Pattern matching in Scala for Java programmers tutorial

[santiagobasulto]

Hello friends,

i'm finishing the translation of this tutorial: https://github.com/scala/scala.github.com/blob/gh-pages/tutorials/scala-for-java-programmers.md

When discussing traits, the authors define this method:

override def equals(that: Any): Boolean = that.isInstanceOf[Date] && {
      val o = that.asInstanceOf[Date]
      o.day == day && o.month == month && o.year == year
}

Shouldn't be better to define the same method this way:

   override def equals(that: Any): Boolean = that match{
     case o:Date => o.day == day && o.month == month && o.year == year
     case _ => false
   }

Maybe a note can be added noting the type erasure problem.

What do you think?

[heathermiller]

Hey Santiago,

So yes, I normally agree- it's better idiomatic Scala to use a pattern match over a cast. However, I think it's probably best in this case to keep the implementation with isInstanceOf and asInstanceOf just to ensure that normal, straightforward casting is also covered in the tutorial. Otherwise, I don't think casting is covered anywhere else in the tutorial.

Alternatively, we could also add a note here to say the same semantics could be achieved with a pattern match- and mention that pattern matching is often used over casting in practice.

Thoughts?

[santiagobasulto]

I think there's something that cannot be avoided. If we use isInstanceOf and asInstanceOf we should explicity say two important things:

• Place a warning regarding type erasure
• A Heather said, place a note saying that pattern matching is prefered in Scala

Pattern matching is explained lines above the example. I would change the example to do it "the right way" and add some paragraph explaining casting.

[heathermiller]

Well, I'm not so sure that we should replace it. I've checked with a college here, and he agrees. I think that the example should remain as it is- but a note would perhaps be useful.

The only issue is that regarding the type erasure note, it's only appropriate to touch on type erasure after generics have been introduced. It's not very sensible to mention type erasure before generics have appeared.

So, I think that we can keep the note suggesting to use pattern matching over casting in the Traits section. Regarding type erasure-- I'm not sure it's entirely necessary to bring up in this tutorial (as the tutorial isn't meant to be very in-depth, it's kind of high-level and a general overview). But if you insist that it must be touched upon, I think the only appropriate place to mention it is by adding a note to the generics section on casting with generics.

[santiagobasulto]

Yes, you're righy Heather. I've been thinking and maybe the type erasure is too much for the tutorial. Would you add the note about the pattern matching?

[SethTisue]

no activity on this issue in a while, so, closing. a PR would be welcome for any changes that were agreed upon.