Use http4s routes as Play framework routes.
What's the benefit? To benefit from the nice features that both Play and http4s offer, without drastic nearly-impossible migrations. Play can still be the outside, while you use http4s for the nice fully pure FP.
See http4s/http4s#1819 for more background.
Run the tests:
$ sbt play-route/test
Running:
$ sbt example/run