If you see the language as the 'barrier' that people have to jump over in order to be accepted as smart then maybe go for something exotic. Clojure or Haskell?
I'm not sure if a test like that is meaningful or not, you can narrow down the field of applicants substantially but you may end up with people that have more theoretical than practical knowledge.
Speaking as someone who has dabbled in both Haskell and Clojure (emphasis "dabbled"), I'd have to currently recommend Clojure over Haskell, for a couple reasons:
* The Clojure community has a strong focus on web stuff, while
the Haskell community has (to date) been primarily concerned with
more math-centric fields.
* Haskell's web stack is still very much a work-in-progress. It
severely lacks when it comes to documentation of even "simple"
stuff necessary for the web, like shuffling data into a database.
Unless you're looking for a type signature. The Haskell community
loves to provide documentation in the form of type signatures.
* Learning curve. While both are functional languages, it was my
experience that Clojure's emphasis on simplicity and ease-of-use
made it very easy to pick up and start tackling real things.
There is some ramp up time if you're getting used to the tools,
such as Emacs or the Java ecosystem, but you don't have to worry
about things like monads slowing you down.
* It's a LISP! LISP is cool.
I'm not sure if a test like that is meaningful or not, you can narrow down the field of applicants substantially but you may end up with people that have more theoretical than practical knowledge.