Last week we released a new version of the Clojure tools with support for directly executing a Clojure function that takes a map.

There is one good example of this sort of program in Clojure itself - clojure.core.server/start-server, which can be used to start a socket server. Clojure users are probably most familiar with using the socket server to run a remote repl (but it can really be used to run any kind of socket server).

Looking closer at this function’s doc:

(doc clojure.core.server/start-server)
-------------------------
clojure.core.server/start-server
([opts])
  Start a socket server given the specified opts:
    :address Host or address, string, defaults to loopback address
    :port Port, integer, required
    :name Name, required
    :accept Namespaced symbol of the accept function to invoke, required
    :args Vector of args to pass to accept function
    :bind-err Bind *err* to socket out stream?, defaults to true
    :server-daemon Is server thread a daemon?, defaults to true
    :client-daemon Are client threads daemons?, defaults to true
   Returns server socket.

The Clojure tools now support the invocation of an ad hoc function using the -X parameter, and then a series of key value parameters. To invoke this function, it’s necessary to pass all of the required arguments (:name, :port, :accept function) plus we need to set :server-daemon to false to prevent the JVM from exiting:

clojure -X clojure.core.server/start-server :name '"server"' :port 5555 :accept clojure.core.server/repl :server-daemon false

Once the repl server has been started we can verify it’s working by connecting with netcat in a separate terminal:

nc localhost 5555
user=> (+ 1 1)
2

Of course, that’s a bit wordy on the command line and we mostly want all of those options every time so instead we can move those arguments into the deps.edn file:

{:aliases
 {:repl-server
  {:exec-fn clojure.core.server/start-server
   :exec-args {:name "server"
               :port 5555
               :accept clojure.core.server/repl
               :server-daemon false}}}}

And then we can just invoke the function via the args in the alias:

clojure -X:repl-server

The execution mode also allows command line overrides of those parameters stored in the exec-args:

clojure -X:repl-server :port 5678

That’s it! This just seemed like a good quick example to understand how -X works…