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Replumb Logo

Replumb ('rɛplʌm) is a plumbing library for your self-hosted ClojureScript Read-Eval-Print-Loops. Live demo available at

This library tries to ease the burden of talking directly to ClojureScript cljs.js and puts together pieces of work contributed by the ClojureScript community at disparate points in time on the cljs-in-cljs subject. It aspires to be a common starting point for custom REPLs and/or REPL-ish apps (educational interactive environments for instance).


As current maintainer of the project I would like to thank each and every present and future contributor, Scalac for actively supporting the (Clojure) open source community and Mikes Fikes for having answered to my incessant questions on Slack, for having aided in brainstorming the project name and of course for the symbiosis with planck.


Versions higher than 0.2.0 break two replumb.core APIs:

  • there is a unified options function for both browser and Node.js
  • result->string, unwrap-result and error->str have a new parameter order

via Clojars

There is one entry namespace, replumb.core, whose functions you should call directly:

(ns ...
  (:require ...
            [replumb.core :as replumb]
            [ :as io]))

(defn handle-result!
  [console result]
  (let [write-fn (if (replumb/success? result) console/write-return! console/write-exception!)]
    (write-fn console (replumb/unwrap-result result))))

(defn cljs-read-eval-print!
  [console repl-opts user-input]
  (replumb/read-eval-call repl-opts
                          (partial handle-result! console)

(defn cljs-console-did-mount
   (fn []
     (let [repl-opts (merge (replumb/options :browser
                                             ["/src/cljs" "/js/compiled/out"]
                            {:warning-as-error true
                             :verbose true})
           jqconsole (console/new-jqconsole "#cljs-console" console-opts)]
       (cljs-console-prompt! jqconsole repl-opts)))))

Note that replumb, like ClojureScript, abstracts over IO details. You will need to provide your own read/write functions for it to work. There are examples of this in both and that you can freely copy over.

Read-eval-call options

The core of it all is read-eval-call, which reads, evaluates and calls back with the evaluation result.

The first parameter is a map of configuration options, currently supporting:

  • :verbose will enable the evaluation logging, defaults to false. To customize how to print, use (set! *print-fn* (fn [& args] ...)

  • :warning-as-error will consider a compiler warning as error.

  • :target :nodejs and :browser supported, the latter is used if missing.

  • :init-fn! user provided initialization function, it will be passed a map:

      :form   ;; the form to evaluate, as data
      :ns     ;; the current namespace, as symbol
      :target ;; the current target
  • :load-fn! will override replumb's default cljs.js/*load-fn*. It rules out :read-file-fn!, losing any perk of using replumb.load helpers. Trickily enough, :load-fn! is never used with load-file. It is the only case where it does not take precedence over :read-file-fn!. Use it if you know what you are doing and follow this protocol:

    Each runtime environment provides a different way to load a library. Whatever function *load-fn* is bound to will be passed two arguments, a map and a callback function: The map will have the following keys:

    :name   - the name of the library (a symbol)
    :macros - modifier signaling a macros namespace load
    :path   - munged relative library path (a string)

    The callback cb, upon resolution, will need to pass the same map:

    :lang       - the language, :clj or :js
    :source     - the source of the library (a string)
    :cache      - optional, if a :clj namespace has been precompiled to
                  :js, can give an analysis cache for faster loads.
    :source-map - optional, if a :clj namespace has been precompiled
                  to :js, can give a V3 source map JSON

    If the resource could not be resolved, the callback should be invoked with nil.

  • :read-file-fn! an asynchronous 2-arity function with signature [file-path src-cb] where src-cb is itself a function (fn [source] ...) that needs to be called with the file content as string (nil if no file is found). It is mutually exclusive with :load-fn! and will be ignored in case both are present

  • :write-file-fn! a synchronous 2-arity function with signature [file-path data] that accepts a file-path and data to write.

  • :src-paths a vector of paths containing source files

  • :cache - a map containing two optional values: the first, :path, indicates the path of the cached files. The second, :src-paths-lookup?, indicates whether look for cached files in :src-paths. If both present, :path will have the priority but both will be inspected.

  • :no-pr-str-on-value in case of :success? avoid converting the return :value to string

  • :context - indicates the evaluation context that will be passed to cljs/eval-str. One in :expr, :statement, :return. Defaults to :expr. If you really feel adventurous check David Nolen's dev notes.

  • :foreign-libs - a way to include foreign libraries. The format is analogous to the compiler option. For more info visit the compiler option page.

  • :static-fns - static dispatch in generated JavaScript. See the compiler option page.

  • :preloads - accepts either a sequence of symbols, akin to the core feature, or a map containing keys to specs, analogous to the :ns form syntax:

    {:preloads {:require '#{[my-ns.core :refer [init]] your-ns.core}
                :use '#{their-ns}
                :cb #(println "Result:" %)}}

    (Note the set, order does not matter)

The second parameter, callback, should be a 1-arity function which receives the result map, whose result keys will be:

{:success?  a boolean indicating if everything went alright
 :value     (if (:success? result)), this key contains the yielded value as
            string, unless :no-pr-str-on-value is true, in which case it
            returns the bare value.
 :error     (if-not (:success? result)) will contain a js/Error
 :warning   in case a warning was thrown and :warning-as-error is falsey
 :form      the evaluated form as data structure (not string)}

The third parameter is the source string to be read and evaluated.

See the browser-repl for an actual implementation using jq-console.


Support is provided, but only :optimizations :none works fine at the moment:

  (replumb/options :node src-paths node-read-file!)
  (fn [res]
    (-> res
        replumb/result->string true
    (.setPrompt rl (replumb/get-prompt))
    (.prompt rl))

Where node-read-file! is the user-provided node implementation for :read-file-fn!.

See replumb.core/options documentation and feel free to reuse code in src/node/replumb/nodejs/io.cljs. Moreover repl-demo/node contains a working example that can be built and executed with lein node-repl.

You can also watch Mike Fikes' demo or peek under the crook of his elbow.


The implementation was designed not to conceal ClojureScript's cljs.js/eval quirks and idiosyncrasies. Therefore tricks like Mike Fikes' "Messing with macros at the REPL" are still part of the game and actually implemented as tests.

This will in the long run be useful for both replumb and the ClojureScript community as it will help in finding and fix issues.

Another challenge faced during the development was about the asynchronous evaluation response: we were torn between keeping the callback model as per cljs.js or adopting the novel and cutting-edge channel model of core.async. We did not see any strong evidence for adding channels here, therefore avoiding inflating the artifact size and delegating the choice to client code.

This project adheres to the SemVer specification.


The documentation referring to the latest version (SNAPSHOT or release) can always be found in the doc folder of this repo or generated through lein codox.

The wiki is also good (growing) source of info.


Contributions are welcome, of any kind. Have a look at for details.

The project uses lein-figwheel and needs a simple lein repl to bootstrap, after which the following customary three commands needs to be executed in the resulting repl:

(require 'figwheel-sidecar.repl-api)

Now you point your browser to http://localhost:3449 and you'll be thrown in the Clojurescript-in-clojurescript parallel world. Enjoy!


Tell us about your amazing Replumb project!

Author Link Repo
@viebel, @RaphaelBoukara KLIPSE: a simple and elegant online cljs compiler and evaluator klipse
@joakin Web repl + notebook cljs-browser-repl
@jaredly Repl w/ autocomplete, parinfer, and more Reepl
@milt Simple Emacs-flavored web repl re-pl


Distributed under the Eclipse Public License, the same as Clojure.

Copyright (C) 2015-16 Scalac Sp. z o.o.

LambdaX is a Clojure-centric software house: functional experts dedicated to Clojure and ClojureScript.

On our blog we share our knowledge with the community. Put (defn) back into programming!