[swift-evolution] [Proposal] mapValues

Jonathan Hull jhull at gbis.com
Sun Apr 17 08:58:23 CDT 2016

I am definitely +1 for adding the initializer in any case.  

I would like to see it (based on Nate’s suggestion) have a “merge" parameter which takes a closure of (Key, Value, Value)throws->Value, which would be called to choose a value whenever a repeated key is encountered.  That parameter should have a default value which just traps.  That way the default behavior is to trap when a key is repeated, but it can still be overridden with a more appropriate behavior for the situation (e.g. keeping the first value, keeping the last, averaging them, etc…)

That said, I would still also like to see the functionality of mapValues (whatever it ends up being called) in the standard library.  It easily applies to 80-90% of my use cases, and allowing re-mapping of keys adds a lot of complexity which must be carefully considered (there is a lot more which can go wrong).  As swift is a practical language, it would be nice to have a quick and foolproof way to do this common task.


> on Tue Apr 12 2016, Jonathan Hull <swift-evolution at swift.org <https://lists.swift.org/mailman/listinfo/swift-evolution>> wrote:
> > 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)
> I agree that we need a way to do this, and was surprised when what I
> tried didn't work.  This should work:
>   Dictionary(d.lazy.map { (k, v) in (k, transform(v)) })
> We should have a proposal that makes the constructor work* if we don't
> already have one.
> I'm inclined against building specialized variants of basic algorithms
> into particular collections, though.  Could be talked out of it if the
> use-case is strong enough.

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