[swift-evolution] [Review #2] SE-0089: Renaming String.init<T>(_: T)

Kevin Ballard kevin at sb.org
Mon Jun 6 18:37:08 CDT 2016

On Sat, Jun 4, 2016, at 11:24 AM, Chris Lattner via swift-evolution wrote:
> 	* What is your evaluation of the proposal?

+1 in general, but I still think String.init(describing:) is annoyingly long and would prefer String.init(from:).

Also, I'm a little concerned by this bit:

> As a performance optimization, the implementation of the string literal interpolation syntax will be changed to prefer the unlabeled initializer when interpolating a type that is LosslessStringConvertible or that otherwise has an unlabeled String initializer, but use the String.init<T>(describing: T) initializer if not.

Right now string interpolation is done via the StringInterpolationConvertible protocol, and in particular the implementation for String contains

  public init<T>(stringInterpolationSegment expr: T) {
    self = String(expr)

This implementation could be changed to test for LosslessStringConvertible, but there's no way to test for "Does String have an unlabelled initializer that accepts type T?", so the only way to implement this proposed optimization is by adding extra compiler magic.

I'm also not convinced that this is a performance optimization at all. It seems reasonable to expect that the unlabeled initializer would basically be sugar for calling the labeled initializer (or accessing the description property directly), which means there's no performance benefit to calling the unlabeled initializer. I can't think of any good reason for a type to implement an unlabeled String initializer that's more efficient than its implementation of CustomStringConvertible or Streamable.

> 	* Is the problem being addressed significant enough to warrant a change to Swift?


> 	* Does this proposal fit well with the feel and direction of Swift?


> 	* How much effort did you put into your review? A glance, a quick reading, or an in-depth study?

A quick reading of the current proposal, and I participated in the original review.

-Kevin Ballard

More information about the swift-evolution mailing list