Kotlin 序列类型推断

Question

我在解决以下问题时遇到问题:

class Node(val next: Node?) {

    fun asSequence(): Sequence<Node> = sequence {
        var node: Node? = this@Node;
        while (node != null) {
            yield(node)
            node = node.next
        }
    }
}

编译器显然失败了:

Type inference failed. Expected type mismatch: inferred type is Sequence<Node?> but Sequence<Node> was expected

我确实希望序列是 Sequence<Node>不是 Sequence<Node?>这应该是可能的，因为 yield()在空检查后面。

当我改变yield(node)至 yield(node!!)或 yield(node as Node) ，类型推断有效，但现在我收到编译器警告告诉我:

Unnecessary non-null assertion (!!) on a non-null receiver of type Node

如何创建 Sequence<Node>没有编译器警告？

Answer 1

看起来像 Kotlin 编译器问题。您可以帮助编译器并显式设置序列的类型，而不是固定的

class Node(val next: Node?) {
    fun asSequence(): Sequence<Node> = sequence<Node> {
        var node: Node? = this@Node;
        while (node != null) {
            yield(node)
            node = node.next
        }
    }
}