running sbt in interactive mode with -feature

This has annoyed me for a while, but it’s actually simple.

While running sbt in interactive mode, sometimes you might want to compile with the ‘-feature’ options, to see MatchError warnings for example.

Typing ‘compile -feature’ won’t work.

~ sbt
> compile -feature
[error] Expected ID character
[error] Not a valid command: compile (similar: completions)
[error] Expected project ID
[error] Expected configuration
[error] Expected ':' (if selecting a configuration)
[error] Expected key
[error] Expected '::'
[error] Expected end of input.
[error] compile -feature
[error]

All you need to do is to add your options to scalacOptions, exactly as you would in your build.sbt.

~ sbt
> set scalacOptions += "-feature"

You can also check or confirm its scalacOptions value.

~ sbt
> show scalacOptions

The option you just set is temporary (for the current session) and won’t affect your project build file. You can check your current session temporary settings.

~ sbt
> session list
  1. scalacOptions += "-feature"

Finally, let’s re-compile everything with the warnings showing.

> sbt
> set scalacOptions += "-feature"
> clean compile

More info on inspecting the build, from the sbt doc.

Edit: you can also save back your session’s options to your project build.sbt file:

> session save

Check this talk from Scala Days 2014 to learn about sbt and how to write an sbt plugin.

Well, that wasn’t so hard.