Where parallels cross

Interesting bits of life

scala-cli-repl: a Scala REPL based on scala-cli

Too long; didn't read

I published scala-cli-repl a mode to interact with scala-cli, the future scala command from within Emacs. It comes with an org-babel extension that makes it easy to try libraries and scala versions in Org Mode src blocks. And it also supports lsp-org goodies!

The problem

I have been reading books about Scala libraries lately and it was fun to see how the transition to Scala 3 is in progress. Since code examples use both Scala 2 and 3, I have struggled testing things out in an easy way. I was following along with Ammonite, which allowed me to try libraries easily in scripts, and then I discovered scala-cli which also allowed me to switch between Scala versions easily.

Since I like to take notes in Org Mode, I thought it would be cool to have a REPL and org babel extension for Emacs running on scala-cli. Apparently nobody felt the need to make a mode yet, so how could I not be the first?

And there is a solution

So far I had been using ammonite-term-repl, a mode for Ammonite and that has been archived recently. After a bit of tweaking I found out that by changing some variables the mode worked fine also for scala-cli. Since it was archived, I cleaned it up a little bit and made a new repository that focuses on scala-cli: scala-cli-repl.

This is how the REPL look:


I added also a function to convert SBT dependencies into the Mill format that scala-cli accepts (scala-cli-conver-and-kill-deps). That should make it easier to try things out.

The interesting bit has been to support org babel for Scala. This relies on the REPL. For my needs I wanted to support setting a Scala version and dependencies as header arguments of a source block.

The extension supports blocks like these:

#+begin_src scala :dep '("com.lihaoyi::os-lib:0.9.0" "com.lihaoyi::utest:0.7.10")
println("This is:" + os.pwd)

Given I like to try things in Org Mode source blocks, I thought that would have been cool to get some code completions that scala-metals usually provide me.

This is how it looks to setup a scala block with my ob-scala-cli-lsp-org:


You can see that I was able to complete os.pwd using scala-metals from an Org Mode block!

The implementation uses lsp-org (which is part of lsp-mode) and scala scripts (file ending with .sc). All in all, is pretty cool to work on Scala examples with full IDE support in your literate programming! And now I can more easily follow my books along :)

Happy hacking!