[swift-evolution] Dictionary Enhancements

Jonathan Hull jhull at gbis.com
Fri Feb 17 20:50:29 CST 2017

Thoughts inline.

> On Feb 16, 2017, at 4:26 PM, Ben Cohen via swift-evolution <swift-evolution at swift.org> wrote:
> Hi swift-evolution,
> Following up on Ted’s post regarding the opening up of stage 2, I’m starting a thread to discuss improvements to the Dictionary type.
> Here is a list of commonly requested changes/enhancements to Dictionary, all of which would probably be appropriate to put together into a single evolution proposal:
> init from/merge in a Sequence of Key/Value pairs (already raised as SE-100: https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md <https://github.com/apple/swift-evolution/blob/master/proposals/0100-add-sequence-based-init-and-merge-to-dictionary.md>).
+1. I have wanted this since Swift 1.

> make the Values view collection a MutableCollection (as in this PR: https://github.com/apple/swift-evolution/pull/555 <https://github.com/apple/swift-evolution/pull/555>).
I think Nate’s proposal covers this case well.

> Add a defaulting subscript get (e.g. counts[key, default: 0] += 1 or grouped(key, default:[]].append(value)).
I am indifferent to this. I am happy using ??.  I guess it could be slightly more efficient because it avoids wrapping and unwrapping the optional.

> Add a group by-like init to create a Dictionary<K,[V]> from a sequence of V and a closure (V)->K.
+1. I would use this.

> Add Dictionary.filter to return a Dictionary.

> Add Dictionary.mapValues to return a Dictionary (can be more efficiently implemented than composition as the storage layout remains the same).
+1000.  I have also been asking for this since the beginning.  I built my own version (and use it frequently), but as you say, the standard library can do it much more efficiently.

I would also like to see an in-place version as well.

One design detail. Even though it is only mapping the values, I would like it to pass the key to the closure as well.  It occasionally figures in to the mapping logic.

> Add capacity property and reserveCapacity() method.
+0.5. I could see it being useful occasionally.

> Have Dictionary.removeAtIndex return the Index of the next entry.
No opinion on this.

> (once we have conditional conformance) Make dictionaries with Equatable values Equatable.

> Please reply here with any comments or questions on the above list, or any additions you believe are important that are missing from it.
I would also like to see a version of map which returns a dictionary and handles key collisions:

	let newDict = myDict.map(collision: {k,v1,v2 in v2}) { (k,v) in ... }

The collision parameter would take a throwing closure and handle the case of a key conflict (by returning the value to use, throwing, or trapping).  It would have a default value so that it would only have to be specified if a different behavior was desired.  

In advanced cases, the collision could be used to accumulate values together.  Because of this, I would actually like to see this on *collection* (not just dictionary).  The map closure is handed each element of the sequence (which in the case of dictionary is a key/value tuple), and expects a return value of a key/value tuple.  The collision block is called when a key is returned which has already been used to figure out what value to use.  This might choose a winner, or it could act like reduce, building a value from the components.

As a concrete example of what this allows, I could take in an array of words/strings [“apple”, “aardvark”, …] and then do the following to get a count of how many words start with each letter:
	let letterFrequency = words.map(collision:{$1+$2}) { (String($0.characters.first ?? “”) ,1)}
	print(letterFrequency[“a”]) //number of words starting with “a"

I am ok using a term other than ‘map' if that is easier on the compiler, but I would like this functionality.  At the simple end, it allows map functionality for both keys and values.  At the advanced end, it acts as a categorizing reduce over a sequence/collection.

You can even trivially implement the proposed groupBy with it (this is on Collection, but you could do an init on dict the same way):

	func grouped<K>(by categorizer: (Element)->K ) -> [K:Element] {
		return self.map(collision:{$1+$2}) {(categorizer($0), [$0])}


> All methods added to the standard library increase complexity, so need a strong justification to reduce the risk of API sprawl. When requesting additions/modifications, please keep the following questions in mind:
> Is the suggested addition a common operation that many would find useful? Can it be flexible enough to cover different needs?
> Will it encourage good practice? Might it be misused or encourage anti-patterns?
> Can the operation be composed simply from existing std lib features? Is that composition intuitive/readable? 
> Is writing the equivalent by hand hard to get right? Are there common correctness traps that this addition would help avoid?
> Is writing the equivalent by hand hard to make efficient? Are there common performance traps that this addition would help avoid?
> Might a native implementation be able to execute more efficiently, by accessing internals, than the equivalent implementation using public APIs?
> As he has already written up SE-100 and another Dictionary proposal, Nate Cook has kindly offered to collate a new omnibus proposal for Dictionary, which will then get pitched here.
> I will send another email about enhancements to Sequence/Collection algorithms shortly.
> Thanks!
> Ben
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> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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