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.
examples/yaml-default-values/all.yaml
name: Foo Bar
email: foo@bar.com
year: 1990
married: true
However if we provide the following file:
MISSING
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:
examples/yaml-default-values/data.yaml
name: Foo Bar
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
examples/yaml-default-values/src/main.rs
use serde::Deserialize;
use std::fs;
#[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
}
fn main() {
let filename = get_filename();
let text = fs::read_to_string(filename).unwrap();
let data: Person = serde_yaml::from_str(&text).unwrap_or_else(|err| {
eprintln!("Could not parse YAML file: {err}");
std::process::exit(1);
});
println!("name: {}", data.name);
println!("email: {}", data.email);
println!("year: {}", data.year);
println!("married: {}", data.married);
}
fn get_filename() -> String {
let args: Vec<String> = std::env::args().collect();
if args.len() != 2 {
eprintln!("Usage: {} FILENAME", args[0]);
std::process::exit(1);
}
args[1].to_string()
}
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
examples/yaml-default-values/typo.yaml
name: Foo Bar
email: foo@bar.com
year: 1990
maried: true
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:
examples/yaml-default-values/Cargo.toml
[package]
name = "yaml-default-values"
version = "0.1.0"
edition = "2021"
# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
[dependencies]
serde = { version = "1.0", features = ["derive"] }
serde_yaml = "0.9"