[swift-evolution] [Draft proposal] Extending read-only properties with setters

[swift at lng.la]

Swift extensions are fantastic for filling in gaps in an API, but they don't currently allow you to convert a read-only property into a read-write property by extending it with your own setter. I'd like to propose that an extension may contain a set-only computed property iff the extended type already has a getter for that property.

My specific motivation for this proposal stems from CGRect's translated API. If you frequently work with rects, it's very convenient to be able to access their x/y/width/height properties directly. It's easy to extend CGRect with x/y properties, but width and height are problematic because of the CGRectGetWidth and CGRectGetHeight functions that get imported as read-only width and height properties on CGRect.

If you extend CGRect with read-write width/height properties, they'll work within the module they're defined because the getters shadow the imported Objective-C versions, but using the getters in other modules becomes impossible due to ambiguity errors. If it was possible to only define the width/height setters, the ambiguity wouldn't exist.

This particular issue is pretty narrow, but it's moderately frustrating if you write a lot of layout code or do other work with rects, and I think this is just one of many situations where adding a setter to a property would be useful.

This proposal is somewhat related to the "support for pure setters" discussion, but since it doesn't involve the tradeoffs of introducing actual set-only properties, I feel that this warrants its own separate discussion.

Looking forward to your input!

Jarod