Feature Request: Parse strings as other types

[joshlf]

The GDAX API full channel is a JSON API whose messages include numerical values that are for some reason encoded as strings. In order to parse these messages, I've had to implement manual parsing, but it'd be nice if there were some attribute that I could use to instruct serde_json to attempt to parse string-typed values as other types.

Example message from that API:

{
    "type": "received",
    "time": "2014-11-09T08:19:27.028459Z",
    "product_id": "BTC-USD",
    "sequence": 12,
    "order_id": "dddec984-77a8-460a-b958-66f114b0de9b",
    "funds": "3000.234",
    "side": "buy",
    "order_type": "market"
}

[dtolnay]

The from_str helper from #317 (comment) looks like it would work for any type that implements FromStr. Does that cover all the types in the GDAX API full channel?

[joshlf]

Yeah, that's roughly what I've done in my implementation.

However, this gets tricky when you want to nest those fields. For example, if instead of having an f64 field, you want it to be optional, making it an Option<f64> field, then the attribute isn't sufficient. I ended up writing my own type - struct F64(f64) - and then using Option<F64>. It'd be nice if instead I could just do something like:

struct Foo {
    #[serde(deserialize_string)]
    bar: Option<f64>,
}

[dtolnay]

Yes the interaction of deserialize_with and Option still needs to be implemented. Let's follow up in serde-rs/serde#723.