[swift-evolution] Proposal Sketch: simplify optional unwrapping syntax

Jeff Kelley slaunchaman at gmail.com
Fri Dec 11 10:19:46 CST 2015

I’ve had similar ideas to this. Instead of ditching the if let syntax altogether, another approach would be to use the existing name if no new name is given, so that this code:

	if let foo = foo { /* use foo */ }

could become this code:

	if let foo { /* use foo */ }

In both cases, foo is non-optional inside the braces. If you gave it another name with the if let syntax, that would work as it does today.

Jeff Kelley

SlaunchaMan at gmail.com | @SlaunchaMan <https://twitter.com/SlaunchaMan> | jeffkelley.org <http://jeffkelley.org/>
> On Dec 11, 2015, at 11:11 AM, Daniel Hooper via swift-evolution <swift-evolution at swift.org> wrote:
> A very common pattern in swift code is to "guard let" or "if let" optionals  - this works by creating a new non-optional variable to be used by future code. Often, the new variable is given the same name as the original optional variable, creating a shadow variable. This approach leads to odd looking code like this:
> if let nearestX = nearestX { closest = nearestX }
> guard let layer = layer else { continue } 
> // use layer
> At a glance, and to Swift newcomers, this code looks completely non-sensical. It's also verbose for simply ensuring the variable is non-nil. 
> The solution:
> Swift should generate unwrapped shadow variables after nil checking. The compiler would treat the below code as if it had created an unwrapped shadow variable.
> if nearestX != nil { closest = nearestX } // notice that nearestX isn't force unwrapped
> guard layer != nil else { continue } 
> // use layer, without force unwrapping
> Why force unwrapping isn't a good alternative: 
> You might suggest force unwrapping variables when they're inside an if or after a guard that checks for nil. While this does allow for the "layer = nil" syntax, it results in code that is less resilient to change. Imagine that this code was written:
> {code:java}
> if layer != nil {
> // lots of code before //
> layer!.backgroundColor = newColor
> // lots of code after //
> }
> {code}
> And much later, you need to use some of the the code in the if body elsewhere, so you copy and paste a huge chunk of it. You likely won't notice the force unwrap, and unless you're lucky, you probably didn't paste it into an if that checked layer for nil. So you get a crash. Because of this, it's important we make safe optional unwrapping as easy and sensical as possible, and minimize the situations that you would need to force unwrap.
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