Why no partial function type literal?

I wonder why there doesn't exist a literal for partial function types. I have to write

val pf: PartialFunction[Int, String] = {
  case 5 => "five"
}

where an literal like :=> would be shorter:

val pf: Int :=> String = {
  case 5 => "five"
}

Partial functions are often used and in Scala already some "special" feature, so why no special syntax for it?