[swift-evolution] [Proposal] mapValues

[Brent Royal-Gordon]

> 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<OutValue>(_ valueTransform: @noescape Value throws -> OutValue) rethrows -> [Key: OutValue] { … }
		
		func mapKeys<OutKey: Hashable>(_ keyTransform: @noescape Key throws -> OutKey) rethrows -> [OutKey: [Value]] { … }

		// Possibly flatMap variants, too?
	}
	
	extension Dictionary where Value: Sequence {
		func reduceValues<OutValue>(_ 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
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