[swift-evolution] rethrows as first-class type annotation

David Waite david at alkaline-solutions.com
Mon Dec 21 23:54:49 CST 2015

I don’t know what the compiler does internally. However, I look at a signature like

func forEach(fn:Element throws->()) rethrows

and imagine that conceptually this exposes two overloads for this “forEach” function:

a)	func forEach(fn:Element->())

b)	func forEach(fn:Element throws->()) throws

In fact, I expected the swift CLI to give me an error when I attempted to define all three of these as functions, but no such luck.


> On Dec 21, 2015, at 7:09 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org> wrote:
> John, IIRC you had some reason why this wasn't a great idea, but I can't remember it. It seems useful to me too, if not something that comes up too often.
> Jordan
>> On Dec 20, 2015, at 2:46 , Alexandre Lopoukhine via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> Hi Dmitri,
>> This is a better example than any that I have come up with so far as to why “rethrows” should be a part of the signature. You shouldn’t have to use “try!” to apply a non-throwing function, like {print($0)} to “forEach”.
>> — Sasha
>>> On 20 Dec 2015, at 13:37, Dmitri Gribenko <gribozavr at gmail.com <mailto:gribozavr at gmail.com>> wrote:
>>> Hi Alexandre,
>>> I think for this use case we don't actually need 'rethrows' to become
>>> a part of the closure type, we just need the compiler to allow and
>>> "instantiate" it in more places.
>>> The case where we would need 'rethrows' to become a first class part
>>> of the type system is if we wanted 'rethrows' to be a part of the
>>> signature of the closure itself, for example:
>>> (swift) let forEach = [ 10, 20, 30 ].forEach
>>> // forEach : (@noescape (Int) throws -> Void) throws -> () = (Function)
>>> Here, a more precise type would be (@noescape (Int) throws -> Void)
>>> rethrows -> Void.
>>> 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 <mailto:gribozavr at gmail.com>>*/
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