Composing PartialFunctions with orElse when there is a wildcard case - scala

Question

Is it possible to orElse compose two PartialFunctions when the first function has case _ => wildcard pattern that matches anything thus in effect being a total function.
For example, given
val pf1: PartialFunction[Int, String] = {
case 1 => "foo"
case 2 => "bar"
case _ => "wildcard"
}
val pf2: PartialFunction[Int, String] = {
case 3 => "baz"
case _ => "wildcard"
}
then, out-of-the-box
(pf1 orElse pf2)(3)
outputs wildcard. However, assuming we cannot modify pf1, can we compose with pf2 using some compfn such that we get in effect
{
case 1 => "foo"
case 2 => "bar"
case 3 => "baz"
case _ => "wildcard"
}
where (pf1 compfn pf2)(3) would output baz?

Answer 1

You can turn pf1 into a true partial function by doing a second match to convert the "wildcard" result to a failed match.
val pf3: PartialFunction[Int, String] = (i: Int) => pf1(i) match {
case s if s != "wildcard" => s
}
And then
(pf3 orElse pf2)(3)
If you want the precise syntax you showed then you need to use an implicit class:
implicit class addCompfn(f1: PartialFunction[Int, String]) {
def compfn(f2: PartialFunction[Int, String]) = (i: Int) => f1(i) match {
case s if s != "wildcard" => s
case s => f2(i)
}
}
And then
(pf1 compfn pf2)(3)

Answer 2

Why not opting for something like this:
val pf1: PartialFunction[Int, String] = {
case 1 => "foo"
case 2 => "bar"
}
val pf2: PartialFunction[Int, String] = {
case 3 => "baz"
}
def composeOrElseWildcard(input: Int) = (pf1 orElse pf2).applyOrElse(input,(_: Int) => "wildcard")
Otherwise, to me, there are no solution to get rid of wildcard pattern effect in pf1.
( composeOrElseWildcard(3) gives "baz" and composeOrElseWildcard(4) gives "wildcard" )