directly applying lift function not give expected function - scala

Question

Below are the two partial function that are expected to perform sme tasks but defined in diffrent ways.
val pf1 : PartialFunction[String, String] = {
case s : String if (s != null) => s.toUpperCase()
}
//> pf1 : PartialFunction[String,String] = <function1>
val lift1 = pf1.lift
//> lift1 : String => Option[String] = <function1>
val d1 = lift1(null)
//> d1 : Option[String] = None
val d2 = lift1("hello world")
//> d2 : Option[String] = Some(hello world)
val pf2 = PartialFunction[String, String] {
case s : String if(s != null) => s.toUpperCase()
}
//> pf2 : PartialFunction[String,String] = <function1>
val lift2 = pf2.lift
//> lift2 : String => Option[String] = <function1>
val d3 = lift2(null)
//> scala.MatchError: null
val d4 = lift2("hii")
//> d4 : Option[String] = Some(hii)
Why passing null to lift2 gives MatchError, when the definition of both lift1 and lift2 is same?

Answer 1

If you look at PartialFunction.apply, you will see that it does a bit something different then what you've expected:
/** Converts ordinary function to partial one
* #since 2.10
*/
def apply[A, B](f: A => B): PartialFunction[A, B] = { case x => f(x) }
So it wraps a normal function into partial function that is defined on all domain. And that's why when you lift, you receive exception - because inside is still a normal not-lifted partial function, that is not defined

Answer 2

Why passing null to lift2 gives MatchError, when the definition of
both lift1 and lift2 is same?
They're not defined the same. When you define:
val pf1: PartialFunction[String, String] = {
case s : String if (s != null) => s.toUpperCase()
}
The compiler creates a PartialFunction, like this:
this.pf1 = ({
new <$anon: Function1>()
}: PartialFunction);
But when you declare pf2 like this:
val pf2 = PartialFunction[String, String] {
case s : String if(s != null) => s.toUpperCase()
}
You're actually telling the compiler "please take this Function1 and pass it to PartialFunction.apply. That is why the compiler does this:
this.pf2 = scala.PartialFunction.apply({
(new <$anon: Function1>(): Function1)
});
Which actually wraps a Function1 inside a partial function. So when you call pf2.lift(null), it will internally call the Function1, causing a MatchError as you're seeing.