Some extensions for RxSwift Variables, motivated by applicatives.

Variable+Helpers.swift

extension Variable {
    func debug() -> Self {
        _ = self.asObservable().debug().subscribeNext { _ in }
        return self
    }
    
    static func combineLatest<A, B>(a: Variable<A>, _ b: Variable<B>, resultSelector: (A, B) -> E) -> Variable<E> {
        let v = Variable(resultSelector(a.value, b.value))
        _ = Observable.combineLatest(a.asObservable().skip(1), b.asObservable().skip(1), resultSelector: resultSelector).subscribeNext { e in
            v.value = e
        }
        return v
    }

    static func combineLatest<A,B,C>(a: Variable<A>, _ b: Variable<B>, _ c: Variable<C>, resultSelector: (A, B, C) -> E) -> Variable<E> {
        let v = Variable(resultSelector(a.value, b.value, c.value))
        _ = Observable.combineLatest(a.asObservable().skip(1), b.asObservable().skip(1), c.asObservable().skip(1), resultSelector: resultSelector).subscribeNext { e in
            v.value = e
        }
        return v
    }
    
    func map<R>(selector: E -> R) -> Variable<R> {
        let v = Variable<R>(selector(self.value))
        _ = self.asObservable().skip(1).subscribeNext { e in
            v.value = selector(e)
        }
        return v
    }
    
    func flatMapLatest<R>(selector: E -> Variable<R>) -> Variable<R> {
        
        let currentVariable = Variable<Variable<R>>(selector(self.value))
        _ = self.asObservable().skip(1).subscribeNext { e in
            currentVariable.value = selector(e)
        }
        
        let currentValue = Variable<R>(currentVariable.value.value)
        _ = currentVariable.value.asObservable().skip(1).subscribeNext { r in
            currentValue.value = r
        }
        
        return currentValue
    }
}