[swift-evolution] Proposal: An Either Type in the STL

Thorsten Seitz tseitz42 at icloud.com
Sat Dec 12 12:48:33 CST 2015

> Am 11.12.2015 um 03:14 schrieb John McCall via swift-evolution <swift-evolution at swift.org>:
> Right, sorry; I’ve gotten too used to the C++ sense.  I mean types which contain free uses of the generic parameters of the context, which therefore appear to be opaque there.  It’s a problem for union typing because the intersection of any opaque types with the other concrete types in a union is not necessarily empty.  It’s not an unsolvable problem, of course.

Do you mean something along these lines (taken from Ceylon)?

 interface Set<These> {
	shared formal Set<These | Those> union<Those>(Set<Those> set);
	shared formal Set<These & Those> intersection<Those>(Set<Those> set);

Set<Foo> setOfFoo = ... ;
Set<Bar> setOfBar = ... ;

Set<Foo | Bar> setUnion = setOfFoo.union(setOfBar); // each element conforms to Foo or Bar
Set<Foo & Bar> setIntersection = setOfFoo.intersection(setOfBar); // each element conforms to Foo and Bar

Types like (Int | Int) reduce to Int, of course, etc.


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