[swift-evolution] [Proposal] mapValues

[Miguel Angel Quinones]

I'm +1 for adding mapValues. Very useful functionality and trivial to implement. 

> > I.e. I suggest to implement and mapKeys() also. It could be also useful in some situations.
> `mapKeys` is much more dangerous, because you could end up mapping many values into a single key. You kind of need to combine the values somehow. Perhaps:
>  
> extension Dictionary {
> func mapValues(_ valueTransform: @noescape Value throws ->OutValue) rethrows ->[Key: OutValue] { … }
>  
> func mapKeys(_ keyTransform: @noescape Key throws ->OutKey) rethrows ->[OutKey: [Value]] { … }
>  
> // Possibly flatMap variants, too?
> }
>  
> extension Dictionary where Value: Sequence {
> func reduceValues(_ initial: OutValue, combine: @noescape (OutValue, Value.Iterator.Element) throws ->OutValue) rethrows ->[Key: OutValue] {
> return mapValues { $0.reduce(initial, combine: combine) }
> }
> }
>  
> Which you would end up using like this:
>  
> let wordFrequencies: [String: Int] = …
> let firstLetterFrequencies: [Character: Int] = wordFrequencies.mapKeys { $0.characters.first! }.reduceValues(0, combine: +)
>  
> --
> Brent Royal-Gordon
> Architechies
>  
>  
>  
>

-- 
Miguel Angel Quinones
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