[swift-evolution] [Pitch] KeyPath based map, flatMap, filter

[Karl Wagner]

> On 11. Jul 2017, at 21:01, Robert Bennett via swift-evolution <swift-evolution at swift.org> wrote:
> 
> In general, I like the idea of making ordinary types callable (although curried functions already accomplish this to some extent), but I hesitate to bring this capability to keypaths because, well, they don’t really feel like functions; I think the point of them is that they work like subscripts, not functions. After all, before keypaths were added, there was already an easy to make a function that does what a keypath does (which makes me wonder whether keypaths were necessary in the first place, but that ship has sailed). The only reason to add callable support to keypaths is for use in map, which I don’t think justifies making them callable.
> 
> Also, since I brought this up, I’d like to be proved wrong about keypaths – what use do they have that isn’t accomplished by the equivalent closure?
> 

I can’t find a formal definition of a “keypath”, so let me explain how I think of them:

Conceptually, I think I would define a KeyPath as a stateless, deferred function application with one unbound argument (the “base"). Anything you do with a KeyPath could be done with a closure of type (Base)->Value which captures all other arguments (e.g. subscript/function parameters). The major benefit that it has over a closure is identity (so you can put it in a dictionary or compare two keypaths), and that property that captures all of its parameters except the base, and that those parameters don’t have stateful side-effects. That makes it really handy for parallel execution and database predicates in ORMs.

There’s also another benefit of KeyPaths: they are de-/serialisable. Again, since it captures all of its (stateless) parameters, it itself is stateless and can be transferred to persistent storage or over a network.

You can actually see those constraints in the KeyPath proposal (which is part of what makes it such a great proposal, IMO): all captured parameters must be Hashable and Codable.

But to come back to your point - in all other respects a KeyPath is conceptually identical to a closure of type (Base)->Value. It’s like a specially-annotated closure, where it’s special construction syntax lets us statically verify that it’s a stateless, deferred function applicable to an instance of the Base type.

The KeyPath proposal said that eventually, the core team would like to be able to support arbitrary function calls in KeyPath expressions, too. For example, it’s "not fair” that \MyObject.firstFiveElements and \MyObject[3] are valid KeyPaths, but \MyObject.prefix(5) is not. It’s also expressible as (Base)->Value, so conceptually it’s also a KeyPath and can be serialised and whatnot.

- Karl 

>> On Jul 11, 2017, at 2:28 PM, Benjamin Herzog via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> I still think using an operator for this conversation would neither increase readability nor transparency. I think my mail on Sunday was lost, so I paste the content here again. It referred to a suggestion to create a possibility for KeyPath to act as a function which would bring other benefits as well:
>> 
>> In Scala you can implement an apply method which makes it possible to call an object just like a function. Example:
>> 
>> case class Foo(x: Int) {
>>    def apply(y: Int) = x + y
>> }
>> 
>> val foo = Foo(3)
>> val bar = foo(4) // 7
>> 
>> That is similar to what you suggested to have a possibility to convert an object to a closure getting called. And I totally see the point for this! I think using a keyword or special name like apply is not a good idea because it's not obvious what it does and it also makes it possible to just call the method with its name: foo.apply(4).
>> 
>> However, having a protocol is kinda hard because it's not possible to have a flexible parameter list. Maybe having a method without a name? Swift example:
>> 
>> class Foo {
>>    var x: Int
>>    init(x: Int) { self.x = x }
>> 
>>    func (y: Int) -> Int {
>>        return self.x + y
>>    }
>> }
>> 
>> let foo = Foo(x: 3)
>> let bar = foo(y: 4) // 7
>> 
>> I actually like that, would be like an anonymous function. It would also be possible to have multiple of those defined for one object (which would have to be unambiguous of course).
>> 
>> So getting back to KeyPath, it could look like this:
>> 
>> class KeyPath<Root, Value> {
>>    func (_ root: Root) -> Value {
>>        return root[keyPath: self]
>>    }  
>> }
>> 
>> I see that this would be a much bigger change and would not justify the syntactic sugar for map, flatMap, etc. But it would still be a nice addition to the Swift programming language, especially for KeyPath, transformers etc.
>> 
>> What do you think?
>> 
>> ______________________
>> 
>> Benjamin Herzog
>> 
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution