Re: Proposal: create an Extractor for PartialFunction

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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