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Middleware

:middleware takes a vector of qualified symbols, each resolving to nREPL middleware:

;; .cljconf/org.clojure/clojure-cli.repl.edn
{:middleware [dev.timing/middleware dev.heap/middleware]}

A symbol can also name a var that holds a vector of middleware symbols, such as cider.nrepl/cider-middleware.

Custom middleware you write lives in the src dir next to the config file. Any 3rd party middleware (e.g., cider-nrepl) needs the dep added to the :clojure-cli.repl/server alias, see Dependencies.

;; .cljconf/org.clojure/clojure-cli.repl/deps.edn
{:aliases {:clojure-cli.repl/server {:extra-deps {cider/cider-nrepl {:mvn/version "0.62.2"}}}}}

A symbol marked ^:optional is used when its namespace and var are found, and skipped when they are not. Any unmarked symbol that is not found is an error, and the REPL will not start.

;; .cljconf/org.clojure/clojure-cli.repl.edn
{:middleware [^:optional org.corfield.rephrase.nrepl/wrap-rephrase]}

Adding data to responses

clojure-cli.repl.middleware/assoc-response is a convenience function to assoc values onto each eval's response map. The client reads response values at clojure-cli.repl.api/last-response.

(ns dev.heap
  (:require [clojure-cli.repl.middleware :as mw]))

(def middleware
  (mw/assoc-response
    (fn []
      (let [rt (Runtime/getRuntime)]
        {:clojure-cli.repl/heap-used (- (.totalMemory rt) (.freeMemory rt))
         :clojure-cli.repl/heap-max (.maxMemory rt)}))))

Your map keys share the response with nREPL's own keys, so namespacing them is recommended. The values must survive nREPL's bencode wire encoding, so use strings or integers.

Long form nREPL middleware

assoc-response covers the common case of adding values after the eval completes. If that does not meet your needs, :middleware accepts any nREPL middleware. For example, timing.clj must capture the start time before the eval runs.