message: docs(client): improve client_json example
diff: @@ -11,14 +11,32 @@ use hyper::rt::{self, Future, Stream};
  fn main() {
  let url = "http://jsonplaceholder.typicode.com/users".parse().unwrap();
  + let fut = fetch_json(url)
  + // use the parsed vector
  + .map(|users| {
  + // print users
  + println!("users: {:#?}", users);
  +
  + // print the sum of ids
  + let sum = users.iter().fold(0, |acc, user| acc + user.id);
  + println!("sum of ids: {}", sum);
  + })
  + // if there was an error print it
  + .map_err(|e| {
  + match e {
  + FetchError::Http(e) => eprintln!("http error: {}", e),
  + FetchError::Json(e) => eprintln!("json parsing error: {}", e),
  + }
  + });
  +
  // Run the runtime with the future trying to fetch, parse and print json.
  //
  // Note that in more complicated use cases, the runtime should probably
  // run on its own, and futures should just be spawned into it.
  - rt::run(fetch_json(url));
  + rt::run(fut);
  }
  -fn fetch_json(url: hyper::Uri) -> impl Future<Item=(), Error=()> {
  +fn fetch_json(url: hyper::Uri) -> impl Future<Item=Vec<User>, Error=FetchError> {
  let client = Client::new();
  client
  @@ -29,18 +47,15 @@ fn fetch_json(url: hyper::Uri) -> impl Future<Item=(), Error=()> {
  // asynchronously concatenate chunks of the body
  res.into_body().concat2()
  })
  + .from_err::<FetchError>()
  // use the body after concatenation
  - .map(|body| {
  + .and_then(|body| {
  // try to parse as json with serde_json
  - let users: Vec<User> = serde_json::from_slice(&body).expect("parse json");
  + let users = serde_json::from_slice(&body)?;
  - // pretty print result
  - println!("{:#?}", users);
  - })
  - // If there was an error, let the user know...
  - .map_err(|err| {
  - eprintln!("Error {}", err);
  + Ok(users)
  })
  + .from_err()
  }
  #[derive(Deserialize, Debug)]
  @@ -48,3 +63,21 @@ struct User {
  id: i32,
  name: String,
  }
  +
  +// Define a type so we can return multile types of errors
  +enum FetchError {
  + Http(hyper::Error),
  + Json(serde_json::Error),
  +}
  +
  +impl From<hyper::Error> for FetchError {
  + fn from(err: hyper::Error) -> FetchError {
  + FetchError::Http(err)
  + }
  +}
  +
  +impl From<serde_json::Error> for FetchError {
  + fn from(err: serde_json::Error) -> FetchError {
  + FetchError::Json(err)
  + }
  +}
  \ No newline at end of file
  