[英]how to circumvent Kotlin's restriction “Type parameter is forbidden for catch parameter”
I have defined the following function: 我定义了以下功能:
inline fun <T> T.tryTo(block: T.() -> Unit): T? {
try {
block()
} catch (ex: IllegalArgumentException) {
return this
}
return null
}
The purpose is to build a chain of try-actions on an object, eg: 目的是在对象上建立一系列尝试操作,例如:
val input: String = getInput();
input.tryTo /* treat as a file name and open the file */ {
Desktop.getDesktop().open(File(this))
}?.tryTo /* treat as a number */ {
try {
doSomethingWithTheNumber(parseInt(this))
} catch (ex: NumberFormatException) {
throw IllegalArgumentException()
}
}?.tryTo {
println("All options tried, none worked out. Don't know how to treat this input.")
}
So far that works fine. 到目前为止,一切正常。
But, as you can see in the middle tryTo -block ("treat as a number"), it is inconvenient to rethrow an "expected" exception as an IllegalArgumentException to keep the schema working. 但是,正如您在中间的tryTo -block (“按数字处理”)中所看到的那样,将“期望的”异常作为IllegalArgumentException抛出以保持架构正常工作是不方便的。 It would be nicer to write:
这样写会更好:
val input: String = getInput();
input.tryTo<IllegalArgumentException> /* treat as a file name and open the file */ {
Desktop.getDesktop().open(File(this))
}?.tryTo<NumberFormatException> /* treat as a number */ {
doSomethingWithTheNumber(parseInt(this))
}?.tryTo<Exception> {
println("All options tried, none worked out. Don't know how to treat this input.")
}
So, I have rewritten the function tryTo to: 因此,我将函数tryTo重写为:
inline fun <T, X: Exception> T.tryTo(block: T.() -> Unit): T? {
try {
block()
} catch (ex: X) {
return this
}
return null
}
Unfortunately, the latter does not compile: "Type parameter is forbidden for catch parameter". 不幸的是,后者不能编译:“捕获参数禁止使用类型参数”。
How to circumvent this restriction? 如何规避这一限制?
Addendum: 附录:
Now I've got it to: 现在,我可以做到:
inline fun <T, reified X: Exception> T.tryTo(block: T.() -> Unit): T? {
try {
block()
} catch (ex: Exception) {
return if (ex is X) this else throw ex
}
return null
}
But I'm still not happy with this because it requires me to specify both types explicitly ("Type inference failed..." / "2 type arguments expected ..."): 但是我对此仍然不满意,因为它要求我显式地指定两种类型(“类型推断失败...” /“需要2个类型参数...”):
input.tryTo<String, IllegalArgumentException> /* treat as a file in the stapel-directory */ {
...
}
though the first type parameter is obvious as inferable from the receiver object. 尽管第一种类型的参数显然可以从接收者对象中推断出来。
I thought this would be possible if you just made the type parameter reified, but apparently it is not. 我认为,如果您只是对类型参数进行了类型化,那将是可能的,但是显然不是。 I did find the source of this check, and it quite clearly errors for any sort of type parameter in a catch clause, whether it's reified or not.
我确实找到了此检查的来源 ,并且很明显,在catch子句中的任何类型参数(无论是否经过了整形)都会出错。
The commit message that added these checks references this issue - apparently the catch clause with a type parameter was catching all thrown Exception
instances, and crashing with a ClassCastException
if the exception wasn't of the specified type. 添加了这些检查的提交消息引用了此问题 -带有类型参数的catch子句显然正在捕获所有抛出的
Exception
实例,如果异常不是指定的类型,则使用ClassCastException
崩溃。
A possible workaround for your case comes from this answer for the similar Java question - if the generic type is reified, you can check if the exception thrown was of that specific type, which I believe makes this function what you're looking for: 对于这种情况,可能的解决方法来自类似Java问题的答案 -如果泛型类型得到了修正,则可以检查引发的异常是否属于该特定类型,我相信这会使该功能成为您所要查找的功能:
inline fun <T, reified X : Exception> T.tryTo(block: T.() -> Unit): T? {
try {
block()
} catch (ex: Exception) {
if (ex is X) {
return this
}
}
return null
}
Although the call site gets quite ugly because you can't just specify the second type parameter of a function call if it has two type parameters: 尽管调用站点变得很丑陋,因为您不能仅指定函数调用的第二个类型参数(如果它具有两个类型参数):
val input: String = getInput()
input.tryTo<String, IllegalArgumentException> /* treat as a file name and open the file */ {
Desktop.getDesktop().open(File(this))
}?.tryTo<String, NumberFormatException> /* treat as a number */ {
doSomethingWithTheNumber(parseInt(this))
}?.tryTo<String, Exception> {
println("All options tried, none worked out. Don't know how to treat this input.")
}
A slightly nicer alternative to the above, and closer to the original Java answer: 一个比上面更好的替代方案,并且更接近原始Java答案:
inline fun <T> T.tryTo(exceptionType: KClass<out Exception>, block: T.() -> Unit): T? {
try {
block()
} catch (ex: Exception) {
if (exceptionType.isInstance(ex)) {
return this
}
}
return null
}
With the KClass
instances passed in like so: 随着
KClass
实例的传递,如下所示:
input.tryTo(IllegalArgumentException::class) /* treat as a file name and open the file */ {
Desktop.getDesktop().open(File(this))
}?.tryTo(NumberFormatException::class) /* treat as a number */ {
doSomethingWithTheNumber(parseInt(this))
}?.tryTo(Exception::class) {
println("All options tried, none worked out. Don't know how to treat this input.")
}
you can simply remove the receiver parameter 您只需删除接收器参数
and it is better to use ?:
then ?.
最好使用
?:
然后?.
for else
semantic 对于
else
语义
inline fun <reified E : Throwable> runIgnoring(block: () -> Unit): Unit? {
return try {
block()
} catch (e: Throwable) {
if (e is E) null else throw e
}
}
val input: String = getInput()
runIgnoring<IllegalArgumentException> /* treat as a file name and open the file */ {
Desktop.getDesktop().open(File(input))
} ?: runIgnoring<NumberFormatException> /* treat as a number */ {
doSomethingWithTheNumber(parseInt(input))
} ?: run {
println("All options tried, none worked out. Don't know how to treat this input.")
}
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