Kotlin：混淆 lambda 和泛型

Question

请查看 cmets 中的错误消息：

interface Printable {}

class Book(val title: String) :Printable

fun bookPrint(b: Book?):String =  "Title: " + b?.title

class Author(val name: String) :Printable

fun authorPrint(a: Author?):String = "Name: " + a?.name

// Unsupported: [modifier on parameter in function type]
// -------------vv
fun printIt(f: (in Printable?) -> String, a:Printable):String {
    return "Unknown: " + f.invoke(null) +
            "Known: " + f.invoke(a)
}

fun main(args: Array<String>) {
    // Type Mismatch:
    // Required: (Printable?) -> String
    // Found: KFunction1<Book?,String>
    // -------vvvvvvvvv
    printIt(::bookPrint, Book("Storm Front"))
    // -------vvvvvvvvvvv
    printIt(::authorPrint, Author("Jim Butcher"))
}

关键点：

• bookPrint() 和authorPrint() 都需要空书/作者
• printIt() 需要使用其中任何一个功能。

因此，考虑“生产者扩展，消费者超级”，我认为我的问题是我希望输入参数是协变的，而当它被硬编码为逆变（“in”）时。

我的这个想法没有奏效：

// Unresolved reference: KFunction1
// --------------vvvvvvvvvv
fun htmlList2(f: KFunction1<Printable?,String>, a:Printable):String {
    return "Unknown: " + f.invoke(null) +
            "Known: " + f.invoke(a)
}

Answer 1

我认为这是正确的：

interface Printable {}

class Book(val title: String) :Printable

fun bookPrint(b: Book?):String =  "Title: " + b?.title

class Author(val name: String) :Printable

fun authorPrint(a: Author?):String = "Name: " + a?.name

// Add type parameter T, upper-bounded by Printable.  This ties
// the type of the first argument to the type of the second and
// ensures they are both Printable.
fun <T:Printable> printIt(f: (T?) -> String, a:T):String {
    return "Unknown: " + f.invoke(null) +
            "Known: " + f.invoke(a)
}

fun main(args: Array<String>) {
    printIt(::bookPrint, Book("Storm Front"))
    printIt(::authorPrint, Author("Jim Butcher"))
}

感谢在Generic Constraints: Upper Bounds 上尝试和阅读文档

如果有更好的方法仍然感兴趣......