Extensibility in Programming Languages: An overview

I here conduct an exploration of programming language extensibility, making an argument for an often overlooked component of conventional language design. Now, this is not a technical detailing of these components, rather, I attempt to provide an overview as I myself have lacked during my time investigating programming languages. Thus, read this as an introduction to the magical world of extensibility. Through a literature review, I identify key extensibility themes - Macros, Modules, Types, and Reflection - highlighting diverse strategies for fostering extensibility. The analysis extends to cross-theme properties such as Parametricism and First-class citizen behaviour, introducing layers of complexity by highlighting the importance of customizability and flexibility in programming language constructs. By outlining these facets of existing programming languages and research, I aim to inspire future language designers to assess and consider the extensibility of their creations critically.