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Require in self-hosted ClojureScript

Replumb is a new library we decided to hack together as single point of reference for future implementations of ClojureScript self-hosted REPLs. At the moment its current version is displayed in all its beauty on clojurescript.io.

The ClojureScript community has already put in a huge amount of effort in order to introduce this very cool feature of having a bootstrapping compiler and as we both are a new member of it and immensely keen on building cool stuff, we thought we could add some glue for it.

This blog post unveils the magic and the gotchas behind require, the omnipresent Clojure(Script) function that loads your symbols in the current namespace.

Require and load namespaces

Everything starts from the amazing job done by David Nolen and Mike Fikes in the ClojureScript core’s namespace cljs.js. The namespace contains pretty much all you need to implement a self-hosted REPL and, needless to say, it is the main building block of replumb.

When a require is triggered, replumb, actually following its predecessors, converts it to a ns call against the current namespace, so that cljs.user=> (require 'foo.bar.baz) evaluates (ns cljs.user (:require 'foo.bar.baz) instead.

There is a valuable reason to do this: ns is very solid and already handles things like dependent namespaces loading and require option parsing. From what I gathered David Nolen suggested there was no need to reinvent this particular wheel and this is very in tune with Clojure’s pragmatic design.

What happens next is not very surprising: the namespace needs to be mapped to a file, this file needs to be loaded somehow and forms need to be evaluated (for example def and defn will define vars).

Duly, cljs.js abstracts the functionality that all JavaScript developers miss from their browser toolbox: IO. This is even more clear when reading the first lines of cljs.js/*load-fn* docstring:

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)

Replumb takes the same approach, as not having ties with the environment in which you execute is a good thing, but it does better.

Alongside with the replumb.core/read-eval-call’s option :load-fn!, which entirely replaces cljs.js/*load-fn*, since version 0.1.3 replumb can be customized with what is the very basis of loading: a function that given a file path, returns its content.

The name of the option is :read-file-fn!, an asynchronous 2-arity function with signature [file-path src-cb]. The function will receive the complete file path and a callback src-cb that should be called with either the file content or nil as argument.

By using this option replumb users can gloss over the repetitive but necessary logic of converting the namespace to the file path for instance and the rest of the load-fn protocol can be internally handled more in a more robust way.

The two handy replumb.core/browser-options and replumb.core/nodejs-options were also born to provide an easy external API for it.

The source job

We already know that namespace segments map to a precise file path and we can now read that file, but we need to know the root folder in which look for our files.

Even in this case, replumb has a configurable key in its option map: :src-paths. It accepts a sequence of strings which represent file paths. Note that it has to be sequential or no *load-fn* will be added, resulting in the dreaded "No *load-fn* set" error.

This opens up another big chapter in the require story, which is how to provide the source files to our environment. It should be now clear that in order for this to work:

The file clojure/string.cljs should be available in :src-paths or replumb won’t be able to employ its :read-file-fn! on it in order to read the docstring for trim.

For the seasoned ClojureScript developer this might trigger one or two alarm bells. ClojureScript core did so much in order to integrate the Google Closure compiler so that it could shed all of the unused dependencies and now you are telling me that I need them even if I don’t use them?

Yes, this is the biggest disadvantage of embedding a REPL in your client-side JavaScript app. You need all your source files for doc, source and other REPL perks to work. Not only that, in the particular case of clojure.string you also need the Google Closure library. The reason is obvious if we peek under the carpet: goog.string is imported as dependency, and might potentially import other Google Closure libraries.

In replumb and clojurescript.io we met and solved this in a hackish way. The ClojureScript compiler, if :optimizations is set to :none, creates the whole set of dependencies for you and copies them in the specified :output-dir folder. We only then needed to mirror this folder on our web server (it could have been any remote location) and implement a file fetching :read-file-fn!.

The problem is that these files were just our app dependencies. We were still missing, for instance, the Clojure core namespaces that we were not explicitely requiring. So a better solution still needed to be thought of.

A Possible Solution

Planck employs the best solution at the moment embedding everything in its executable file. In a browser of course you have to minimize the size of the files you serve and moreover you have the cloud at your disposal.

An idea would be then to simply centralize in a kind of oracle all the necessary source files needed for all the REPL apps out there.

The solution of course is costly in terms of maintenance and for this reason it should be shared by the whole Clojure community. However, we have already examples like Clojars and cider which are crowd-founded and self-hosting REPL could be part of this world as well.

Conclusion

To wrap up, self-hosted solutions are definitely worth exploring as they educate on how things work under the hood.

We just scratched the surface, but enough to appreciate what require needs in order to work, which is very nice and useful.

We now understand that a fully functioning REPL in your client-side code needs some effort but that replumb can aid you in that.

That’s all folks!

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by Andrea Richiardi
December 21, 2015
Tags : Clojure ClojureScript Replumb


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