Simple way to create Scala scripts

This post is a description of a small project idea developed by a friend of mine: Przemysław Pokrywka, I’m just writing down the idea as a blog post.

There are many ways one can execute Scala code, most people use sbt to create a some kind of build, for example fat jar or something similar or just sbt-native-packager to build the application in more native formats.

But what options do you have in case you want to write Scala scripts?

You can use the scala command to execute something, for example like this:

[email protected] ~/ $ cat hello.scala 
println("Hello from scala")

[email protected] ~/ $ scala hello.scala
Hello from scala

This is quite useful when you are using only the standard library, but when your script requires more dependencies you have to figure out how to properly manage them, and very quickly this becomes troublesome

You can achieve similar results by using Ammonite, especially with the “Scala Scripts” extensions. I find this a little bit troublesome, especially because it makes it harder to edit files inside IDE.

The option I’m suggesting allows you to take this one step further:

  • Only single file, that’s both valid Bash and Scala
  • Manage dependencies via coursier
  • IDE support with sbt
  • only Bash and JVM required to run

The whole script is available here: https://github.com/przemek-pokrywka/play-framework-app-in-a-single-file

You can execute it now and you should see following output:

[email protected] ~/ $ bash play-app.scala 
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
Server started! Please go to http://127.0.0.1:9000/hello/world to see the result

After going to the url you should see this:

Hello world 127.0.0.1:9000

So by just running a simple Bash script we were able to start very simple Play application and accept HTTP requests!

I’m going to go over it step by step and explain everything.

Line 1:

trait valid_both_in_bash_and_in_scala /* 2>/dev/null

First line of the file added because Bash attempts to execute it

Lines 5-7

# Making sure Coursier is available
cr=~/.coursier
test -e $cr/cr || (mkdir $cr && wget -q -O $cr/cr https://git.io/vgvpD && chmod +x $cr/cr)

This downloads coursier library if it’s not available

Lines 9-14

dependencies=(
  com.typesafe.play:play-netty-server_2.11:2.5.0
  com.typesafe.play:play_2.11:2.5.0
  com.lihaoyi:ammonite-repl_2.11.7:0.5.5    # Mandatory for running Scala script
)

This is a normal Bash array that stores Scala dependencies, we’ll be using it later. Note that we are actually using ammonite-repl here to execute the script.

Lines 16-20

# Generate simple build.sbt for editing in IDEs locally. Run with `--build.sbt` (now also on Macs)
test "$1" == "--build.sbt" && \
  printf '%s\n' "${dependencies[@]}" | \
  sed 's/\(.*\):\(.*\):\(.*\)/libraryDependencies += "\1" % "\2" % "\3"/g' > build.sbt && \
  exit

This allows you to generate build.sbt file with libraryDependencies section so that you are able to import and edit the script inside IDE and all dependencies will be resolved correctly.

Lines 22-24

# Small enhancement to the Scalapolis version. Enabling Ammonite to cache compilation output:
just_scala_file=${TMPDIR:-/tmp}/$(basename $0)
(sed -n '/^object script/,$ p' $0; echo "script.run()") > $just_scala_file

Ammonite uses caching mechanism to prevent unnecessary recompilation, also we are saving evertyhing after object script into the temporary file

Line 26

CLASSPATH="$($cr/cr fetch -q -p ${dependencies[*]} )" \

This is where the magic happens: The cr fetch command downloads all dependencies (if they were not downloaded before) listed in the previously defined Bash array. The output of the command is a list of those JARs, this output is captured in the CLASSPATH variable, which we’ll be using in the next step.

Lines 27-31

  java \
    -Dplay.crypto.secret=foo.bar.baz \
    -Dconfig.resource=reference.conf \
    ammonite.repl.Main $just_scala_file # hide Bash part from Ammonite
                                        # and make it run the Scala part

We are executing straightforward Java process (with CLASSPATH from the previous step), we are starting Ammonite REPL feeding it the scala part of the script

Line 33

exit $?

This prevents Bash from processing the rest of the file.

Lines 36-57

This is the actual Scala code that we are running.

Applications

The most immediate application for a script like that is some sort of a quick start guide where by just running one Bash command you can run some Scala code or setup environment for development.
Also it should be suitable for running more advanced Scala CLI scripts that fetch multiple dependencies and the scripts are not intended to be used for a long time (if you happen to have that scenario, you might be better of by generating a fat jar)