FunctorRaise

FunctorRaise[F, E] expresses the ability to raise errors of type E in a functorial F[_] context. This means that a value of type F[A] may contain no A values but instead an E error value, and further map calls will not have any values to execute the passed function on.

FunctorRaise has no external laws.

FunctorRaise has two internal laws:

def catchNonFatalDefault[A](a: => A)(f: Throwable => E)(implicit A: Applicative[F]) = {
  catchNonFatal(a)(f) <-> try {
    A.pure(a)
  } catch {
    case NonFatal(ex) => raise(f(ex))
  }
}

def ensureDefault[A](fa: F[A])(error: => E)(predicate: A => Boolean)(implicit A: Monad[F]) = {
  ensure(fa)(error)(predicate) <-> for {
    a <- fa
    _ <- if (predicate(a)) pure(()) else raise(error)
  } yield a
}

FunctorRaise has one free law, i.e. a law guaranteed by parametricity:

def failThenFlatMapFails[A, B](ex: E, f: A => F[B]) = {
  fail(ex).flatMap(f) <-> fail(ex)
}
guaranteed by:
  fail[X](ex) <-> fail[F[Y]](ex) // parametricity
  fail[X](ex).map(f) <-> fail[F[Y]](ex)  // map must have no effect, because there's no X value
  fail[X](ex).map(f).join <-> fail[F[Y]].join // add join to both sides
  fail(ex).flatMap(f) <-> fail(ex) // join is equal, because there's no inner value to flatten effects from
  // QED.