Scala PartialFunction with isDefinedA and apply not working

Question

I am new in Scala, i was trying PartialFunctions, is this correct way to test functionality, as some tutorials followed this to work, but not working for me?

code:

object MyScalaApp extends App {
def try29{
    val r = new PartialFunction[Int, Int]  
    { 
        def isDefinedAt(q: Int) = q < 0 // Applying isDefinedAt method  
        def apply(q: Int) = 12 * q // Applying apply method 
    }        
    val rr = new PartialFunction[Double, Double]  
    { 
        def isDefinedAt(q: Double) = {q < 0}
        def apply(q: Double) = 12 * q 
    }

    println(r(1))
    println(r(2))        
    println(rr(-1))
    println(rr(-2))
  }
  }
  try29
}

output:

12
24
-12.0
-24.0

Why apply get call when its not matching first condition?
When I write def isDefinedAt(q: Int) = q != 0 it gives println(r(0)) as output 0

Answer 1

According to the ScalaDocs page:

It is the responsibility of the caller to call isDefinedAt before calling apply...

Let's try your r() Partial Function in a context where isDefinedAt() is called automatically.

val r = new PartialFunction[Int, Int] {
  def isDefinedAt(q: Int) = q < 0
  def apply(q: Int) = 12 * q
}
List(4,-3,22,-9,0).collect(r)
//res0: List[Int] = List(-36, -108)

Seems to work as expected.

Answer 2

It is not exactly what you asked, but I think it is worth mentioning the existence of applyOrElse. if later on you plan to write code such as:

if(r.isDefinedAt(1)) r(1) else 10

It is better to use applyOrElse, as elaborated in the docs:

Note that expression pf.applyOrElse(x, default) is equivalent to

if(pf isDefinedAt x) pf(x) else default(x)

except that applyOrElse method can be implemented more efficiently. For all partial function literals the compiler generates an applyOrElse implementation which avoids double evaluation of pattern matchers and guards. This makes applyOrElse the basis for the efficient implementation for many operations and scenarios.