Pitfalls with Mill Build
Here are some challenges and pitfalls I ran into when using Mill.
Circe JSON: Use JsonObject instead of cursors
I’m using the Circe JSON library at work. I am not a fan of it. I recommend to stay away from it. Luckily I rarely have to touch JSON parsing code paths.
What makes my blood boil the most with Circe is small ad-hoc JSON parsing, where I need to extra some values from JSON, without going to create boiler plate class for it. [1]
My expectation for something like that I get a Map, maybe an extended
map with some convenience functions. However, the Circe documentation
guides you towards their cursor API
for that. That API is (censored/insert swearwords/rant here).
Mill Build for Java Devs
Mill has its origin in the Scala world. However, it is well suited to build Java projects. Recently the official Mill documentation gained a growing Java section with many examples. So, I’m keeping this post short, as the official documentation has more information than I can cover.
You might be scared that Mill builds are written in Scala. Luckily, Mill is conservative with its Scala use to constructs most Java developers are already familiar with.
Build Questions answered by Mill builds
I’ve started this series with a list of questions you will encounter in non trivial builds. I claimed that Mill had great at answering most of these questions, but left the actual answers open. Well, after showing Mill for a bit, it is time to answer them.
Mill Basic Building Blocks
Lets explore the Mill building blocks. We start with a hello world:
import mill._
import scalalib._
def buildGreetings = T{ // T stands for Target ;)
val greeting = "Hello Mill!"
val outFile = T.dest / "greeting.txt" // Note, we use T.dest as our output directory
os.write(outFile, greeting)
println(s"File is in $outFile")
PathRef(outFile)
}Then we build it:
$ ./mill buildGreetings
...
[1/1] buildGreetings
File is in /home/roman/dev/private-dev/mill-demo/out/buildGreetings.dest/greeting.txt
$ cat /home/roman/dev/private-dev/mill-demo/out/buildGreetings.dest/greeting.txt
Hello Mill!That works. Note that we used T.dest to give each target its own directory to write to disk
without trampling over other targets. The location in the 'out' directory is always the task names with a .dest suffix.