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

Robert Bennett rltbennett at icloud.com
Tue Jul 11 20:22:49 CDT 2017

Sure, but you could just as easily call a keypath “a thing that gives you read+write access to one of an object’s members”. Sounds like an inout function to me. The difference being that in general an inout function *can* do a lot more than just give access to a single member of a base object. But a keypath is still just a (constrained) inout function.

> On Jul 11, 2017, at 9:14 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> on Tue Jul 11 2017, Robert Bennett <swift-evolution at swift.org> wrote:
>> Er, yes, I now realize I diverged into two different keypaths-as-functions ideas there.
>> I think that the best implementation of deferred access is keypaths as
>> callable first-class objects, like you (Karl) said. — although I
>> wonder whether callability should be restricted to KeyPath, or
>> instead, if the notion of a callable type should gain first-class
>> language support.
>> If not possible, then a conversion sigil to make a KeyPath into a function. After that, giving
>> KeyPath a function `apply` is probably next best.
>> I like the subscript idea the least because: I don’t like the look of
>> the syntax, keypaths feel more function-y than subscript-y, 
>> and it diminishes the flexibility of keypaths (as this thread has
>> revealed).
> Because they're parameterized by a base object and a key, and (in
> general) they're writable, they're semantically very much like
> subscripts.
>>> On Jul 11, 2017, at 7:56 PM, Karl Wagner <razielim at gmail.com> wrote:
>>>> On 12. Jul 2017, at 01:20, Robert Bennett
>>>> <rltbennett at icloud.com
>>>> <mailto:rltbennett at icloud.com>> wrote:
>>>> Well, if they really are first-class deferred method calls or
>>>> member accesses, then do seem pretty function-y after all. Then
>>>> again, if they were meant to be functions, it seems like their
>>>> design would reflect that – instead of being used like subscripts,
>>>> they would be called like functions, and since new syntax had to be
>>>> created either way, the fact that they *weren't* just made into
>>>> callable objects seems to indicate that that was not the intent,
>>>> although I’d have to go back and read the discussion to see exactly
>>>> what was discussed.
>>> I agree, and I suspect I’m not alone in disliking the subscript syntax.
>>>> That said, I agree with Benjamin that having an `apply` method for
>>>> KeyPath seems like the right way to make (or have made) keypaths
>>>> work. keypath.apply(to: instance) (or keypath.evaluate(on:), or
>>>> some other name that gets the idea across) reads just as nice as
>>>> instance[keyPath: keypath] and has the added benefit of allowing
>>>> collection.map(keypath.apply) at no cost. But it’s probably too
>>>> late to even bother having a discussion about this, right?
>>> I don’t think so. That’s why we have a beta. Real-world experience
>>> has shown that we would often like to erase a KeyPath and use it as
>>> if it were a closure. Maybe we want to pass a KeyPath as a parameter
>>> to a function such as “map", which accepts a closure, or we want to
>>> set a variable with a closure-type using a KeyPath. Those functions
>>> and stored properties don’t care about the special properties of
>>> KeyPaths (e.g. that they are Codable) - they only care that they are
>>> executable on a base object to produce a result. You can wrap the
>>> key-paths inside closures, but it’s cumbersome and some developers
>>> are asking for a shorthand to perform that erasure.
>>> Personally, I would be in favour of making the erasure implicit and allowing KeyPaths to be invoked using function-syntax:
>>> // Invocation using function-syntax:
>>> let _: Value = someClosure(parameter)
>>> let _: Value = someKeypath(parameter)
>>> let _: Value = { $0.something.anotherThing }(parameter)
>>> let _: Value = (\MyObj.something.anotherThing)(parameter)
>>> // Implicit erasure to closure-type:
>>> class PrettyTableCell<T> {
>>>    let titleFormatter: (T) -> String
>>> }
>>> let cell            = PrettyTableCell<MyObj>()
>>> cell.titleFormatter = \MyObj.something.name
>>> let stuff = myObjects.map(\.something.anotherThing)
>>> - Karl
>>>>> On Jul 11, 2017, at 6:27 PM, Karl Wagner
>>>>> <razielim at gmail.com
>>>>> <mailto:razielim at gmail.com>> wrote:
>>>>>> On 11. Jul 2017, at 21:01, Robert Bennett via swift-evolution
>>>>>> <swift-evolution at swift.org
>>>>>> <mailto: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 <mailto: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
>> <mailto:swift-evolution at swift.org>
>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
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> -- 
> -Dave
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