Functional, Object-Oriented, and Concurrent Programming by Michel Charpentier

Author:Michel Charpentier
Language: eng
Format: epub
Publisher: Addison-Wesley Professional
Published: 2023-10-15T00:00:00+00:00

This definition is rejected by the Scala compiler because values of this Book type have no title, and therefore cannot be part of the set of publications.

As an alternative, you can define books without titles by taking them out of the Publication type. Then, of course, these books cannot be used as arguments of type Publication:

Scala

case class Book(author: String, pageCount: Int) // OK val book: Book = ... printTitle(book) // rejected by the compiler

Semantically, however, there is nothing in a programming language that forces a value of a subtype to behave like a value of the supertype—you can implement method title in class Book to do anything, as long as it returns a string. If a value of type S cannot meaningfully be used in code that expects a value of type T, then type S should not be a subtype of type T, and you must be careful not to introduce such a subtyping relationship inadvertently (see the aside on composition and inheritance).

The discussion of subtypes so far in this section refers to nominal subtyping: A type S is a subtype of a supertype T because the definition of S refers to T explicitly—by name. Some programming languages use a different notion, known as structural subtyping or duck typing.9 As an illustration, you can define a type Book with all the characteristics of a publication, but which is not a subtype of type Publication:

9Named after the saying that if something looks like a duck, quacks like a duck, and walks like a duck, it is probably a duck, even if you call it something else.

Scala

case class Book(title: String, author: String, pageCount: Int)

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