type Set = Int => Boolean

def singletonSet(x: Int): Set = Set(x)
val three = singletonSet(3)
three(3) //True
three(5) //False

def map(s: Set, p: Int => Int): Set

def map(s: Set, p: Int => Int): Set = {
  x =>
    if (s(x)) singletonSet(p(x))
    else p(x) => false
}

The error that I'm getting with this format is:
    error: not a legal formal parameter.
    Note: Tuples cannot be directly destructured in method or function parameters.
    Either create a single parameter accepting the Tuple1,
    or consider a pattern matching anonymous function: `{ case (param1, param1) => ... }
        else p(x) => false