First pass at Handlers chapter.

examples/either/Cargo.toml

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+[package]
+name = "either"
+version = "0.1.0"
+edition = "2018"
+
+[dependencies]
+actix-web = "1.0"
+futures = "0.1"

examples/either/src/main.rs

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+// <main>
+use actix_web::{web, App, Either, Error, HttpRequest, HttpResponse};
+use futures::future::{ok, Future};
+
+type RegisterResult =
+    Either<HttpResponse, Box<Future<Item = HttpResponse, Error = Error>>>;
+
+fn index(_req: HttpRequest) -> RegisterResult {
+    if is_a_variant() {
+        // <- choose variant A
+        Either::A(HttpResponse::BadRequest().body("Bad data"))
+    } else {
+        Either::B(
+            // <- variant B
+            Box::new(ok(HttpResponse::Ok()
+                .content_type("text/html")
+                .body(format!("Hello!")))),
+        )
+    }
+}
+
+fn main() {
+    App::new().route("/", web::get().to(index));
+}
+// </main>
+
+fn is_a_variant() -> bool {
+    true
+}