FooStruct.swift

struct Foo {
	
	var names: Set<String> = []
	var numbers: Set<Int> = []
	var otherNumbers: Set<Double> = []
	
	func setNames<S: SequenceType where S.Generator.Element == String>(seq: S) -> Foo {
		var new = self
		new.names = Set(seq)
		return new
	}
	
	func setNumbers<S: SequenceType where S.Generator.Element == Int>(seq: S) -> Foo {
		var new = self
		new.numbers = Set(seq)
		return new
	}
	
	func setOtherNumbers<S: SequenceType where S.Generator.Element == Double>(seq: S) -> Foo {
		var new = self
		new.otherNumbers = Set(seq)
		return new
	}
}

var MyFoo = Foo().setNumbers(Set([1, 2, 3])).setOtherNumbers([1.1, 2.2, 3.3])

print(MyFoo.names, MyFoo.numbers, MyFoo.otherNumbers)	// output: "[] [2, 3, 1] [3.3, 2.2, 1.1]\n"

// try #1 - I would expect this to modify MyFoo
MyFoo.setNames(["A", "B", "C"])

print(MyFoo.names, MyFoo.numbers, MyFoo.otherNumbers)	// ??? output is still: "[] [2, 3, 1] [3.3, 2.2, 1.1]\n"

// try #2
MyFoo = MyFoo.setNames(["A", "B", "C"])

print(MyFoo.names, MyFoo.numbers, MyFoo.otherNumbers)	// ok, now, output is: "["C", "B", "A"] [2, 3, 1] [3.3, 2.2, 1.1]\n"

// try #3
MyFoo.names = ["X", "Y", "Z"]

print(MyFoo.names, MyFoo.numbers, MyFoo.otherNumbers)	// as expected, output is: "["X", "Z", "Y"] [2, 3, 1] [3.3, 2.2, 1.1]\n"