Bracha proposes that choice of
type system be made independent of choice of language; that these type systems should be co-existing modules that "
plug" into said language as needed. He believes this is advantageous, because what he calls mandatory type systems make languages less expressive and code more fragile. The requirement that types do not affect the semantics of the language is challenging to fulfill; for example, constructs like type-based overloading are disallowed. Pluggable type systems are similar to other categories of type systems, such as
optional typing, type hinting, type annotations, and
gradual type checking. The difference lies in allowing for multiple type systems to co-exist in the same codebase. Examples of these similar type approaches include Smalltalk,
Python 3,
Ruby, Clojure, and
PHP 5. For most of the languages there is only one implementation of the type system. The exception is Python 3 where the type system is truly pluggable, because there are several implementations and the programmer can use the one that is the best fit. The only language designed with a pluggable type system from the start is Newspeak. Java 8 provides the ability to extend the type system, but does not allow flexibility in the basics of the type system, such as choosing between static and dynamic typing. ==References==