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.
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?
> On Jul 11, 2017, at 6:27 PM, Karl Wagner <razie...@gmail.com> wrote:
>
>
>> On 11. Jul 2017, at 21:01, Robert Bennett via swift-evolution
>> <swift-evolution@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@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@swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> _______________________________________________
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>> swift-evolution@swift.org
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>
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