LazyUniquedSequence.swift

struct LazyUniquedSequence<Element>: Sequence {
    
    let base: AnySequence<Element>
    let isEqual: (Element, Element) -> Bool
    
    init<S: Sequence>(_ sequence: S, isEqual: @escaping (Element, Element) -> Bool) where S.Element == Element {
        self.base = AnySequence(sequence)
        self.isEqual = isEqual
    }
    
    func makeIterator() -> AnyIterator<Element> {
        var seen: [Element] = []
        let iterator = base.makeIterator()
        return AnyIterator({
            while let next = iterator.next() {
                if !seen.contains(where: { self.isEqual($0, next) }) {
                    seen.append(next)
                    return next
                }
            }
            return nil
        })
    }
}

extension LazySequenceProtocol where Elements.Element: Equatable {
    func uniqued() -> LazyUniquedSequence<Element> {
        return LazyUniquedSequence(self, isEqual: ==)
    }
}

extension LazySequenceProtocol {
    func uniqued(by isEqual: @escaping (Element, Element) -> Bool) -> LazyUniquedSequence<Element> {
        return LazyUniquedSequence(self, isEqual: isEqual)
    }
}

let thing = AnySequence([1, 1, 1, 1, 2, 2, 2, 2, 3, -1, 2, 4, -1])
    .lazy
    .map({ int -> String in print("doing\(int)"); return String(int) })
    .uniqued()

thing.forEach({ print($0) })

print("done")