That's great, but most JSON libraries don't give you that level of access... they just decode in to a bunch of nested maps/dicts consisting of your languages native data types. So one day you swap out your JSON library and the behaviour of your application silently changes.

This is what unit testing is for. Test the methods that validate and return your expected object structures. This catches other things like a structure changing (as simple as a property being modified), ensuring you can handle malformed input (even if it's valid JSON) and any other data validation rules you have.

Use a better JSON library :)

(There are competent JSON implementations, though they're few and far between. PostgreSQL's is the only good example I can point to off the top of my head.)

Here’s what Rust’s `json` crate does for the `Number` type: https://docs.rs/json/0.11.12/json/number/struct.Number.html

It stores it as positive, mantissa and exponent, allowing you to pull it apart in that way if you need to. Meanwhile, normal usage is via JsonValue (https://docs.rs/json/0.11.12/json/enum.JsonValue.html) which allows you to attempt to convert it into an f64, i32, u8, &c. as you desire, returning `None` if it doesn’t fit inside that type (e.g. -7 won’t go into an i32, 576 won’t go into a u8).

By what’s being talked about here, that looks to be a competent JSON implementation.

Wonderful. Good to know that about Rust, I haven't used it but everything I read about it looks good. Yes that is exactly the approach I meant (and a similar mechanism to that used by my own C++ JSON library I use for side projects).

Doesn't most of the Rust ecosystem use serde though?

It looks like serde-json does the same: https://docs.serde.rs/serde_json/struct.Number.html

Well, not the same; it eagerly interprets it as PosInt(u64), NegInt(i64) or Float(f64) rather than storing the mantissa and exponent. This is enough for JavaScript compatibility (its Number type being essentially a 64-bit float), but it’s not quite as flexible as the json crate’s approach.

Shouldn't they return a list of warnings/errors on the parse? Just blindly accepting a machine parse of some random input data is how you crash programs with bad input.