You don't have to reduce an array to get it into a set; just create the set with an array: let objectSet = Set(objects.map { $0.URL }).

With Swift 5.1, you can use one of the three following examples in order to solve your problem.

#1. Using Array's map(_:) method and Set's init(_:) initializer

In the simplest case, you can map you initial array to an array of urls (String) then create a set from that array. The Playground below code shows how to do it:

struct MyObject {    let url: String}let objectArray = [    MyObject(url: "mozilla.org"),    MyObject(url: "gnu.org"),    MyObject(url: "git-scm.com")]let urlArray = objectArray.map({ $0.url })let urlSet = Set(urlArray)dump(urlSet)// ▿ 3 members//   - "git-scm.com"//   - "mozilla.org"//   - "gnu.org"

#2. Using Array's reduce(into:_:) method

struct MyObject {    let url: String}let objectArray = [    MyObject(url: "mozilla.org"),    MyObject(url: "gnu.org"),    MyObject(url: "git-scm.com")]let urlSet = objectArray.reduce(into: Set<String>(), { (urls, object) in    urls.insert(object.url)})dump(urlSet)// ▿ 3 members//   - "git-scm.com"//   - "mozilla.org"//   - "gnu.org"

As an alternative, you can use Array's reduce(_:_:) method:

struct MyObject {    let url: String}let objectArray = [    MyObject(url: "mozilla.org"),    MyObject(url: "gnu.org"),    MyObject(url: "git-scm.com")]let urlSet = objectArray.reduce(Set<String>(), { (partialSet, object) in    var urls = partialSet    urls.insert(object.url)    return urls})dump(urlSet)// ▿ 3 members//   - "git-scm.com"//   - "mozilla.org"//   - "gnu.org"

#3. Using an Array extension

If necessary, you can create a mapToSet method for Array that takes a transform closure parameter and returns a Set. The Playground below code shows how to use it:

extension Array {    func mapToSet<T: Hashable>(_ transform: (Element) -> T) -> Set<T> {        var result = Set<T>()        for item in self {            result.insert(transform(item))        }        return result    }}struct MyObject {    let url: String}let objectArray = [    MyObject(url: "mozilla.org"),    MyObject(url: "gnu.org"),    MyObject(url: "git-scm.com")]let urlSet = objectArray.mapToSet({ $0.url })dump(urlSet)// ▿ 3 members//   - "git-scm.com"//   - "mozilla.org"//   - "gnu.org"

reduce() method expects a closure that returns a combined value, while insert() methods of Set value does not return anything but instead it inserts a new element into the existing set.

In order to make it work you would need to do something like:

objects.reduce(Set<String>()) {    $0.union(CollectionOfOne($1.URL))}

But the above is a bit of an unnecessary complication. If you have a big array, that would mean quite a number of ever-growing sets to be created while Swift goes over all the elements from objects. Better follow the advice from @NRitH and use map() as that would make a resulting set in one go.