为什么验证不是Monad? [英] Why isn't Validation a Monad?

问题描述

一个示例用例:

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

错误:无法将类型scalaz.Validation[String,Int]应用于类型为scalaz.Bind

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

我猜测，该错误是由编译器找不到Validation[String, Int]的Monad实例引起的.

I guess the error is caused by the compiler can't find a Monad instance for Validation[String, Int]

我可以为自己做一个，例如:

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
}
}

但是为什么Validation还没有呢?毕竟Validation已经定义了bind方法.

but why doesn't Validation have it already? after all, Validation already has the bind method defined.

此外，由于另一个复杂 错误import Validation._和import Instances._放在一起了(这使我无所适从...) > ...
不明确的隐式值:类似validationMonad(我的实例)和特征ValidationInstances2中的方法ValidationInstances1的东西都匹配某些Functor of Validation ...

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...

我应该修改scalaz的来源吗?还是我完全想不到的东西〜?
请帮忙〜

should I modify the source of scalaz? or I'm completely missing something~?
please help~

我正在使用scalaz 7.0.0-M2

I'm using scalaz 7.0.0-M2

推荐答案

作为 在Scalaz组中，问题似乎在于ap会积累错误，而(伪)单原子组合只会对Validation的值部分起作用.

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.

因此，一个不能用另一个来表示，因此Validation不存在monad实例.

Therefore, one cannot be expressed in terms of the other and thus no monad instance exists for Validation.

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