jre used by scala-cli

[padhia]

https://github.com/typelevel/typelevel-nix/blob/27b58dea50fcd1d185df9ea3024bb2e914dce0b4/modules/typelevelShell.nix#L39

I am new to Nix and Scala, so please excuse me if I got this wrong.

It seems to me that the scala-cli is built without overriding the jre attribute, and thus it defaults to the one from nixpkgs, which in my case currently happens to be zulu19.30.11-ca-jdk-19.0.1. So by using this flake, I have both jdk-19 and jdk-17 installed. I couldn't think of why scala-cli cannot use jdk-17. If this isn't an oversight, I'd appreciate it if someone could help me understand why this flake needs two different jdks. Thanks!

[rossabaker]

I thought this would be an easy fix, but see #86. We can add another config option with a smarter default, but the idea of "one JRE for the whole devshell" doesn't work when SBT needs to be on something below Java 17.

[padhia]

Thank you for your quick response! Now it makes sense why scala-cli might have been left out of use the chosen JRE.

I like your suggestion -- even though one JDK would have been ideal, multiple JDKs only when necessary would be a good (enough) option.

[rossabaker]

If I make it refer to the default, it works great for application and can be overriden in library. But anyone who creates their own shell, like for a Java 11 library, gets a crash at startup until they set this new option.

Alternatively, I can just pin scala-cli to jdk17, which works great... until someone creates a shell with a default > 17 and wants them to align. I don't yet have a clean way to detect the version of the default JRE and override it by default if and only if it's too old. 🤔

[domenkozar]

here's how I handled it in devenv: https://github.com/cachix/devenv/pull/311/files#diff-741cdd590957888463a214f5325a8cdb5c1aba2da3c4d2c80d29474216586487R33

and how it works: https://github.com/cachix/devenv/blob/main/examples/scala/devenv.nix