I've done about a hundred coding interviews in the past couple of years, and one thing I've noticed is that unless you are really, really good at functional programming, don't do functional programming in an interview. Many people try it, because they think it is more impressive or proper or whatever, and then it doesn't work correctly, and they flounder because debugging within a functional idiom is different, if not more difficult. Then they switch to imperative, and things get better. I've seen maybe 2 or 3 people that can handle debugging when using the functional approach.

As a side note, those 2 or 3 were not any more or less capable, in the end, than other interviewers.

I don't quite know what to do with that datapoint, but it is interesting.