Kotlin中的简单泛型函数失败 [英] Simple Generic Function in Kotlin Fails

问题描述

这是Kotlin中一个简单的泛型函数:

Here's a simple generic function in Kotlin:

fun <T> twice(x: T) : T { return 2 * x }

尝试在项目或REPL中进行构建会导致以下错误:

Attempting to build this (either in a project or REPL) results in the following errors:

error: none of the following functions can be called with the arguments supplied: 
public final operator fun times(other: Byte): Int defined in kotlin.Int
public final operator fun times(other: Double): Double defined in kotlin.Int
public final operator fun times(other: Float): Float defined in kotlin.Int
public final operator fun times(other: Int): Int defined in kotlin.Int
public final operator fun times(other: Long): Long defined in kotlin.Int
public final operator fun times(other: Short): Int defined in kotlin.Int
fun <T> twice(x: T) : T { return 2 * x }
                                   ^

如果将return语句的操作数切换为x * 2，则错误消息将变为:

If I switch the return statement operands to x * 2, the error messages become:

error: unresolved reference. None of the following candidates is applicable because of receiver type mismatch: 
@InlineOnly public inline operator fun BigDecimal.times(other: BigDecimal): BigDecimal defined in kotlin
@InlineOnly public inline operator fun BigInteger.times(other: BigInteger): BigInteger defined in kotlin
fun <T> twice(x: T) : T { return x * 2 }
                                   ^

我在这里想念什么?

推荐答案

由于T可能是任何东西，因此编译器无法找到匹配的times运算符.正如您在错误消息中看到的那样，对于Int，有多种可用的替代方法

Since T could be anything, the compiler is not able to find a matching times operator. As you can see in the error message, for Int, there a multiple alternatives available

public final operator fun times(other: Byte): Int defined in kotlin.Int
public final operator fun times(other: Double): Double defined in kotlin.Int
public final operator fun times(other: Float): Float defined in kotlin.Int
public final operator fun times(other: Int): Int defined in kotlin.Int
public final operator fun times(other: Long): Long defined in kotlin.Int
public final operator fun times(other: Short): Int defined in kotlin.Int

但是不幸的是，没有通用的times函数，例如Number.恐怕在这种情况下，您必须为每个要处理的类型(例如Double，Int等)创建一个重载.

But unfortunately there's no generic times function, which can be used with e.g. Number. I'm afraid in this case, you would have to create an overload for each type you want to be handling, i.e., Double, Int, etc.

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