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

[John McCall]

> On Dec 21, 2015, at 6:09 PM, Jordan Rose <jordan_rose at apple.com> 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.

I don’t remember off-hand.  I think it’s theoretically supportable, but it adds extra complexity to the type system that I wanted to avoid if possible.

John.

> 
> 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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