Question

I’m trying to understand this def method :

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

Which is referred to at question : Scala set function

This is my understanding :

The method takes two parameters of type Set – a & b
A Set is returned which the union of the two sets a & b.

Here is where I am particularly confused : Set = i => a(i) || b(i)

The returned Set itself contains the ‘or’ of Set a & b . Is the Set ‘i’ being populated by an implicit for loop ?

Since ‘i’ is a Set why is it possible to or a ‘set of sets’, is this something like whats being generated in the background :

a(i) || b(i) 
becomes
SetA(Set) || SetB(Set)

Answer 1

Maybe what’s confusing you is the syntax. We can rewrite this as:

type Set = (Int => Boolean)

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

So this might be easier to sort out. We are defining a method union that takes to Sets and returns a new Set. In our implementation, Set is just another name for a function from Int to Boolean (ie, a function telling us if the argument is “in the set”).

The body of the the union method creates an anonymous function from Int to Boolean (which is a Set as we have defined it). This anonymous function accepts a parameter i, an Int, and returns true if, and only if, i is in set a (a(i)) OR i is in set b (b(i)).