What is cider?

CIDER is a plugin for Emacs, a long-standing text editor popular among programmers, that turns Emacs into a full development environment for the Clojure programming language. Clojure is a language that runs on the Java platform and is often used for data processing, web backends, and other general programming tasks. Without CIDER, Emacs can open and edit Clojure files but has no special understanding of the language. With CIDER installed, it gains a rich set of capabilities for working with Clojure interactively. The central feature is a live connection between the editor and a running Clojure program. CIDER starts or connects to a server component called nREPL, which stands for network REPL. A REPL is an interactive session where a programmer can type code and immediately see the result. With CIDER, this interaction happens from inside Emacs rather than a separate terminal window. Developers can send code from a file directly to the running program, test individual functions, inspect results, and continue working without restarting anything. Beyond the interactive REPL, CIDER provides code completion, documentation lookup, jump-to-definition (which opens the source of any function with a single keypress), a debugger that lets you step through code line by line, test running, and error messages with links back to the relevant source lines. These are the kinds of features that many modern code editors include by default, adapted for Emacs and specifically tuned for how Clojure development tends to work. Installing CIDER is done through Emacs's built-in package manager. Once installed, opening a Clojure project file and running a single command is enough to launch a connected session. The project has comprehensive online documentation and a one-page reference card listing all the keyboard shortcuts. CIDER is free software under the GPL 3 license and is maintained by a small core team. The primary maintainer actively requests financial contributions to support continued development.