# Lifting the "Self or associated type" constraint on existentials

**URL:** <https://forums.swift.org/t/lifting-the-self-or-associated-type-constraint-on-existentials/18025>\
**Category:** Pitches\
**Created:** [November 15, 2018, 10:13pm UTC](https://forums.swift.org/t/lifting-the-self-or-associated-type-constraint-on-existentials/18025 "2018-11-15T22:13:59Z")\
**Posts on this page:** 1\
**Showing post:** 20

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**Author:** ![anandabits](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/anandabits/32/355_2.png) [@anandabits](https://forums.swift.org/u/anandabits)\
**Post date:** [November 16, 2018, 3:58pm UTC](https://forums.swift.org/t/lifting-the-self-or-associated-type-constraint-on-existentials/18025/20 "2018-11-16T15:58:18Z")

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This is a pretty awesome trick I have used a few times. Unfortunately, because Swift does not have higher-ranked types we cannot pass a generic function to call. The good news is that we don't have to hard code the function that gets called (`foo` in the above example). There are many variations on this trick and a very useful one to know is how to use a "receiver" protocol to do double dispatch:

```swift
protocol _ReceiveP {
    func _receiveP<T: P>(_: T)
}

extension P {
    func _openP<Receiver: _ReceiveP>(for receiver: Receiver) {
        receiver._receiveP(self) 
    }
}

extension Bar {
    func doSomethingWithPs(_ ps: [P]) {
        for p in ps {
            p._openP(for: self)
        }
    }
  
    func _receiveP<T: P>(_ value: T) {
        // do somehing with value
    }
}

```

I hope you can imagine several variations on this theme. For example, you can pass dynamic context through the dispatch, you can have a receive method that returns a value which is directly returned by the open method, etc.

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