Many languages provide their own purpose-built runtime; popular examples of this approach include Python (CPython), Ruby (MRI), and Java (the JVM).^[271] In contrast, Clojure is fundamentally a hosted language, meaning that it targets an existing runtime, also the JVM.^[272] This means that rather than reimplementing a variety of foundational facilities (e.g., garbage collection, just-in-time compilation, threading, graphics contexts, etc.) and libraries of all sorts (from basics like String handling to esoterica like cryptographic functions), Clojure simply reuses all of the work that’s been done around the JVM.

Aside from simply being an implementation time saver, targeting a mature host is advantageous for the Clojure programmer as well: