Unit Testing in a Different Language Than Actual Code

I've been at talks before where it is encouraged if you're interested in learning a new language to write your unit tests in that language, even though your source code is in another language. For example, if your code base is in Java the suggestion is that you should write your unit tests in Groovy, or Scala, or something else that is awesome.

I understand certain pragmatic reasons for this. Sometimes, it's hard to find the time to learn a new language, and it's less risky to experiment with a new language in your unit tests than in your production code.

But if you're trying to practice TDD (which I think is a pretty good idea) writing tests in a language that differs from your source code is risky. TDD allows you to explore and try out your new APIs like the clients of those object would. It's not only about making sure the code as you envision works, it's a methodology for writing new code. But if you're using a different language to unit test your code, you are loosing out on that exploratory capability. Sure you're using the API, but you're not using it in any way that resembles how your clients would use it.

It's a different story when writing integration / acceptance tests. These tests are at a higher level and are more about general application behavior than specific APIs. I think it's very appropriate in these cases to use a Groovy or a Scala or a DSL or a framework (e.g. selenium).