[swift-evolution] [Proposal] mapValues

[Jonathan Hull]

I would really like to see something like the following added to the standard library:

extension Dictionary {
    
    func mapValues<U>(transform:(Key,Value)->U)->[Key:U] {
        var output:[Key:U] = [:]
        for (k,v) in self {
            output[k] = transform(k,v)
        }
        return output
    }
    
}

It comes up enough that I have had to add it to pretty much every one of my projects.  I also don’t feel comfortable adding it to my frameworks, since I figure a lot of people are also adding something like this to their projects, and I don’t want to cause a conflict with their version.  Prime candidate for the standard library.

I like calling it ‘mapValues' as opposed to providing an override for map, since it makes the specific behavior more clear.  I would expect ‘map' to possibly map the keys as well (though there are issues where the new keys overlap).  I suppose you could just have a bunch of overrides for map if the compiler becomes good enough at differentiating return types: (Value)->(Value), (Key,Value)->Value, (Key, Value)->(Key,Value)

Thanks,
Jon
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