Can I clean the repl?

I have played with a lot of code in a repl console, how can I clear it? I would like a fresh one without restarting it. Can that be done?

If you want to clear the current namespace of all temporary variables and functions you declared you can use this one liner (or make a function of it) :

(map #(ns-unmap *ns* %) (keys (ns-interns *ns*)))

or

(ns myutil)
(defn ns-clean
       "Remove all internal mappings from a given name space or the current one if no parameter given."
   ([] (ns-clean *ns*)) 
   ([ns] (map #(ns-unmap ns %) (keys (ns-interns ns)))))
(ns mytest)

... make loads of junk ...

(myutil/ns-clean)

... great!!! I can now make all new junk ... 

It does not claim to give you a squeaky clean namespace, just one with less of the junk which usually accumulates in a typical repl session.

Use with caution : do not pull the rug from under your feet!

If you are running the repl through a terminal window (eg: Terminal.app on MacOS or xterm/aterm/urxvt etc on linux) then you can type Control-L and it should clear the terminal window and give you a new repl prompt. However all macros/atoms you previously defined are still going to be in memory, so this is just a "Cosmetic" clear.

In EMACS/slime REPLs C-c C-o clears the last output (in case you've typed something that gave a very long answer) C-c M-o clears the whole thing

In GNOME terminals, you've got a menu option Terminal/Reset and Clear

The shorcut to clean the whole buffer : C-u C-c C-o

The shortcut to clean the last output : C-c C-o

Note, the old way was : C-c M-o

Also, cider-repl-clear-buffer (which is bound to C-ENTER s-c on my machine)

If you are using Emacs + nREPL, you can either:

• Run Mx nrepl-clear-buffer or
• Run Cc Mo

If the key binding is not enabled, run Mxnrepl-interaction-mode to enable it. You can find other useful shortcuts in nrepl.el and/or customize the key bindings to fit your needs.

Note: you can find all nREPL's key bindings in your system by running M-x v nrepl-mode-map and following the nrepl.el link.

I use the Emacs command cider-repl-clear-buffer via M-x. One might also use cider-repl-clear-output with a prefix argument: C-u C-c C-o.

It depends what you mean by 'clean'. To remove all namespaces within a 'package' you can use:

(mapv remove-ns 
  (map symbol 
    (filter #(.startsWith % "org.mycompany") 
      (map str (all-ns)))))

For users of the Cursive IDE plugin for IntelliJ IDEA who end up here, like me:

You can "cosmetically" clear the REPL (keeping your def'd symbols etc.) by clicking on this icon at the top of the REPL frame:

There's no default key binding for this, but you can easily add one by opening your preferences, navigating to Keymap > Plugins > Cursive, and adding a binding for "Clear output for current REPL".

Alternatively, you can right-click in the editor and access "Clear output for current REPL" via the REPL commands.