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 { $0[keyPath: rhs] } }
guys.map($\.name)
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
> <[email protected]> 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
>> <[email protected] <mailto:[email protected]>> 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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