How to deal with weak in closure in Swift

Issue #326

Traditionally, from Swift 4.2 we need guard let self

addButton.didTouch = { [weak self] in
    guard
        let self = self,
        let product = self.purchasedProduct()
    else {
        return

    self.delegate?.productViewController(self, didAdd: product)
}

This is cumbersome, we can invent a higher order function to zip and unwrap the optionals

func with<A, B>(_ op1: A?, _ op2: B?, _ closure: (A, B) -> Void) {
    if let value1 = op1, let value2 = op2 {
        closure(value1, value2)
    }
}

addButton.didTouch = { [weak self] in
    with(self, self?.purchasedProduct()) {
        $0.delegate?.productViewController($0, didAdd: $1)
    }
}