isdefinedat.scala

scala> def isEven(i: Int): Boolean = { println("checking %d".format(i)); i % 2 == 0 }                              
isEven: (i: Int)Boolean

scala> val pf: PartialFunction[Int, Boolean] = { case i if isEven(i) => { println("%d is even".format(i)); true } }
pf: PartialFunction[Int,Boolean] = <function1>

scala> pf.isDefinedAt(0)                                                                                           
checking 0
res0: Boolean = true

scala> pf(0)                                                                                                       
checking 0
0 is even
res1: Boolean = true

scala>