struct RequestBuilder {
    method: String,
    url: String,
    headers: Vec<(String, String)>,
    body: Option<String>,
}

impl RequestBuilder {
    pub fn new(method: &str, url: &str) -> Self {
        Self {
            method: method.to_string(),
            url: url.to_string(),
            headers: Vec::new(),
            body: None,
        }
    }

    pub fn add_header(mut self, key: &str, value: &str) -> Self {
        self.headers.push((key.to_string(), value.to_string()));
        self
    }

    pub fn set_body(mut self, body: &str) -> Self {
        self.body = Some(body.to_string());
        self
    }

    pub fn build(self) -> Result<Request, &'static str> {
        let request = Request {
            method: self.method,
            url: self.url,
            headers: self.headers,
            body: self.body,
        };

        if request_is_valid(&request) {
            Ok(request)
        } else {
            Err("Invalid request")
        }
    }
}

let request = RequestBuilder::new("GET", "https://api.example.com")
    .add_header("Authorization", "Bearer token")
    .set_body("Hello, world!")
    .build()?;

println!("Request: {:?}", request);