Cute, essentially equivalent to Python's inner functions and Go's closures, e.g in Go:

    func foo(cfg_file string) (parsed, error) {
        config := func() {
            return json.Parse(cfg_file)
        }
        return config.parsed
    }

In any case, the real solution here is to simply allow proper nested functions that behave exactly like freestanding functions in that they can only access what's passed to them:

    func foo(cfg_file string) (parsed, error) {
        func config(cfg_file) config {
            return json.Parse(cfg_file)
        }
        return config.parsed
    }