actix-web/guide/src/qs_4_5.md

# Errors

Actix uses [`Error` type](../actix_web/error/struct.Error.html) 
and [`ResponseError` trait](../actix_web/error/trait.ResponseError.html) 
for handling handler's errors.
Any error that implements `ResponseError` trait can be returned as error value.
*Handler* can return *Result* object, actix by default provides 
`Responder` implemenation for compatible result object. Here is implementation
definition:

```rust,ignore
impl<T: Responder, E: Into<Error>> Responder for Result<T, E>
```

And any error that implements `ResponseError` can be converted into `Error` object.
For example if *handler* function returns `io::Error`, it would be converted
into `HTTPInternalServerError` response. Implementation for `io::Error` is provided 
by default.

```rust
# extern crate actix_web;
# use actix_web::*;
use std::io;

fn index(req: HttpRequest) -> io::Result<fs::NamedFile> {
    Ok(fs::NamedFile::open("static/index.html")?)
}
#
# fn main() {
#     Application::new()
#         .resource(r"/a/index.html", |r| r.f(index))
#         .finish();
# }
```

## Custom error response

To add support for custom errors, all we need to do is just implement `ResponseError` trait
for custom error. `ResponseError` trait has default implementation
for `error_response()` method, it generates *500* response.

```rust
# extern crate actix_web;
#[macro_use] extern crate failure;
use actix_web::*;

#[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"})
}
#
# fn main() {
#     Application::new()
#         .resource(r"/a/index.html", |r| r.f(index))
#         .finish();
# }
```

In this example *index* handler will always return *500* response. But it is easy
to return different responses for different type of errors.

```rust
# extern crate actix_web;
#[macro_use] extern crate failure;
use actix_web::*;

#[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(
              StatusCode::INTERNAL_SERVER_ERROR, Body::Empty),
          MyError::BadClientData => HttpResponse::new(
              StatusCode::BAD_REQUEST, Body::Empty),
          MyError::Timeout => HttpResponse::new(
              StatusCode::GATEWAY_TIMEOUT, Body::Empty),
       }
    }
}

fn index(req: HttpRequest) -> Result<&'static str, MyError> {
    Err(MyError::BadClientData)
}
#
# fn main() {
#     Application::new()
#         .resource(r"/a/index.html", |r| r.f(index))
#         .finish();
# }
```

## Error helpers

Actix provides set of error helper types. It is possible to use them to generate
specific error response. We can use helper types for first example with custom error.

```rust
# extern crate actix_web;
#[macro_use] extern crate failure;
use actix_web::*;

#[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(error::ErrorBadRequest)?)
}
# fn main() {
#     Application::new()
#         .resource(r"/a/index.html", |r| r.f(index))
#         .finish();
# }
```

In this example *BAD REQUEST* response get generated for `MyError` error.