Planck can be very fast, especially since it depends on JavaScriptCore, which is great for minimizing startup latency. This makes it very useable for quickly starting up a REPL or running simple scripts.


Planck executes scripts by compiling the ClojureScript to JavaScript for execution in JavaScriptCore. This is done dynamically and is usually very fast.

But, if you have scripts that don't change frequently, or are making use of large libraries, and the ClojureScript is expensive to compile, it may make sense to save the resulting JavaScript so that subsequent script execution can bypass compilation.

This means that if you re-run Planck and use namespaces that have been cached, the JavaScript representing those namespaces is simply loaded into JavaScriptCore.

To enable compilation caching in Planck, you simply need to pass the -K or -​-​auto-cache option. This will automatically create a .planck_cache directory in the current working directory. (Alternatively, you can specify an existing directory into which Planck can write cache files using -k or -​-​cache.)

Here's an example: Let's say you have a foo.cljs script file that you run via

planck foo.cljs

Instead, you can instruct Planck to cache:

planck -K foo.cljs

The first time you run Planck this way, it will save the results of compilation into .planck_cache. Then subsequent executions with -K will use the cached results instead.

In addition to caching compiled JavaScript, the associated analysis metadata and source mapping information is cached. This makes it possible for Planck to know the symbols in a namespace, their docstrings, etc., without having to consult the original source. And, if an exception occurs, the source mapping info is used in forming stack traces. For additional speed, this cached info is written using Transit.

This caching works for

The caching mechanism works whether your are running planck to execute a script, or if you are invoking require in an interactive REPL session.

Planck uses a (naïve) file timestamp mechanism to know if cache files are stale, and it additionally looks at comments like the following

// Compiled by ClojureScript 1.9.494 {:static-fns true, :elide-asserts true}

in the compiled JavaScript to see if the files are applicable. If a file can’t be used, it is replaced with an updated copy.

Planck's cache invalidation strategy is naïve because it doesn’t attempt to do sophisticated dependency graph analysis. So, there may be corner cases where you have to manually delete the contents of your cache directory, especially if the cached code involved macroexpansion and macro definitions have changed, for example.

Planck's caching mechanism is compatible with the static function dispatch and assert mechanisms described below. In short, if you have cached code that does not match the current settings for static functions or asserts, then it will not be eligible for loading and will be replaced with freshly-compiled JavaScript as needed.

Static Dispatch

Planck supports the :static-fns ClojureScript compiler option via the -s or -​-​static-fns command-line flag.

With :static-fns disabled (the default), the generated JavaScript for (foo 1 2) will look like,1,2)

and with it enabled you will get,2)

David Nolen commented on the differences

It's an option mostly because of REPL development to allow for redefinition. For example if :static-fns true we statically dispatch to specific fn arities—but what if you redef to a different set of arities? What if you change the var to store a deftype instance that implements IFn. That kind of thing.

So for compilation :static-fns can nearly always be true, but for the REPL it's not desirable.

In short, enabling it can lead to performance benefits, being more amenable to inlining, etc., but usually you want to leave it turned off during dev.

And—importantly for Planck—it can be used to work around a particularly severe JavaScriptCore perf bug that you can encounter when evaluating the JavaScript generated for lengthy literal list forms.

Removing Asserts

ClojureScript allows you to embed runtime assertions into your code. Here is an example of triggering an assert at the Planck REPL:

cljs.user=> (assert (= 1 2) "Uh oh!")
Assert failed: Uh oh!
(= 1 2)

Also, if you use pre- and post-conditions in your code, then behind the scenes, these involve the use of assert.

By default, the *assert* var is set to true and calls to assert can trigger as illustrated above. But if you set this var to false then asserts will not trigger, with the macro expanding to nil.

cljs.user=> (set! *assert* false)
cljs.user=> (assert (= 1 2) "Uh oh!")

Note that the *assert* var is consulted by the assert macro at macroexpansion time. JVM-based ClojureScript does not currently support setting *assert* dynamically as is illustrated above. And, while you can set *assert* in source code being loaded into bootstrap environments like Planck, it will not affect that code because it is being compiled using the previously-set value for *assert*.

If you are curious, this is because a set! call on *assert* is not trapped by the compiler as is done for *unchecked-if*.

With JVM ClojureScript, asserts are disabled globally via the :elide-asserts compiler option. Planck supports the :elide-asserts compiler option via the -a or -​-​elide-asserts command-line flag. This flag simply initializes the *assert* var to false upon startup.

Note: If you'd like to disable asserts in some source code that you've already loaded at the Planck REPL, you can first (set! *assert* false) and then require that namespace passing the :reload flag.