scads | Scala Algorithms and Data Structures | Learning library

If you were able to write a parser for Request c a, that would imply that the result of parsing the JSON is polymorphic in c and a, so a caller could take the result and use it as a Request Int Double and then as a Request String Bool, and both of those would make sense. This probably isn't what you want.

I'm going to guess here that you have a collection of environment types Env c, base types b, and step types s, each of which has a standalone FromJSON instance that can parse it, in ignorance of any other types. (So, for example, a particular base type MyBase can be parsed into a MyBase value without knowing the environment c or state a types it will be used with.)

Clearly, a specific concrete request involves exactly one environment type Env c and exactly one base type b. I'm a little fuzzy on whether the list of steps are intended to all be steps of the same type or a heterogeneous list of steps of different types, but I'll assume the latter. If so, the desired final result of your parse will be the nested existential request type: