sbt fails to compile anything

C:\...\src>type vhdl_parser.scala

object use extends App {
  println ("fuck off")
}
C:\...\src>scalac vhdl_parser.scala

C:\...\src>scala use
fuck off

C:\...\src>sbt
Java HotSpot(TM) Client VM warning: ignoring option MaxPermSize=256m; support was removed in 8.0
[info] Set current project to src (in build file:C:/.../src/)
[INFO] Unable to bind key for unsupported operation: display-shell-version
> run
[warn] No main class detected
java.lang.RuntimeException: No main class detected.
        at scala.sys.package$.error(package.scala:27)
[trace] Stack trace suppressed: run last compile:run for the full output.
[error] (compile:run) No main class detected.
[error] Total time: 0 s, completed 20.11.2015 17:42:26
[INFO] Unable to bind key for unsupported operation: display-shell-version
> [INFO] Unable to bind key for unsupported operation: display-shell-version