How does Idris' Java compatibility compare? Simply compiling to the JVM is nice, but unless it can cleanly interoperate with the rest of the Java ecosystem, it simply cannot take the place of something like Scala.
I recently experimented with the Idris JavaScript backend and it is not ready for prime-time. In particular, the FFI doesn't handle functions well. I was unable to call into Idris code from JavaScript which is a deal-breaker, IMO.