Well, nothing stops Turing-complete languages from being declarative: lambda calculus is both, for instance, as well as some of the languages based on it (in pa

Well, nothing stops Turing-complete languages from being declarative: lambda calculus is both, for instance, as well as some of the languages based on it (in particular parser combinators in Haskell and similar languages are neat and flexible). And there's indeed a practice of embedding some languages into others, though I think it ends up quite awkward to use, and in this case potential limitations (actually I don't know if kaitai is Turing-complete) may be undesirable. I imagine in many cases it may be simpler to ship, say, a parser/decoder (and/or printer/encoder) library providing a C API that is easy to call via FFI, rather than a description in a less common language, to be used with a particular library. But then again, probably that'd have its uses too.