OCaml Parser combinator library as powerful as fastparse(Scala)?

Yes, I think that that’s what @jhw is getting at (and that’s what JSON Schema ought to be for, if they’d only designed it right). If you look at the way JSON gets used in cloud computing (for instance), it’s clear that that data needs to be strongly-typed, and that it’s meant to be consumed by programs which themselves ought to be demarshalling into precise application data-structures.