Re: Proposal: create an Extractor for PartialFunction

Thank you. But I think Computes for Function1 is not that useful unless we
allow eta-conversion as a pattern. e.g.

case foo.someMethod.Computes(x) =>



Rex Kerr <[email protected]>于2015年7月11日周六 上午3:32写道：

> I think it's a good idea.  I'd make it more general, actually: call it
> `Computes`, and make a (lower priority) version on Function1 also.
>
> You often want to do things like
>
> foo match {
>   case bar.Computes(x) if baz(x) => x
>   case _ => quux
> }
>
> where the only difference with the PartialFunction is that you don't
> always need the if clause since it's embedded.
>
> It wouldn't be fast for primitives, for instance, since you always have
> the overhead of at least a virtual Some wrapper (and maybe a real one,
> depending on the JVM).  But it'd sure be handy.
>
>   --Rex
>
>
>
>
> On Thu, Jul 9, 2015 at 8:06 AM, 杨博 <[email protected]> wrote:
>
>>
>> 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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-- 
杨博 (Yang Bo)

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