Swift Functional Programming - Second Edition

LSP

The LSP states that objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.

The LSP is essentially a restricted instance of subtyping, which aims to guarantee semantic portability across class hierarchies. Portability is achieved by ensuring that whatever is true of a base type is also true of all subtypes. Subclasses must not strengthen preconditions. They must accept all input and initial states that the base class accepts, and subclasses must not weaken post-conditions. Behavioral expectations declared by the super class must be met by the subclass. These characteristics cannot be enforced by the type system alone.

The LSP as a relation of inheritance is thus deceptive, ...