When interviewing for Haskell jobs in the past, I’ve usually asked interviewees to implement a parser combinator library from scratch. I still like that question because it can be adjusted to the level of the interviewee – if it’s more of a beginner Haskeller, I can offer them some guidance and maybe we’ll end up with a simple list based monadic parser similar to Graham Hutton’s famous functional pearl. If it’s someone with more experience, we can talk about performance and maybe do a CPS based parser, or talk about how to return good error messages.
This was for a compiler-related job though, so if you’re interviewing for a e.g. a mostly web-based company, I’d recommend looking into that area a bit more.