[swift-evolution] [Preemptive Proposal] Operator for Lenses

[Andrey Tarantsov]

Could you please explain what a lens is/means? I only found [1], but quick skimming didn't reveal any lenses.

[1] https://www.youtube.com/watch?v=estNbh2TF3E

A.


> On Jan 15, 2016, at 6:10 AM, Michael Henson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> There hasn't been a major Lens thread on the list so far, but I'm intrigued by the idea and was tinkering with a related proposal until I stumbled across Brandon William's "Lenses in Swift" video and discovered that Lenses cover everything I wanted to do and more.
> 
> The one thing that stuck with me, though, is the question of syntax. If Lenses are to be a native part of the Swift language, how would one declare them?
> 
> We've currently got not-so-secret methods at the type level of Structs and Classes for every member function:
> 
> class Example {
>     func action() {}
> }
> 
> let example = Example()
> example.action // () -> Void
> Example.action // (Example) -> Void
> 
> That looks a lot like a Lens on the member function, though if I understand correctly the current plan is for those to go away in a future version.
> 
> We also have to deal with Type-level members and functions:
> 
> class Example {
>    static action() {}
>    static name: String
> }
> 
> Example.action() // () -> Void
> Example.name // String
> 
> So, using a dot-operator as the way to get a Lens could be problematic due to name collisions.
> 
> The Proposal:
> 
> Use the '#' character in place of the dot operator to retrieve a Lens for a Type/member pair:
> 
> Example#action() // (Example) -> Void
> Example#name    // Lens<Example,String>, autogenerated to work on the 'name' member
> 
> Member function names should be fully-specified with the mechanism from Doug Gregor's method naming proposal.
> 
> Some notes:
> * A specific operator stands out and is easier to scan for as a code reader.
> * The octothorpe seems to be available in Swift.
> * It also has a current meaning in a technology familiar to most everyone - the Document Fragment in HTML - which could make the idea easier to explain to newcomers, by analogy.
> 
> What about Lenses on Type-level members? My first thought is that we don't have to support that because they're more like namespaced globals rather than parts of a data type. They can always be referenced directly. That might be a vacuous observation to people more familiar with the Lens concept, but I noted it for completeness.
> 
> Mike
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