Not that it matters now, but I’ve recently learned about generic-data-surgery [1], a library that let’s you take a record X and return a record X with an added field Y. Pretty cool stuff, considering it’s all strongly typed. You would need to do the JSON serialisation _after_ the addToken step, but that shouldn’t be a problem.

[1]: https://hackage.haskell.org/package/generic-data-surgery-0.3...