Yeah I was also confused. I wrote a bidirectional parser/writer layer for yaml in a haskell program I had at work. The yaml structure mappings were all declarative in the code and even allowed documentation for the structures to be printed out. It's not that hard once you define the primitive bidirectional (higher-order) mapping function to go from a `Configurable a` to a `Configurable b`, the rest kind of unfolds from there.