[swift-evolution] Proposal: An assignment operator := that ensures nil before assignment.

[Amir Michail]

> On Feb 27, 2016, at 9:46 AM, Ross O'Brien <narrativium+swift at gmail.com> wrote:
> 
> How is this different from the rejected ??= proposal?
> 

If the value is not nil, the program stops with an assertion error.

> On Sat, Feb 27, 2016 at 2:34 PM, Amir Michail via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> Consider the use of “!” in the following:
> 
> var x: Int? = 5
> 
> x! = 2 // “!” is used to ensure non-nil value before assignment
> 
> The idea is to have a similar method for ensuring a nil value.
> 
> So instead of writing:
> 
> precondition( x == nil )
> x = 3
> 
> You could write:
> 
> x := 3
> 
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