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If someone could explain this behavior in plain english I would really appreciate it. Specifically, is there any difference between case _:Foo and case Foo()? 

myvariable match {
  case _: Foo => println("its a foo!")
  case Foo() => println("It's also a Foo")
  case Foo => println("Oops, this will *not* match a Foo! It will never be called")
  case _ => println("This will be called for all unmatched items")
}
Hamy
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1 Answers1

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This is a normal type match, sort of like a instanceOf in Java for example.

case x: Foo => ???

It guarantees that you can use x as a Foo on the right hand side

case Foo(a,b,c) => ???

On the other hand tries to call unapply (which you get for free on a case class) and allows you to "reach into" the instance and catch all fields as vals that you can use on the right hand side.

Side note: defining a case class with no fields is essentially identical to using a case object, which is what most people do in Scala to just represent one possible value. So your second case is a bit strange, and would probably rather be done using case objects:

case object Foo

case Foo => ???
johanandren
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  • Ahhh I get it now, `case Foo()` is actually calling `Foo.unapply`, but because it's an empty parameter list there's nothing to extract. I know the third case is a bit odd, but I think it's a common mistake so I wanted to include it as an "oops" for others reading – Hamy Sep 20 '14 at 18:06
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    @Hamy The third case is not odd at all -- you do this when you want to match a *value*, rather than a structure. So if you have a case of a `sealed trait` that doesn't carry any data, as johanandren alluded to, you could make it a `case object` and match it that way. You can also match literal values that way too. There's another type of case too which uses backticks to match against a previously defined variable. – acjay Sep 21 '14 at 00:37
  • @acjay - Sure, all great advice. Perhaps odd was the wrong word, I meant that I believe people often confuse `case Foo` with `case Foo()`, not realizing that the former is probably not doing what they intended. For example, see http://stackoverflow.com/questions/2254710/why-were-the-case-classes-without-a-parameter-list-deprecated where the first answer mentions this confusion – Hamy Sep 21 '14 at 01:27