使用foldLeft时,Scala假定类型错误 [英] Scala assumes wrong type when using foldLeft
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问题描述
我正在尝试在Scala中创建叉积函数,其中k
是构建叉积的次数.
I am trying to create a cross product function in Scala, where k
is the number of times I build the cross product.
val l = List(List(1), List(2), List(3))
(1 to k).foldLeft[List[List[Int]]](l) { (acc: List[List[Int]], _) =>
for (x <- acc; y <- l)
yield x ::: l
}
但是,此代码无法编译:
However, this code does not compile:
test.scala:9: error: type mismatch;
found : List[List[Any]]
required: List[List[Int]]
for (x <- acc; y <- l)
^
为什么曾经认为我有List[Any]
?显然,我要处理的是Int
的List
.
Why does it ever think I have a List[Any]
's there? Clearly everything I am dealing with is List
s of Int
s.
推荐答案
您的理解力实际上是产生List [List [Int或List [Int]]]],因此推断的类型是List [List [Any]].这是来自repl的示例:
Your for comprehension is effectively yielding List[List[Int or List[Int]]] hence the inferred type is List[List[Any]]. Here's an example from the repl:
scala> val l = List(List(1), List(2), List(3))
l: List[List[Int]] = List(List(1), List(2), List(3))
val x = for {
| x <- l
| y <- l
| } yield x ::: l
x: List[List[Any]] = List(List(1, List(1), List(2), List(3)), List(1, List(1), List(2), List(3)), List(1, List(1), List(2), List(3)), List(2, List(1), List(2), List(3)), List(2, List(1), List(2), List(3)), List(2, List(1), List(2), List(3)), List(3, List(1), List(2), List(3)), List(3, List(1), List(2), List(3)), List(3, List(1), List(2), List(3)))
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