Parsing yaml in Rust using serde_yaml - rust

Question

I am new to Rust. I am trying to parse yaml in Rust using serde_yaml, but can't make the code compile:
My Cargo.toml:
[package]
name = "apmdeps"
version = "0.1.0"
authors = ["Roger Rabbit"]
edition = "2018"
[dependencies]
git2 = "0.10"
serde = { version = "1.0", features = ["derive"] }
serde_yaml = "0.8"
I tried to adapt the code sample found on the serde_yaml website, to no avail:
use serde::{Deserialize, Serialize};
#[derive(Debug, PartialEq, Serialize, Deserialize)]
struct Dependency {
url: String,
tag: String,
}
#[derive(Debug, PartialEq, Serialize, Deserialize)]
struct Project {
dependencies: Vec<Dependency>,
}
fn main() {
let s = "---\ndependencies:\n--url:http://test1\ntag:tag1\n--url:http://test2\ntag:tag2";
let project: Project = serde_yaml::from_str(&s);
}
I get the following error:
error[E0308]: mismatched types
--> src/main.rs:17:28
|
17 | let project: Project = serde_yaml::from_str(&s);
| ^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `Project`, found enum `std::result::Result`
|
= note: expected type `Project`
found type `std::result::Result<_, serde_yaml::error::Error>`

Answer 1

Your problem is that serde_yaml::from_str(&s) does not return a Dependency struct directly as you expect, but a Result struct.
Result structs are rust's way of error handling. Results are either Ok(value) or an Err and you need to check which one it is. Typically in a match expression.
In your case the parsed dependency is wrapped in Ok(project) if parsing the string is successful.
I could get your code to compile with the following match expression:
let project_result : Result<Project, _> = serde_yaml::from_str(&s);
match project_result {
Ok(project) => println!("dependencies = {:?}", project),
Err(_) => println!("Error!")
}
The next problem however is that your string seemed not proper yaml, at least not as expected from serde, and I do get "Error!" from the program.
I changed your string to the following to get some useful output. I don't know if that is your intended yaml though.
let s = "---\ndependencies:\n - {url: http://test1, tag: tag1}\n - {url: http://test2, tag: tag2}\n";