[swift-users] How flatMap's signature is able to translate from Optional<Type> to [Type]?

[Slava Pestov]

> On Feb 13, 2017, at 4:40 PM, Maxim Veksler via swift-users <swift-users at swift.org> wrote:
> 
> I'm trying to understand the signature of flapMap that allows it to "strip down" Optional from a type, when called on an Optional collection but remain same type when called on a non option collection.
> 
> How would flatMap be implemented if would have been stand alone?
> 
> This for ex. I can't get to compile:
> 
> func flatMap<ElementOfResult, Element, T: Sequence>(on: T, _ transform: (Element) throws -> ElementOfResult?
>     ) rethrows -> [ElementOfResult] {
>     var result: [ElementOfResult] = []
>     for element in on {
>         if let newElement = try transform(element) {
>             result.append(newElement)
>         }
>     }
>     return result
> }

This doesn’t compile because there’s no relation between T.Element.Iterator and Element. I think you want to remove the Element type parameter altogether, and make the input type of flatMap into T.Element.Iterator.

Slava

> 
> 
> Additionally please see 2 related SO questions, for background.
> 
> http://stackoverflow.com/q/42213656 <http://stackoverflow.com/q/42213656>
> http://stackoverflow.com/q/42214880 <http://stackoverflow.com/q/42214880>
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