I think the intention is that Playgrounds will cover this use case but I can see where the set up is the result of complex and disparate code it might be hard to pull into the Playground and repl in the code is helpful. I do it myself in LLDB and use Pry when working with Ruby/Rails to dig into the state at the context and try things out. I can see this working with a debug build with LLVM/LLDB support.

The OP seemed to be picturing a service that could operate in released software and I find that harder to imagine.