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

[Karl Wagner]

 I agree that it should be completely implicit.
  

  
KeyPaths are simply chains of partially-applied properties and subscripts. At the same time, it was noted in the KeyPath proposal that a similar mechanism might be used to model chains of partially-applied functions. I think that having both types be convertible to a closure would be sensible.
  

  
In fact, you could argue for a general-purpose “Executable” protocol which would allow any conforming object to be implicitly used as a function/closure. Command-style objects such as predictes and transformers would also benefit from such a feature.
  

  
- Karl
  

  

  

  

  
  

  
  
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> On Jul 8, 2017 at 11:56 pm,  <Benjamin Herzog via swift-evolution (mailto:swift-evolution at swift.org)>  wrote:
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> 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.
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> Benjamin Herzog
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> > On 8. Jul 2017, at 22:10, Hooman Mehr via swift-evolution  <swift-evolution at swift.org (mailto:swift-evolution at swift.org)>  wrote:
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> > 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.  
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> > The problem with the postfix operator is that it does not currently work without an extra set of parenthesis:
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> > postfix  operator  ^
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> > postfix   func  ^<T,U>(lhs:  KeyPath<T,U>) ->  (T)->U  {  return  { $0[keyPath: lhs] } }
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> > struct  Guy {  let  name:  String  }
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> > let  guys = [
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> >         Guy(name:  "Benjamin"),
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> >         Guy(name:  "Dave"),
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> >         Guy(name:  "Brent"),
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> >         Guy(name:  "Max")
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> > ]
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> > guys.map(\.name^)  // Error: Invalid component of Swift key path
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> > guys.map((\.name)^)  // This works
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> > Is this a bug?   
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> > 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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