[swift-evolution] [Pitch] Add `mapValues` method to Dictionary

[Nicola Salmoria]

Nate Cook via swift-evolution <swift-evolution at ...> writes:

> 
> 
> > On May 24, 2016, at 7:43 AM, Matthew Johnson via swift-evolution
<swift-evolution <at> swift.org> wrote:
> > 
> > Sent from my iPad
> > 
> > On May 24, 2016, at 12:59 AM, Brent Royal-Gordon via swift-evolution
<swift-evolution <at> swift.org> wrote:
> > 
> >>> I have a small remark though, wouldn’t it be better to let transform
be of type (Key, Value) throws -> T
> instead of (Value) throws -> T? You can just ignore the key (with _) if
you don’t need it, but I think it
> might come in handy in some cases.
> >> 
> >> The problem is, that closes the door to writing many simple maps in
functional style. For instance, this:
> >> 
> >>   dictionaryOfNumbers.mapValues(abs)
> >> 
> >> Would have to become this:
> >> 
> >>   dictionaryOfNumbers.mapValues { _, v in abs(v) }
> >> 
> >> (It *might* be possible to do it with `$1`, but I'm not sure; there are
some limitations around that.)
> >> 
> >> A value-value map is just simpler and cleaner, while almost always
giving you what you need.
> > 
> > +1.  
> > 
> > I don't think I have ever mapped keys.  Incidentally, that doesn't have
the usual semantics of a map
> operation as you can produce duplicate keys.
> 
> I think this conversation suffers from not knowing whether Dictionary is
going to receive a
> sequence-based initializer. There's been a proposal for that in the PR
queue for quite a while. Could
> someone in the core team please either merge or close that proposal?
> 
> https://github.com/apple/swift-evolution/pull/125

+1 for scheduling that proposal!

> 
> If there's no sequence-based init, mapValues should probably produce a new
Dictionary. If there is,
> having mapValues produce a dictionary would be redundant. I'd rather see a
MapDictionary type that wraps
> a dictionary and only evaluates the transformation on access, rather than
eagerly allocating storage
> and performing all the transformations.

I don't think it would be redundant, because producing a new Dictionary from
a Sequence should have complexity O(N log N) if I'm not mistaken, while in
principle mapValues() could be implemented more efficiently, producing a new
Dictionary with the same structure and different values with just O(N)
complexity.

Nicola

> 
> You can take a look at a rudimentary MapDictionary in this gist:
https://gist.github.com/natecook1000/7cfc5c04f18237066f7aca6e9658f276
> 
> Best,
> Nate
> 
> >> -- 
> >> Brent Royal-Gordon
> >> Architechies
> >> 
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