Add RemovableMatchChecker

[yzgw]

Add checker to detect whether match statement is removable or not.

This checker finds removable "match" statements for collections.
Effective Scala suggests to use pattern matching directly in function definitions if possible.
For example, the checker accepts

list map {
  case Some(x) => x
  case None => default
}

but rejects

list map { item =>
  item match {
    case Some(x) => x
    case None => default
  }
}

This helps to write pattern matchings clearly.

[HairyFotr]

👍

I have a suggestion for one more case:

list map { _ match {
  case Some(x) => x
  case None => default
}}

[yzgw]

Sounds good. But I found some bug of the checker.
After fixing it, I want to try it.

[matthewfarwell]

I don't think that this is the right approach for Scalastyle. For instance we can never guarantee that someone won't create a function called map or foreach, which this would then apply to. I think this sort of check would be better as a compiler plugin or similar.

[HairyFotr]

I've now added this check to Linter (a static analysis compiler plugin): HairyFotr/linter@ae17ec4

[yzgw]

Ok, I got it.
But I want a easy way to add own checker...

@HairyFotr
Great!
I have not known Linter. I will consider using it!

[matthewfarwell]

I think that this check fits better in Linter than in Scalastyle - the solution for Scalastyle seems far too general. A different solution for Scalastyle would be to allow the use of external rules. See #25. Would this be an acceptable solution @yzgw?

[matthewfarwell]

This check fits better into Linter than into Scalastyle. If you really wish to add this rule to Scalastyle, then you can add it as an external rule. See #25. This isn't implemented at the minute, but would be fairly easy to do.