Add `BigDecimal` <-> `serde_json::Value` conversions under `serde-json` feature

[lovasoa]

Hi,

It would be useful to support direct conversions between BigDecimal and serde_json::Value when the serde-json feature is enabled.

Currently, we can serialize/deserialize via Serde, but not convert directly without manual handling. Proposed additions:

• impl From<BigDecimal> for Value

  • Use Value::Number when representable; otherwise fallback to Value::String.

• impl TryFrom for BigDecimal`

  • store Numbers directly and parse Strings

Both gated under #[cfg(feature = "serde-json")].

This would remove boilerplate and probably improve performance compared to custom implementations by the user.

[akubera]

Hi,

It seems to me there's some choices when it comes to serialization (for example when to represent as int/float/string) so I've left the choice up to the user how to handle that, but I understand that's probably more tedious than people would prefer.

I haven't actually used serde/serde-json conversion code outside of the tests of this library, so I'm not sure what all the options are. There's the string-only "feature" which was already there when I became maintainer, and that indicates that BigDecimals are always serialized to strings, never the integers. I don't like that it's a library "feature", and plan to change that eventually.

What are the implications for impl From<BigDecimal> for Value? I assume the choices there would affect all users of serde-json, but is there a way to override that behavior?

Is there a way to have an intermediate struct or trait that the user could chose when annotating fields?

JSON is especially funny because it doesn't specify how numbers should be interpreted, so we can store {"n": 0.3} and as long as both sides are using some kind of decimal library to do the parsing, the behavior is correct, but if there's an implicit conversion to f64 somewhere then you're likely to end up with:
{"n": 0.299999999999999988897769753748434595763683319091796875}, and if that's the case why use BigDecimal to begin with.

Also I see serde-json is using u64 as an integer variant for Number, which again is a little dangerous to be writing big-decimals as I image many json implementations store all numbers as f64 so you only get 53 bits of accuracy. (Dangerous as in naively converting any u64 BigDecimal to Number).

Anyways, I feel default should be string in scientific notation, which always preserves precision.

Please share any ideas or example repositories where I can see how BigDecimal + serde_json is used.

[akubera]

Now that v0.4.9 has been released I can focus on serialization.

#151 #141 also reference serde and #117/#119 adds arbitrary-precision feature, forwarding to serde-json, but doesn't implement From<BigDecimal> for Value. I'm not sure what the implications are there.

[lovasoa]

serde-json itself has customizable number storage. An impl From<BigDecimal> for Value that just serializes to a string and then parses the string with serde-json would be both simple and most useful, I believe. The string would be stored as-is by serde-json when its own arbitrary precision crate feature is activated.

[akubera]

From<BigDecimal> for Value or From<&BigDecimal> for Value or both? Also there's BigDecimalRef<'a> which I think I'll do.

[akubera]

Ok, created branch feature/improved-serde-json with a pretty trivial implementation of impl From for Value. Can you compile against that and see how that works?

[lovasoa]

That's great thanks!