Looks really nice but I haven’t tried it yet. I wrote a small book on my take on Haskell programming using a small subset of the language (read for free on my web site https://markwatson.com). When I update my book I might add an Okapi example. I have played around with Spock for simple web apps and that is nice enough, but I am really impressed by the care in putting together the Okapi documentation web site. Great material, nicely presented.