关于理解和协方差错误的解释

Question

问题

希望获得帮助以了解错误原因。原文来自Coursera Scala Design Functional Random Generators.

任务

使用随机 int 和随机布尔值的工厂，尝试实现随机树工厂。

trait Factory[+T] {
self => // alias of 'this'
    def generate: T
    def map[S](f: T => S): Factory[S] = new Factory[S] { 
        def generate = f(self.generate) 
    }
    def flatMap[S](f: T => Factory[S]): Factory[S] = new Factory[S] { 
        def generate = f(self.generate).generate 
    }
}
val intFactory = new Factory[Int] {
    val rand = new java.util.Random
    def generate = rand.nextInt()
}
val boolFactory = intFactory.map(i => i > 0)

问题

第一个块中的实现会导致错误，但如果它更改为第二个块，则不会。我相信 Factory[+T] 意味着 Factory[Inner] 和 Factory[Leaf] 都可以被视为 Factory[Tree].

我不知道为什么 same if 表达式在 for 块中可以，但在 yield 块中却不行。感谢解释。

trait Tree
case class Inner(left: Tree, right: Tree) extends Tree
case class Leaf(x: Int) extends Tree

def leafFactory: Factory[Leaf] = intFactory.map(i => new Leaf(i))
def innerFactory: Factory[Inner] = new Factory[Inner] {
  def generate = new Inner(treeFactory.generate, treeFactory.generate)
}

def treeFactory: Factory[Tree] = for {
  isLeaf <- boolFactory
} yield if (isLeaf) leafFactory else innerFactory
                    ^^^^^^^^^^^      ^^^^^^^^^^^^
                    type mismatch; found : Factory[Inner] required: Tree
                    type mismatch; found : Factory[Leaf]  required: Tree

但是，下面是可行的。

def treeFactory: Factory[Tree] = for {
  isLeaf <- boolFactory
  tree   <- if (isLeaf) leafFactory else innerFactory
} yield tree

Answer 1

I have no idea why the same if expression in for block is OK but it is not OK in yield block

因为编译器对它们的翻译不同。前面的例子翻译成：

boolFactory.flatMap((isLeaf: Boolean) => if (isLeaf) leafFactory else innerFactor)

产生预期的 Factory[Tree]，而后者被翻译成：

boolFactory.map((isLeaf: Boolean) => if (isLeaf) leafFactory else innerFactory)

这会产生 Factory[Factory[Tree]]，而不是 Factory[Tree]，因此不符合您的方法签名。这与协方差无关，而是关于理解如何以不同方式翻译这些语句。

关于理解和协方差错误的解释

Explanation on the error with for comprehension and co-variance

scala

for-loop

covariant

问题

任务

问题