many-requests-surf: Add example from Chapter 20, Asynchronous Programming.

many-requests-surf/.gitignore

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+/target/

many-requests-surf/Cargo.toml

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+[package]
+name = "many-requests-surf"
+version = "0.1.0"
+authors = ["You <you@example.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+async-std = "1.7"
+surf = "1.0"

many-requests-surf/src/main.rs

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+pub async fn many_requests(urls: &[String])
+                           -> Vec<Result<String, surf::Exception>>
+{
+    let client = surf::Client::new();
+
+    let mut handles = vec![];
+    for url in urls {
+        let request = client.get(&url).recv_string();
+        handles.push(async_std::task::spawn(request));
+    }
+
+    let mut results = vec![];
+    for handle in handles {
+        results.push(handle.await);
+    }
+
+    results
+}
+
+fn main() {
+    let requests = &["http://example.com".to_string(),
+                     "https://www.red-bean.com".to_string(),
+                     "https://en.wikipedia.org/wiki/Main_Page".to_string()];
+
+    let results = async_std::task::block_on(many_requests(requests));
+    for result in results {
+        match result {
+            Ok(response) => println!("*** {}\n", response),
+            Err(err) => eprintln!("error: {}\n", err),
+        }
+    }
+}