[swift-evolution] More flexible guard statement

[Jeff Kelley]

The first line of your example would need to use self.tableView, which is optional, so col would be optional as well. The assignment happens after the unwrap.

I find this a lot in my code where I need to chain together a string of optional unwrapping and assignment from those optionals, like so:

> if let foo = foo, bar = foo.bar, baz =foo.baz {
> 
> }

In that case, if bar is a non-optional property of foo, I need to rewrite it like this:

> if let foo = foo, baz = foo.baz {
> 	let bar = foo.bar
> 
> }

These examples aren’t too bad, but as you add more layers of unwrapping, it gets more difficult to do without multiple levels of indentation—though this is probably more a code smell than anything else.


Jeff Kelley

SlaunchaMan at gmail.com | @SlaunchaMan <https://twitter.com/SlaunchaMan> | jeffkelley.org <http://jeffkelley.org/>
> On Dec 12, 2015, at 3:58 PM, Zef Houssney via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Jacob, you actually don’t need to have the redundant guard statement. If you want to assign something to a constant or variable, you just do that before the `guard let`, and you can still use it in the `where`:
> 
> let col = tableView.columnWithIdentifier("MyColumn")
> guard let tableView = self.tableView where col != -1 else {
>  NSBeep()
>  print("an error occurred")
>  return
> }
> 
> Zef
> 
> 
> 
>> On Dec 12, 2015, at 11:23 AM, Al Skipp via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> How about extending ‘NSTableView’ with a subscript?
>> 
>> extension NSTableView {
>> subscript(columnID columnID: String) -> Int? {
>>   get {
>>     let c = columnWithIdentifier(columnID)
>>     return c >= 0 ? c : .None
>>   }
>> }
>> }
>> 
>> tableView[columnID: "MyColumn"]
>> 
>> 
>> It doesn’t address the general case, but it does introduce a more Swifty way of dealing with non-existent return values in this use case and would enable you to use it with guard.
>> 
>> Al
>> 
>>> On 12 Dec 2015, at 17:43, Jakob Egger via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> At the moment "guard let" can only be used for unwrapping optionals. It
>>> would be really nice if it could also be used with non-optional values.
>>> For example, I'd like to write code like the following
>>> 
>>> guard 
>>>  let tableView = self.tableView,
>>>  let col = tableView.columnWithIdentifier("MyColumn") where col != -1
>>> else {
>>>  NSBeep()
>>>  print("an error occurred")
>>>  return
>>> }
>>> 
>>> This is not possible, because the second let assignment is non-optional,
>>> so I have to write it like this:
>>> 
>>> guard let tableView = self.tableView else {
>>>  NSBeep()
>>>  print("an error occurred")
>>>  return
>>> }
>>> let col = tableView.columnWithIdentifier("MyColumn")
>>> guard col != -1 else {
>>>  NSBeep()
>>>  print("an error occurred")
>>>  return
>>> }
>>> 
>>> This leads to a lot of duplicated error handling code.
>>> 
>>> Ideally, I'd also like to mix let & where clauses in a single guard
>>> statement, like this:
>>> 
>>> guard 
>>>  let tableView = self.tableView,
>>>  let col = tableView.columnWithIdentifier("MyColumn") where col !=
>>>  -1,
>>>  let headerCell = tableView.tableColumns[col].headerCell as?
>>>  MyTableHeaderCell
>>> else {
>>>  NSBeep()
>>>  print("an error occurred")
>>>  return
>>> }
>>> 
>>> What do you think? Right now I always end up writing a lot of separate
>>> guard statement, and I have a lot of repeated error handling code.
>>> 
>>> 
>>> Best regards,
>>> Jakob
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> 
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