It seems cool at first sight, but I feel it is overengineered for a configuration language: why would one need to indicate whether a variable is a list, a vector or a set? These are implementation details, they should be represented as lists, and it is the application underneath that choses the appropriate representation.

You only need Scheme, quoting and quasi-quoting, and s-expressions.

    `((username "foo")
      (password ,(getenv "PASS")))

Want to be fully declarative because you are afraid of Turing-complete configs? Simply abort if the top-level list is quasi-quoted.