# Arrow Choice

API Documentation: Choice

Usually we deal with function more often, we're so familiar with `A => B`

.

If we have two functions `A => C`

and `B => C`

, how can we compose them into a single function that can take either A or B and produce a C?

So basically we just look for a function that has type `(A => C) => (B => C) => (Either[A, B] => C)`

.

This is exactly typeclass `Choice`

provided, if we make `=>`

more generic such as `F[_,_]`

, you will get a `Choice`

```
trait Choice[F[_, _]] {
def choice[A, B, C](fac: F[A, C], fbc: F[B, C]): F[Either[A, B], C]
}
```

Note the **infix** notation of `choice`

is `|||`

.

**Middleware

A very useful case of `Choice`

is middleware in HTTP server.

Take Http4s for example:

`HttpRoutes[F]`

in Http4s is defined as Kleisli

```
type HttpRoutes[F[_]] = Kleisli[OptionT[F, *], Request[F], Response[F]]
// defined type HttpRoutes
def routes[F[_]]: HttpRoutes[F] = ???
// defined function routes
```

If we like to have an authentication middleware that composes the route, we can simply define middleware as:

```
type Middleware[F[_]] = Kleisli[OptionT[F, *], Request[F], Either[Response[F], Request[F]]]
// defined type Middleware
def auth[F[_]]: Middleware[F] = ???
// defined function auth
```

Which means the `Request[F]`

goes through the middleware, will become option of `Either[Response[F], Request[F]]`

, where `Left`

means the request is denied and return immediately, `Right`

means the authentication is OK and request will get pass.

Now we need to define what we should do when middleware returns `Left`

:

```
def reject[F[_]:Monad]: Kleisli[OptionT[F, *], Response[F], Response[F]] = Kleisli.ask[OptionT[F, *], Response[F]]
// defined function reject
```

Now compose middleware with route

```
def authedRoute[F[_]:Monad] = auth[F] andThen (reject[F] ||| routes[F])
// defined function authedRoute
```

You will then get a new route that has authentication ability by composing Kleisli.

**HTTP Response

Another example will be HTTP response handler.

`val resp: IO[Either[Throwable, String]] = httpClient.expect[String](uri"https://google.com/").attempt`

`attempt`

is syntax from `MonadError`

When we need to handle error, without `Choice`

the handler would be something like:

```
resp.flatMap{
case Left => ???
case Right => ???
}
```

With `Choice`

there will be more composable solution without embedded logic in pattern matching:

```
def recover[A](error: Throwable): IO[A] = ???
def processResp[A](resp: String): IO[A] = ???
resp >>= (recover _ ||| processResp _)
```

**`ArrowChoice`

`ArrowChoice`

is an extended version of `Choice`

, which has one more method `choose`

, with syntax `+++`

```
trait ArrowChoice[F[_, _]] extends Choice[F] {
def choose[A, B, C, D](f: F[A, C])(g: F[B, D]): F[Either[A, B], Either[C, D]]
}
```

With the middleware example, you can think `ArrowChoice`

is middleware of middleware.

For example if we want to append log to middleware of `auth`

, that can log both when rejected response and pass request:

```
def logReject[F[_]]: Kleisli[OptionT[F, *], Response[F], Response[F]] = ???
// defined function logReject
def logThrough[F[_]]: Kleisli[OptionT[F, *], Request[F], Request[F]] = ???
// defined function logThrough
```

See how easy to compose log functionality into our `authedRoute`

:

```
def authedRoute[F[_]:Monad] = auth[F] andThen (logReject[F] +++ logThrough[F]) andThen (reject[F] ||| routes[F])
// defined function authedRoute
```