[swift-evolution] [Proposal] mapValues

[Nate Cook]

> On Apr 13, 2016, at 12:02 PM, Jacob Bandes-Storch via swift-evolution <swift-evolution at swift.org> wrote:
> 
> To adhere to the API Design Guidelines, I think it should be named "mappingValues", right?
> 
> On Wed, Apr 13, 2016 at 4:23 AM, Vladimir.S via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 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.

There's a proposal (awaiting merging) to add Dictionary initializers and methods that work with key/value pairs. These provide different ways of dealing with the duplicate key issue after a call to the regular Collection.map method.
https://github.com/natecook1000/swift-evolution/blob/natecook-dictionary-merge/proposals/0000-add-sequence-based-init-and-merge-to-dictionary.md

I'd be interested in a `mapValues` or `transformValues` method that would modify values in place while leaving keys alone.

Another useful method (that could be used to build mapValues more efficiently) would be `Dictionary.updateValue(value: Value, at index: DictionaryIndex)`, so you could write:

var dict = ["a": 1, "b": 2, "c": 3]
if let i = dict.index(where: { $0.value == 3 }) {
    dict.updateValue(100, at: i)
}
// dict == ["a": 1, "b": 2, "c": 100]

-Nate


> 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> <mailto: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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