In a call "retrieve(account: string)", nobody checks the contents of "account". What if we could specify its type not just as a string, but as a formal language - say, a regex "[0-9]+"? In our new paper "FLAT: Formal Languages as Types" with Fengmin Zhu, we do exactly this - for better type checking and even test generation: https://doi.acm.org/?doi=3799978