To be fair, production (vs. library) code has to usually care much less about being extensible, since whoever needs to use it can also usually change it. In se

To be fair, production (vs. library) code has to usually care much less about being extensible, since whoever needs to use it can also usually change it. In servant dealing with the expression problem (albeit in a pretty different way than the usual tagless final approach) was in my opinion a pretty big win - it means you can add terms the the API DSL as well as interpretations for it. I pretty regularly do the former when using it, and there are lots of libraries doing the latter. (Disclaimer: I'm one of the authors of servant.)