Rust and serde deserializing using generics

Question

I am trying to use generics to deserialize structs from file for use with a Swagger generated API. So I have hacked together this which almost works, but I am unable to unpack t

Answer 1

just declare that T is DeserializeOwned:

fn readfile<T: de::DeserializeOwned>(filename: String) -> Result<Box<T>, Box<std::error::Error>> {
    let f = std::fs::File::open(filename)?;
    let config_data: Outer<T> = serde_yaml::from_reader(f)?;
    match config_data.ptr {
        Ptr::Owned(data) => Ok(data),
        _ => unimplemented!(),
    }
}

same with readconfig

Answer 2

When you use a type parameter like T here:

fn readfile<T>(filename: String) -> Result<Box<T>, Box<std::error::Error>>;

The concrete type of T is determined by the caller. The compiler doesn't just look at all available type and make a guess at what makes sense.

First of all, you need to tell the compiler that any T passed here will actually work. That means constraining T to be something that is deserializable, within compatible lifetimes:

// filename should be &str here
fn readfile<'a, T: ?Sized>(filename: &str) -> Result<Box<Outer<'a, T>>, Box<std::error::Error>>
where
    for<'de> T: Deserialize<'de> + 'a
{
    let f = std::fs::File::open(filename)?;
    let config_data: Outer<T> = serde_yaml::from_reader(f)?;
    Ok(Box::new(config_data))
}

// filename should be &str here
fn readconfig<'a, T: ?Sized>(filename: &str) -> Result<Box<Outer<'a, T>>, &'static str>
where
    for<'de> T: Deserialize<'de> + 'a
{
    // read the config file
    let config_data = readfile(filename);
    match config_data {
        Ok(e) => {
            Ok(Box::new(*e)) // need to deref the Box before reboxing
        },
        Err(_) => {
            Err("nadda")
        }
    }
}

Next, when you call this, you need to tell it a concrete type:

let result: Box<Outer<ExternalStructA>> = readconfig("config.yaml")?;

This will fail if the input cannot be parsed into a Box<Outer<ExternalStructA>>, in which case you can try parsing it to Box<Outer<ExternalStructB>>, perhaps using Result::or_else.