Custom Serializer for Foreign Type

help
#1

I'm creating an app that takes input from a file. The file will be edited by hand and read using toml-rs.

Here is an example:

foo = 7300
bar = "0x4563918244f40000"

This is the struct:

use serde::Deserialize;
use web3::types::U256;

#[derive(Deserialize)]
struct FooBar {
    foo: U256,
    bar: u32,
}

Here, U256 is a foreign type. It isn't part of my code base.

This works, but there are two problems. First, foo is scaled quantity. It represents "7.3 percent". Second, bar = "0x4563918244f40000" is a more than I really want to type.

I would rather do this:

foo = "7.3" # implied conversation
bar = "5.0" # implied conversation

How can this be achieved?

#2

You can always implement Deserialize manually instead of relying on the derive. By doing so, you can perform any conversion you need right in the deserialization code.

#3

I believe that you should also be able to use the deserialize_with field attribute.

untested, but something like:

use serde::Deserialize;
use web3::types::U256;

#[derive(Deserialize)]
struct FooBar {
    #[serde(deserialize_with = "deserialize_percentage")]
    foo: U256,
    bar: u32,
}

fn deserialize_percentage<'de, D>(deserializer: D) -> Result<U256, D::Error>
where
    D: Deserializer<'de>,
{
  // Something
}
1 Like
#4

What is deserializer?

#5

I don't think I can implement a visitor for U256. I don't own the type.

#6

You can create a visitor, because the visitor type would be local to your crate, as you created the visitor.

However in this case there's no need to.

fn deserialize_percentage<'de, D>(deserializer: D) -> Result<U256, D::Error>
where
    D: Deserializer<'de>,
{
    let s: String = Deserialize::deserialize(deserializer)?;
    Ok(convert_string_to_u256(&s))
}

fn convert_string_to_u256(s: &str) -> U256 {
    ...
}
3 Likes