Attribute Directives

http4s keeps per-request attributes in a Vault keyed by org.typelevel.vault.Key; servers use it for the connection information, and middleware or outer directives can store anything there with mapRequest.

attribute

Extracts the attribute stored under the given key. If it is absent the request is rejected with a MissingAttributeRejection, which the default handler turns into 500 Internal Server Error: a missing attribute is a server-side wiring error, not a client mistake.

import cats.effect.SyncIO
import org.typelevel.vault.Key

val userKey: Key[String] = Key.newKey[SyncIO, String].unsafeRunSync()

val route: Route =
  mapRequest(request => request.withAttributes(request.attributes.insert(userKey, "alice"))) {
    attribute(userKey) { user =>
      complete(s"Hello, $user")
    }
  }

optionalAttribute

Extracts the attribute as an Option[T], None when it is absent.

val route: Route = optionalAttribute(userKey) {
  case Some(user) => complete(s"Hello, $user")
  case None       => complete("Hello, stranger")
}