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Core http4s library for servers and clients

There is a newer version: 0.16.6a

package org.http4s

import scalaz.{Equal, Monoid}

/**
  * Encapsulates the notion of fallthrough orElse for a Service
  * For any given B, if a Fallthrough[B] exists within implicit context
  * then [[Service#orElse]] can be used.
  */
trait Fallthrough[B] {
  def isFallthrough(a: B): Boolean
  def fallthrough[A](fst: B, snd: Service[A, B]): Service[A, B] =
    if (isFallthrough(fst)) snd else Service.constVal(fst)
}

/** Houses the principal [[Fallthrough]] typeclass instances. */
object Fallthrough {
  /** Syntax for recovering the [[Fallthrough]] currently in scope. */
  def apply[B](implicit F : Fallthrough[B]): Fallthrough[B] = F

  /** Attribute key that signals that a `HttpService` didn't result in a definitive [[Response]] */
  val fallthroughKey = AttributeKey.http4s[Unit]("fallthroughKey")

  /** A [[Fallthrough]] for any Monoid with an Equals. */
  implicit def forMonoid[B : Monoid : Equal]: Fallthrough[B] = new Fallthrough[B] {
    def isFallthrough(a: B): Boolean = Monoid[B].isMZero(a)
  }

  /** Considers any response with a 404 status code and the [[fallthroughKey]]
    * attribute as a fallthrough. */
  implicit def forResponse: Fallthrough[Response] = new Fallthrough[Response] {
    def isFallthrough(r: Response): Boolean =
      r.status.code == 404 && r.attributes.contains(fallthroughKey)
  }

  /** A [[Fallthrough]] which never falls through. */
  def never[B]: Fallthrough[B] = new Fallthrough[B] {
    def isFallthrough(a: B): Boolean = false
  }

}