Re: Proposal: create an Extractor for PartialFunction

https://github.com/search?q=if+catcher.isDefinedAt&ref=reposearch&type=Code&utf8=%E2%9C%93

There are too many people who have to use case e if catcher.isDefinedAt(e) 
=>

Does that matter?

在 2015年7月9日星期四 UTC+8下午10:57:40，杨博写道：
>
> lift and applyOrElse are hard to work with other cases in a match 
> expression, especially when dealing with Exceptions. For example: 
> https://github.com/Atry/memcontinuationed/blob/master/src/main/scala/com/dongxiguo/memcontinuationed/Memcontinuationed.scala#L599
>
> 在 2015年7月9日星期四 UTC+8下午10:43:15，Vlad Patryshev写道：
>>
>> Here's some discussion of this issue: 
>> http://stackoverflow.com/questions/5902266/inverse-of-partialfunctions-lift-method
>>
>> Instead of going back into the world of booleans and conditions, I'd 
>> suggest to think about why there's no such thing already available. Maybe 
>> this is the right direction. applyOrElse is total; if it does not work 
>> for you, lift should. It is pretty scientific, and gets you to a Kleisli 
>> category. All these ifs and match/cases are a little bit like crutches.
>>
>> Thanks,
>> -Vlad
>>
>> On Wed, Jul 8, 2015 at 9:56 PM, 杨博 <[email protected]> wrote:
>>
>>> Sometimes, when I create a method, I need an extractor parameter for 
>>> pattern matching, while there is not a generic extractor trait in the Scala 
>>> Standard Library. So we have to use a PartialFunction as the parameter 
>>> instead.
>>>
>>>
>>> When I use the PartialFunction, I had to create pattern matching 
>>> expression like this.
>>>
>>>
>>> input match {
>>>
>>>   case x if partialFunction.isDefined(x) =>
>>>
>>>     val data = partialFunction.apply(x)
>>>
>>>     doSomethingWith(data)
>>>
>>> }
>>>
>>>
>>> This approach is not perfect because:
>>>
>>>
>>>    1. partialFunction.isDefined and partialFunction.apply checked x 
>>>    twice, which is slow if isDefined has complex logical, checking 
>>>    whether a file exists for example.
>>>    2. The if condition around temporary variable x is too trivial.
>>>
>>> I think the better approach is converting the PartialFunction to an 
>>> extractor and matching with that extractor
>>>
>>>
>>> implicit final class PartialFunctionAsExtractor[A, B](pf: 
>>> PartialFunction[A, B]) {
>>>
>>>   object Extractor {
>>>
>>>     def unapply(a: A) = PartialFunction.condOpt(a)(pf)
>>>
>>>   }
>>>
>>> }
>>>
>>>
>>> input match {
>>>   case partialFunction.Extractor(data) =>
>>>     doSomethingWith(data)
>>> }
>>>
>>> Since this is a common approach, I hope adding the 
>>> PartialFunctionAsExtractor to scala.Predef .
>>>
>>> I would like to create a pull request if people like this idea.
>>>
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>>
>>

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