是否有可能在when语句中返回与type参数相同的类型 [英] Is it possible to return the same type as the type parameter in when statement

问题描述

例如：

For example:

fun <T> f(a: T): T =
    when (a) {
        a is Int -> 0  // if T is Int, then return Int
        a is String -> ""  // if T is String, then return String
        else -> throw RuntimeException()  // Otherwise, throw an exception so that the return type does not matter.
    }

它会产生编译错误：

It gives compile errors:

Error:(3, 20) The integer literal does not conform to the expected type T
Error:(4, 23) Type mismatch: inferred type is String but T was expected

推荐答案

您可以将结果转换为<$之后再c $ c> T 。你不会得到任何编译器的帮助，你会得到警告，但至少它会编译：

You can cast the result to T afterwards. You won't get any compiler assistance with that and you will get warnings, but at least it does compile:

fun <T> f(a: T): T =
    when {
        a is Int -> 0  // if T is Int, then return Int
        a is String -> ""  // if T is String, then return String
        else -> throw RuntimeException()  // Otherwise, throw an exception so that the return type does not matter.
    } as T

请注意，当（a）这里是不必要的，当{足够时，只需。

Note that when (a) here is unnecessary, just when { is enough.

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