[swift-evolution] Add an ifPresent function to Optional

[Thorsten Seitz]

In the Smalltalk projects I worked on, we defined a method #collect:as: on Collection which allowed to pass the desired type of the result container and #collect:into: where a collection to be extended by the mapped element would be passed (Smalltalk uses the name #collect: instead of map).

-Thorsten 


> Am 21.03.2016 um 07:01 schrieb David Waite via swift-evolution <swift-evolution at swift.org>:
> 
> I tend to look at Sequence.map (and the like) as an optimized shortcut for
> 
> sequence.lazy.map(mappingFn).array
> 
> and where array, count maxElement, etc are optimized shortcuts for reductions
> 
> In that light, I don’t know if other collection types are needed for optimized forms. I might prefer promoting the use of reductions instead.
> 
> For instance, imagine I have a Set of employees, and I map based on wage. I want the flexibility for the behavior to be:
> - Create a new Set, collapsing duplicate wages
> - Create a new Array, allowing for duplicate wages
> - Create a new Set, failing if two employees have the same wage (for some definition of failing)
> - something else
> 
> examples of promoting the use of reductions would be supporting mutable reduction, and possibly having a protocol for reducers as types (example that springs to mind is java’s stream Collector)
> 
> -DW
> 
>> On Mar 20, 2016, at 10:04 PM, Howard Lovatt via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> RE: map etc. returning Array
>> 
>> You could currently return AnyCollection though, and that would then let Optional.map also return AnyCollection.
>> 
>> There is also talk in completing generics so that map could return Any<CollectionType where Element == Element> which would be even better than AnyCollection since a wrapper is removed.
>> 
>>   -- Howard.
>> 
>>> On 15 March 2016 at 12:29, Dmitri Gribenko via swift-evolution <swift-evolution at swift.org> wrote:
>>> On Mon, Mar 14, 2016 at 5:51 PM,  <davesweeris at mac.com> wrote:
>>> >
>>> > On Mar 14, 2016, at 5:51 PM, Dmitri Gribenko via swift-evolution
>>> > <swift-evolution at swift.org> wrote:
>>> >
>>> > Optional.map returns an Optional.
>>> >
>>> > Array.map returns an Array.
>>> > Set.map returns an Array.
>>> > <any other collection>.map returns an Array.
>>> >
>>> > I can't say that it is not valid to think about an Optional as a tiny
>>> > collection, but as implemented in Swift, .map() does objectively
>>> > behave differently...
>>> >
>>> > That behavior is, at least partially, because protocols don’t currently
>>> > support this:
>>> > protocol CollectionType {
>>> >     typealias T
>>> >     func map<U>(transform: T->U) -> Self<U> // error: Cannot specialize
>>> > non-generic type 'Self'
>>> > }
>>> 
>>> This feature is called higher-kinded types, and it is not planned for
>>> design or implementation in the near future.
>>> 
>>> > I *think* I remember reading on here somewhere that the intent is to change
>>> > map and flatMap to return “Self<U>" pretty much as soon as the language
>>> > supports it.
>>> 
>>> As a standard library engineer I can with certainty that this is not
>>> the plan.  We like the current formulation of map() that returns an
>>> Array.
>>> 
>>> https://github.com/apple/swift/blob/master/docs/StdlibRationales.rst#high-order-functions-on-collections-return-arrays
>>> 
>>> I don't think that having HKTs in the language would change the rationale.
>>> 
>>> Dmitri
>>> 
>>> --
>>> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
>>> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/
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>> 
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