> On Dec 29, 2015, at 2:37 PM, David Waite via swift-evolution
> <[email protected]> wrote:
>
>
>> Anyway, it would not be correct to ".enumerate()" returns (Index, Element)
>> instead of (n, Element)?
>>
>> I believe that the current behavior was thought when Slices had indices
>> starting with zero.
>>
>
> The behavior of enumerate is easiest to explain when you give everything a
> name and lay them all out on the table: In particular, there is a difference
> between a Counter, an Index, a Key, a Value, and an Element.
>
> Enumerate always works in terms of adding a counter, not an index. It was
> perhaps better served as a global method, since one cannot really improve its
> default implementation.
>
> The rest are as follows:
>
> ╔════════════╦═════════════════╦═══════════════╦═════════════════╦════════════════════╗
> ║ Type ║ Index* ║ Key ║ Value ║ Element**
> ║
> ╠════════════╬═════════════════╬═══════════════╬═════════════════╬════════════════════╣
> ║ Array ║ 0-based offset ║ N/A ║ N/A ║ Generic
> "T" ║
> ╠════════════╬═════════════════╬═══════════════╬═════════════════╬════════════════════╣
> ║ ArraySlice ║ non-zero offset ║ N/A ║ N/A ║ Generic
> "T" ║
> ╠════════════╬═════════════════╬═══════════════╬═════════════════╬════════════════════╣
> ║ Dictionary ║ DictionaryIndex ║ Generic "Key" ║ Generic "Value" ║ Tuple
> (Key, Value) ║
> ╚════════════╩═════════════════╩═══════════════╩═════════════════╩════════════════════╝
>
> * Index is declared on CollectionType
> ** Element is declared on GeneratorType and referenced by SequenceType
>
> That Array [T] does not behave like a Dictionary [Int:T] is possibly a sign
> that an AssociativeCollectionType is needed, something like:
>
> protocol AssociativeCollectionType : CollectionType {
> typealias Key
> typealias Value
> typealias Element = (Key, Value)
> typealias KeySequenceType = AnySequence<Key>
> typealias ValueSequenceType = AnySequence<Value>
>
> var keys: KeySequenceType { get }
> var values: ValueSequenceType { get }
> subscript (key: Key) -> Value? { get set }
> func indexForKey(key:Key) -> Index
>
> mutating func removeValueForKey(key: Key) -> Value?
> mutating func updateValue(value: Value, forKey key: Key) -> Value?
> }
What is the use-case for this protocol?
>
> Dictionary would support such a protocol directly. Array and ArraySlice (or
> even every CollectionType) might have a mapping function (lets bike shed
> “associative()” for now) to return an implementation of the interface,
> mapping:
>
> - AssociativeCollectionType.Index = old Index
> - AssociativeCollectionType.Key = old Index
> - AssociativeCollectionType.Value = old Element
> - AssociativeCollectionType.Element = old (Index, Element)
>
> So maybe:
>
> for (index, element) in someArray.associative() { … }
>
> would do what the original idea requested: provide a (Index, Element)
> sequence for CollectionTypes.
>
> -DW
>
>
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-Dave
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