[fiedzia]

> If the pieces of state are all well known at build time - and trusted in terms of their content

.. than use library, because you should not rely on the assumption that next developer adding one more piece to this code will magically remember to validate it with json spec.

[maxbond]

No magic necessary. Factor your hand-rolling into a function that returns a string (instead of printing as in the example), and write a test that parses it's return with a proper JSON library. Assert that the parsing was successful and that the extracted values are correct. Ideally you'd use a property test.