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

Robert Bennett rltbennett at icloud.com
Sat Jul 8 17:00:40 CDT 2017

I’m not sure about needing a universal promotion operator in Swift. However, in the case of KeyPaths, I think a leading $ (not currently a valid operator) would work well.

prefix operator $
prefix func $<T,U>(rhs: KeyPath<T,U>) -> (T)->U { return { $0[keyPath: rhs] } }


This reads really well to me because the $ is suggestive of the “functionization” of the KeyPath. Also, this is guaranteed to have no compatibility issues (right?) because it’s currently forbidden.

(I’m only suggesting giving the leading $ this functionality, not necessarily achieving this by making it a valid operator — in fact, it would probably be best if this functionality were “hard-coded” just as it is currently hard-coded for use in $0.)

> On Jul 8, 2017, at 5:46 PM, Benjamin Herzog via swift-evolution <swift-evolution at swift.org> wrote:
> Is this operator common in other languages? I would actually expect that the conversation is not 'almost-implicit' but completely implicit instead. I think both - a prefix and postfix operator - are not obvious enough what happens here, especially because this kind of conversion is not happening in other parts of the language.
> All conversions are implicit (from explicit type to protocol, from Swift stdlib types to Objective-C types, from any type to Any, …) currently.
> ______________________
> Benjamin Herzog
>> On 8. Jul 2017, at 22:10, Hooman Mehr via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> I like this promote operator idea. I have been defining similar operators for specific projects almost at random. It makes sense to come up with a well-defined behavior and name for such operators, as a common practice as you suggest.
>> The problem with the postfix operator is that it does not currently work without an extra set of parenthesis:
>> postfix operator ^
>> postfix func ^<T,U>(lhs: KeyPath<T,U>) -> (T)->U { return { $0[keyPath: lhs] } }
>> struct Guy { let name: String }
>> let guys = [
>>     Guy(name: "Benjamin"),
>>     Guy(name: "Dave"),
>>     Guy(name: "Brent"),
>>     Guy(name: "Max")
>> ]
>> guys.map(\.name^) // Error: Invalid component of Swift key path
>> guys.map((\.name)^) // This works
>> Is this a bug? 
>> That is the reason I used a prefix operator (~) in my suggestion in the a previous e-mail on this thread.
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