I've only played around with k and APL in my spare time so I can't speak to real world problems. It is a ridiculously powerful query language, where in SQL you have only started writing `SELECT ...`, in k you are already done. But you need to have very good tacit knowledge of algorithms and the weird syntax to be productive, like oh I need to calculate an integral-image of this time-series, but that just a pre-scan over addition, boom and you are done. The theory of array programming with a focus in combinators is also an interesting perspective on functional programming. IMHO not something you should write full program in, but that hasn't stopped some from trying.