Library.
CRTP is a design pattern in which a class is inherited from a base class with a generic parameter,
and the derived class is passed as the parameter.
This is extensively used in this project, for example, in the Authorable trait.
The project can be run from the command line using the following command:
sbt run