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an example use case:
def div2(i: Int): Validation[String, Int] = if (i%2 == 0) Validation.success(i/2) else Validation.failure("odd") def div4(i: Int) = for { a <- div2(i) b <- div2(a) } yield b error: Unable to unapply type scalaz.Validation[String,Int] into a type constructor of kind M[_] that is classified by the type class scalaz.Bind
I guess the error is caused by the compiler can't find a Monad instance for Validation[String, Int]
I can make one for myself, like:
object Instances { implicit def validationMonad[E] = new Monad[({type L[A] = Validation[E, A]})#L] { override def point[A](a: => A) = Validation.success(a) override def bind[A, B](fa: Validation[E, A])(f: A => Validation[E, B]) = fa bind f } } but why doesn't Validation have it already? after all, Validation already has the bind method defined.
moreover, I can't have import Validation._ and import Instances._ together anymore (this took me looong to figure out...), because of another complicated error...
ambiguous implicit values: something like both validationMonad (my instance), and method ValidationInstances1 in trait ValidationInstances2... both match some Functor of Validation...
should I modify the source of scalaz? or I'm completely missing something~?
please help~
I'm using scalaz 7.0.0-M2
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Validation is not quite a monad as it doesn't quite follow the monad rules, even though it has the monad methods. Validation is similar to a Monad, and the difference is academic. Since we're only interested in practice, not theory, let's make Validation the next step in our monadic adventures in JavaScript.
List as a data structure is not a Monad, but the fact that Scala's List implements flatMap is what gives it its monadic super-powers. It also needs to fulfil associativity, left unit and right unit laws to qualify as a Monad.
The Monad class As of GHC 7.10, the Applicative typeclass is a superclass of Monad , and the Functor typeclass is a superclass of Applicative . This means that all monads are applicatives, all applicatives are functors, and therefore all monads are also functors.
Either is a monad, which has a map and flatMap functionality. We don't notice it now how handy Either becomes after it becomes a Monad. Either is right-biased, meaning the map and flatMap method can execute if the value is a “right” or “happy scenario”.
As discussed in the Scalaz group, the problem seems to be that ap would accumulate errors whereas (pseudo-)monadic composition would only operate on the value part of Validation.
Therefore, one cannot be expressed in terms of the other and thus no monad instance exists for Validation.
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The issue is that the applicative functor as implied by the monad does not equal the actual applicative functor
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