docs(client): improve client_json example

examples/client_json.rs

@@ -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
@@ -29,22 +47,37 @@ 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)]
 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)
+    }
 }