title: Main Script layout: docs category: docs order: 18

Main Scripts

If you have a ClojureScript namespace on the classpath with a -main function in it, you can execute that function from the command line.

For example if you have the following namespace:

(ns example.hello)

(defn -main [& args]
  (println "ARGS:" (pr-str args)))

You can execute it from the command line like this:

$ clj -m figwheel.main -m example.hello hi there

You will see the command will compile and eventually print out:

ARGS: ("hi" "there")

You may wonder why you might need a feature like this in the first place. Being able to run one-off arbitrary scripts can be a major boon to your tool chain. You will most likely use a main script to run tests, but it could also be useful for running tooling like Webpack from Node.

Being able to accept arbitrary arguments at the end of the command line is why CLI option order is important in clojure.main, cljs.main and figwheel.main. I know that this can be confusing at times, but being able to provide scripts and main scripts with arbitrary options, including ones that could mistakenly be recognized by figwheel.main, is very important for flexible expression. And is the reason why figwheel.main can reuse many of the same CLI args as clojure.main.

Figwheel and compile options when using --main

Since the -m and -b options cannot be used at the same time you may be left wondering how to supply the options that you would normally supply with the --build option to the --main option.

You can do this by providing your [build-id].cljs.edn file to the -co option. Figwheel will pick up both the metadata config along with the CLJS compiler config.

For example the following will allow you to use the config in your dev.cljs.edn file:

$ clj -m figwheel.main -co dev.cljs.edn -m example.hello

Asynchronous execution and non-zero exits

Only available in figwheel.main >= 0.1.9-SNAPSHOT

This --main CLI option behavior is unfortunately hampered in its ability to be useful because many ClojureScript tasks (including running tests) are asynchronous. Not only that, but we'd prefer that if a command line task execution fails in ClojureScript we'd like that failure to be communicated by returning a non-zero exit status from the clj process.

The good news is that currently if you throw a JavaScript Error from synchronous code in your -main function the Clojure process will exit with a non-zero exit status.

This means that when you are running test code that is synchronous, you can determine if the tests failed at the end of the run and throw an error to cause a non-zero exit from the Clojure process.

What do we do when the process is asynchronous?

figwheel.main extends the cljs.main behavior to provide a means of waiting for an asynchronous process to complete or throw an error.

The following 3 tools help you do this:

• the Figwheel Clojure process will block and wait when the -main function returns a vector [:figwheel.main.result/async-wait optional-timeout optional-timeout-value]
• the figwheel.main.result/send ClojureScript function will send back a value to the blocked Figwheel process
• the figwheel.main.result/throw-ex ClojureScript function will send back an exception to the blocked Figwheel process

Figwheel will look at the value returned by the -main function and alter its behavior if it receives something like [:figwheel.main.result/async-wait 5000]. This value will cause the Clojure process to block and wait for a result or timeout after 5 seconds.

For Example:

(ns example.hello)

(defn -main [& args]
  [:figwheel.main.async-result/wait 5000])

When you run the above namespace with the following:

$ clj -m figwheel.main -m example.hello

The Clojure process will block for 5 seconds while it waits for some asynchronous result to be sent back. In this example we are not sending back a result value, so the process will throw a timed-out exception which will cause a non-zero exit.

Now let's modify the above -main function to send back an asynchronous result.

(ns example.hello
  (:require [figwheel.main.async-result :as async-result]))

(defn -main [& args]
  (js/setTimeout #(async-result/send args) 3000)
  [:figwheel.main.async-result/wait 5000])

Again run the -main function with:

$ clj -m figwheel.main -m example.hello

Now the Clojure process will block for 3 seconds and ultimately print out the value of args sent by the figwheel.main.async-result/send function.

Let's modify the namespace one more time to send an asynchronous failure:

(ns example.hello
  (:require [figwheel.main.async-result :as async-result]))

(defn -main [& args]
  (js/setTimeout #(async-result/throw-ex (ex-info "This failed in ClojureScript!" {})) 3000)
  [:figwheel.main.async-result/wait 5000])

After you run the above example, the Clojure process will block for 3 seconds and then fail with an exception.

More details

The figwheel.main.async-result/send and figwheel.main.async-result/throw-ex functions are single-use actions that are only intended to be used in a --main script. The first asynchronous value or exception that is received will unblock the process.

The returned :figwheel.main.async-result/wait message has two optional parameters.

• timeout defaults to 5000 milliseconds
• timeout-value defaults to :figwheel.main.async-result/timed-out

When Figwheel runs a -main script it prints the final result, cljs.main does not do this.