理解产量会导致类型不匹配的编译器错误

问题描述：

我想从Iterable[Try[Int]]中提取所有有效值(Iterable[Int])的列表

I want to extract from Iterable[Try[Int]] a list of all valid values (Iterable[Int])

val test = List(
    Try(8), 
    Try(throw new RuntimeException("foo")), 
    Try(42), 
    Try(throw new RuntimeException("bar"))
)

以下是从test打印所有有效值的方法:

The following is the way to print all valid values from test:

for {
    n <- test
    p <- n
} println(p)

// Output
// 8
// 42

但是，当我尝试将有效值保存到列表中时，出现了错误:

However, when I tried to save the valid values to list I have received an error:

val nums: Seq[Int] = for {
    n <- list
    p <- n    // Type mismatch. Required: IterableOnce[Int], found Try[Int]
} yield(p)
println(nums)

如何解决该错误以及为何引发该错误?

How to fix the error and why it was raised?

答

尝试收集

test.collect { case Success(value) => value }
// res0: List[Int] = List(8, 42)

采用对应的理解格式

for { Success(p) <- test } yield p

两者都使用构造函数在引擎盖下执行isInstanceOf型式测试并随后进行asInstanceOf型式转换的图案.也许对应于类似

Both make use of Constructor Patterns which under the hood perform isInstanceOf type test followed by asInstanceOf type cast. Verbosly that corresponds to something like

test
  .filter (_.isInstanceOf[Success[Int]])
  .map    (_.asInstanceOf[Success[Int]].value)

下面的理解不起作用，因为其中的单子必须对齐

The following for-comprehension does not work because the monads in it have to align

for {
  n <- test  // List monad
  p <- n     // does not align with Try monad
} yield (p)

上述理解的解决方法

test.flatMap((n: Try[Int]) => n.map((p: Int) => p))

并查看flatMap的签名，我们看到它期望一个功能

and looking at the signature of flatMap we see it expects a function

Try[Int] => IterableOnce[Int]

我们提供

Try[Int] => Try[Int]

因为n.map((p: Int) => p)返回Try[Int].现在，以下的理解是完全不同的野兽

because n.map((p: Int) => p) returns Try[Int]. Now the following for-comprehension is a whole different beast

for {
    n <- test
    p <- n
} println(p)

由于缺少yield，因此对

because of the absence of yield it desugars to

test.foreach((n: Try[Int]) => n.foreach((p: Int) => println(p)))

其中foreach需要一个类型的函数

where foreach expects a function of type

Try[Int] => Unit

我们确实提供了

，因为n.foreach((p: Int) => println(p))确实返回了Unit.

which we indeed provide because n.foreach((p: Int) => println(p)) indeed returns Unit.

理解产量会导致类型不匹配的编译器错误

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