default-values-deserializing-yaml.md

title	timestamp	author	published	description	tags
Set default values while deserializing YAML in Rust	2023-12-18 07:15:01 -0800	szabgab	true	Some YAML files might be missing some value. In some cases we might want to set default values in the deserialized struct.	YAML default serde

We saw how to read a deserialize a simple YAML into a struct. What happens if some of the fields we are expecting in the YAML file are missing?

For the following example we created a struct like this

#[derive(Deserialize)]
struct Person {
    name: String,
    email: String,
    year: u32,
    married: bool,
}

If se supply the following YAML file, each field is filled.

{% include file="examples/yaml-default-values/all.yaml" %}

However if we provide the following file:

{% include file="examples/yaml-default-values/nameless.yml" %}

We will get an error message in the err variable:

missing field `name`

We get a similar error if more than one field is missing:

{% include file="examples/yaml-default-values/data.yaml" %}

Set the default values

One of the solutions is to set default values for some or all of the fields. We can do that by using the default attribute and passing in the name of a function that is going to return the default value.

#[derive(Deserialize)]
struct Person {
    name: String,

    #[serde(default = "get_default_email")]
    email: String,

    #[serde(default = "get_default_year")]
    year: u32,

    #[serde(default = "get_default_married")]
    married: bool,
}

fn get_default_email() -> String {
    String::from("default@address")
}

fn get_default_year() -> u32 {
    2000
}

fn get_default_married() -> bool {
    false
}

The full example

{% include file="examples/yaml-default-values/src/main.rs" %}

In this example we have a function called get_filename that gets the name of the file from the command line.

A problem - what if we have a typo?

What if this is the YAML file

{% include file="examples/yaml-default-values/typo.yaml" %}

Have you noticed the typo I made in one of the fields? I typed in "maried" instead of "married", but I could have mixed up the field called "color" and typed in "colour", if there indeed was such a field.

The current code will happily disregard the field with the typo and use the default value for the "married" field.

That's not ideal.

Dependencies

See the Cargo.toml` we had:

{% include file="examples/yaml-default-values/Cargo.toml" %}