[swift-evolution] Proposal: Add a sequence-based initializer to Dictionary

[Nate Cook]

Adding a `combine` closure with Donnacha's default seems like a pretty good solution for this, but giving a `try`-marked closure parameter a default  messes with the rethrows behavior and requires a try on every call. I think it's important that when used by default, this initializer has the same behavior as looping over the sequence and setting values for keys. That is, it should replicate:

for (key, value) in sequence { 
    newDictionary[key] = value 
}

and use the last value for any duplicate keys, rather than failing or trapping.

To handle this properly we'd need two new initializers:

init<S: SequenceType where S.Generator.Element == Generator.Element>(_ sequence: S)

init<S: SequenceType where S.Generator.Element == Generator.Element>
    (_ sequence: S, @noescape combine: (Value, Value) throws -> Value) rethrows

Perhaps we also need a mutating `mergeContentsOf` function with the same signatures as the initializers:

mutating func mergeContentsOf<S: SequenceType where S.Generator.Element == Generator.Element>(_ sequence: S)

mutating func mergeContentsOf<S: SequenceType where S.Generator.Element == Generator.Element>
    (_ sequence: S, @noescape combine: (Value, Value) throws -> Value) rethrows

I'm pretty sure I would use all four of those, and that would bring Dictionary more into alignment with how you can use Array and Set. Would a slightly expanded proposal make sense?

Nate


> On Jan 15, 2016, at 9:01 AM, Nicola Salmoria via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I'm ambivalent about the preconditionFailure. Since there would otherwise be silent loss of data, I think it fits Swift's "safe by default" paradigm. It's also consistent with what the normal initialization from a DictionaryLiteral does.
> However, I can also see how it might be more convenient to just pick the last value.
> 
> Nicola
> 
> On Fri, Jan 15, 2016 at 11:53 AM, Alan Skipp <al_skipp at icloud.com <mailto:al_skipp at icloud.com>> wrote:
> I’ve been absorbed in the world of Monoids lately, so I find the suggestion below to be particularly brilliant. : )
> It solves the issue of arbitrarily choosing the value for duplicate keys rather nicely. Only thing I’m not too sure about is the idea of failing by default on duplicate keys?
> 
>> On 15 Jan 2016, at 10:18, Nicola Salmoria via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> To handle the case of duplicate keys, why not allow to pass in a 'combine' function?
>> The function could default to a preconditionFailure to be consistent with the DictionaryLiteral behavior, but be overridden by the caller as needed.
>> 
>> extension Dictionary {
>>     /// Creates a dictionary with the keys and values in the given sequence.
>>     init<S: SequenceType where S.Generator.Element == Generator.Element>(_ sequence: S, combine: (existing: Value, other: Value) -> Value = { preconditionFailure("Sequence contains duplicate keys"); return $1 } ) {
>>         self.init()
>>         for (key, value) in sequence {
>>             if let existing = updateValue(value, forKey: key) {
>>                 updateValue(combine(existing: existing, other: value), forKey: key)
>>             }
>>         }
>>     }
>> }
>> 
>> 
>> usage examples:
>> 
>> let samples = [("Rome", 40.2), ("New York", 35.1), ("Rome", 42.5), ("New York", 32.8)]
>> let minTemperatures = Dictionary(samples, combine: min)
>> // ["Rome": 40.2, "New York": 32.8]
>> let maxTemperatures = Dictionary(samples, combine: max)
>> // ["Rome": 42.5, "New York": 35.1]
>> 
>> let probabilities = [("a", 0.25), ("b", 0.25), ("c", 0.25), ("a", 0.25)]
>> let stateProbabilities = Dictionary(probabilities, combine: +)
>> // ["b": 0.25, "a": 0.5, "c": 0.25]
>> 
>> 
>> Nicola
> 
> It’d be great if there was also an init that restricted the Values to Monoids, which would mean the combine function would be taken from the supplied Monoid values (I understand I’ve departed to fantasy island at this point, but one can dream : )
> 
> Al
> 
> 
> 
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