That said, it’s a dynamic language and therefore it behaves significantly differently to Java. Developers need to be aware that the compiler won’t pick up type errors, which often causes confusion due to the optional static types. They also have to get used to not being able to navigate to method implementations when they are dynamically added to a class or object. On top of that, it still takes time to learn idiomatic Groovy.

Scala is very different to Java, even though it runs on the JVM. It has its own core library and it includes functional programming features, which some imperative programmers find very difficult to get their heads round.

As you say, though, we’re still talking the Java platform in both cases (unless you use the .Net version of Scala), so there are few deployment and integration issues. I think we just need to clearly distinguish between polyglotism (multiple languages) and multiple platforms.

But imagine a project where you are implementing a grails website using groovy, use java to implement services, use scala to handle message passing because of actor support, use clojure to implement the AI logic to predict the flight delay. Imagine what the code review session would be like.

The above is of course, a silly made up example. But in my current project, we are in fact using groovy, java and clojure. It has lead to some, uh, interesting code review sessions.