[swift-evolution] [Proposal] mapValues

[Vladimir.S]

As for mapKeys and many values into a single key. I believe we should have 
a choice - do we expect multiply values for the same key or not. Just like 
"+" and integer overflow : by default it raises the error, but if "&+" - we 
expect the overflow. I can imagine situations when it is ok for me to have 
different values for the same key(if I don't care which of values should be 
for that key in result dictionary).
So my proposal is some additional mapKey(allowMultiplyValues: true) {...} 
or in any other form/name.

On 13.04.2016 13:38, Ross O'Brien via swift-evolution wrote:
> +1 on mapValues.
>
> DictionaryLiteral already throws an exception if it includes duplicate
> keys, so I'd expect mapKeys to throw an error if multiple source keys
> mapped to the same destination key.
>
> On Wed, Apr 13, 2016 at 11:28 AM, Miguel Angel Quinones via swift-evolution
> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>
>     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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