Understanding Case Class's 'unapply'

Question

In the following code, I'm trying to understand the usage of Foo#unapply : Option[Int] with respect to pattern matching.

$ls
Test.scala
$cat Test.scala 
object Test {

  case class Foo(x: Int)

  def f(foo: Foo): Unit = foo match {
    case Foo(_) => ()
  }

}

It compiles:

$scalac Test.scala 
$ls
Test$.class Test$Foo$.class Test$Foo.class  Test.class  Test.scala

I ran javap -c -v X.class | grep unapply on each *class file. But, I did not find any results.

Please explain how Foo#unapply gets used in pattern matching. Also, where can I find the corresponding JVM bytecode instructions?

Answer 1

If you compile with -Xprint:typer or -print, you'll see the generated code. This is the cleaned up version of apply and unapply, compiled with scalac -Xprint:typer Test.scala:

case def apply(x: Int): com.yuval.Test.Foo = new Foo(x);
case def unapply(x$0: com.yuval.Test.Foo): Option[Int] = if (x$0.==(null))
    scala.this.None
  else
    Some.apply[Int](x$0.x);

And f remains the same:

def f(foo: com.yuval.Test.Foo): Unit = foo match {
  case (x: Int)com.yuval.Test.Foo(_) => ()
}

And this is the Java decompiled code from Test$.class:

public void f(Test.Foo foo) {
    Test.Foo localFoo = foo;
    if (localFoo != null) {
        BoxedUnit localBoxedUnit = BoxedUnit.UNIT;
    } else {
        throw new MatchError(localFoo);
    }
}

And this is the bytecode produced with java -c -p Test$.class:

public void f(com.yuval.Test$Foo);
  Code:
   0: aload_1
   1: astore_2
   2: aload_2
   3: ifnull        11
   6: getstatic     #20    // Field scala/runtime/BoxedUnit.UNIT:Lscala/runtime/BoxedUnit;
   9: astore_3
  10: return
  11: new           #22    // class scala/MatchError
  14: dup
  15: aload_2
  16: invokespecial #25    // Method scala/MatchError."<init>":(Ljava/lang/Object;)V
  19: athrow

The pattern match here turns into a simple type check.