Union of two sets in Scala

Question

From the question linked here, I found this implementation of Union in Scala:

def union(a: Set, b: Set): Set = i => a(i) || b(i)

And Set is

Answer 1

And when it finds a match between to sets, it returns true, if it indeed does, where do I filter it?

union does not find a match between two sets, it creates a new set which contains the values of both sets. Example:

val a = (i) => i == 2 // a contains 2 as a(2) == True
val b = (i) => i == 5 // b contains 5 as b(5) == True
val u = union(a, b)   // u contains 2 and 5 as u(2) == True and u(5) == True

So the 'filtering' just happens on the way. This function is not iterating over each set, filtering specific things out, it just returns a combination of two functions which then can executed later to query for the actual values.

Example of querying the values of a union:

val a = (i) => i == 2
val b = (i) => i == 5
val u = union(a, b)

for(i <- 1 to 10 if u(i)) yield i     // returns Vector(2, 5)

And yes, this is not an optimal way of storing values in sets as you have to check values by guessing but it is a good way to demonstrate how combining functions adds complex functionality without writing very complex code.