First pass at errors section.

examples/errors/Cargo.toml

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

examples/errors/src/helpers.rs

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+// <helpers>
+use actix_web::{error, HttpRequest, Result};
+
+#[derive(Debug)]
+struct MyError {
+    name: &'static str,
+}
+
+fn index(req: &HttpRequest) -> Result<&'static str> {
+    let result: Result<&'static str, MyError> = Err(MyError { name: "test" });
+
+    Ok(result.map_err(|e| error::ErrorBadRequest(e.name))?)
+}
+// </helpers>

examples/errors/src/main.rs

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+mod helpers;
+mod override_error;
+mod recommend_one;
+// <response-error>
+use actix_web::{error, HttpRequest};
+use failure::Fail;
+
+#[derive(Fail, Debug)]
+#[fail(display = "my error")]
+struct MyError {
+    name: &'static str,
+}
+
+// Use default implementation for `error_response()` method
+impl error::ResponseError for MyError {}
+
+fn index(req: HttpRequest) -> Result<&'static str, MyError> {
+    Err(MyError { name: "test" })
+}
+// </response-error>
+fn main() {}

examples/errors/src/override_error.rs

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+// <override>
+use actix_web::{error, http, HttpRequest, HttpResponse};
+use failure::Fail;
+
+#[derive(Fail, Debug)]
+enum MyError {
+    #[fail(display = "internal error")]
+    InternalError,
+    #[fail(display = "bad request")]
+    BadClientData,
+    #[fail(display = "timeout")]
+    Timeout,
+}
+
+impl error::ResponseError for MyError {
+    fn error_response(&self) -> HttpResponse {
+        match *self {
+            MyError::InternalError => {
+                HttpResponse::new(http::StatusCode::INTERNAL_SERVER_ERROR)
+            }
+            MyError::BadClientData => HttpResponse::new(http::StatusCode::BAD_REQUEST),
+            MyError::Timeout => HttpResponse::new(http::StatusCode::GATEWAY_TIMEOUT),
+        }
+    }
+}
+
+fn index(req: &HttpRequest) -> Result<&'static str, MyError> {
+    Err(MyError::BadClientData)
+}
+// </override>

examples/errors/src/recommend_one.rs

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+// <recommend-one>
+use actix_web::{error, http, HttpResponse};
+use failure::Fail;
+
+#[derive(Fail, Debug)]
+enum UserError {
+    #[fail(display = "Validation error on field: {}", field)]
+    ValidationError { field: String },
+}
+
+impl error::ResponseError for UserError {
+    fn error_response(&self) -> HttpResponse {
+        match *self {
+            UserError::ValidationError { .. } => {
+                HttpResponse::new(http::StatusCode::BAD_REQUEST)
+            }
+        }
+    }
+}
+// </recommend-one>

examples/errors/src/recommend_two.rs

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+// <recommend-two>
+use actix_web::{error, fs, http, App, HttpRequest, HttpResponse};
+use failure::Fail;
+
+#[derive(Fail, Debug)]
+enum UserError {
+    #[fail(display = "An internal error occurred. Please try again later.")]
+    InternalError,
+}
+
+impl error::ResponseError for UserError {
+    fn error_response(&self) -> HttpResponse {
+        match *self {
+            UserError::InternalError => {
+                HttpResponse::new(http::StatusCode::INTERNAL_SERVER_ERROR)
+            }
+        }
+    }
+}
+
+fn index(_req: HttpRequest) -> Result<&'static str, UserError> {
+    fs::NamedFile::open("static/index.html").map_err(|_e| UserError::InternalError)?;
+    Ok("success!")
+}
+// </recommend-two>