[swift-evolution] Rename "guard" to "unless"

[Greg Parker]

> On Jan 6, 2016, at 2:09 AM, James Campbell via swift-evolution <swift-evolution at swift.org> wrote:
> 
> What is "guard"? why its the opposite to "if"!
> 
> So in other languages what have they called this, well in Ruby they called it "unless" which I feel is a much clearer form of intent and lower barrier of entry for a user. See this code.
> 
> guard name != nil else {
>    fatalError("Please enter a name")
> }
> 
> What does that actually say if you look at it from a linguistic point of view, "guard that name doesn't equal nil otherwise fail", that feels very obtuse. 
> 
> With my proposal the syntax could become this:
> 
> unless name != nil {
>   fatalError("Please enter a name")
> }
> 
> This now reads as "unless name doesn't equal nil then fail" which I think is a much clearer syntax. I think this would be a great change for Swift 3. For me I think this is more friendly for beginners.
> 
> It would support the same structure as an if block:
> 
> unless name != nil {
> }
> else
> {
> }
> 
> Going forward it would allow us to be more flexible if we added inline if/unless statement, as in Ruby.
> 
> callThisMethod() if age > 0
> throwThisError() unless age <= 0

`guard` is not the opposite of `if`. `guard` has effects that `if` does not. We deliberately chose not to provide syntax that was identical to `if not X`.

History: The very first proposal of this feature was called `unless`. Fifty-odd messages of discussion later it was called `require`, in part because we did not want to match the `if not` constructs seen in languages like Perl and Ruby. That name was still controversial. A few weeks later it was changed to `guard..else` to general acclaim because the intended use is commonly known as a "guard clause".


-- 
Greg Parker     gparker at apple.com     Runtime Wrangler