[swift-corelibs-dev] NSTask and try!

[Tony Parker]

> On May 13, 2016, at 1:05 PM, James Lee <james at jelee.co.uk> wrote:
> 
> Cheers for the clarification. I started assuming there may be a reason when changing the guard let on the launch args to use the InvalidArgumentException.
> 
> Could this be a position where we may need os checking to cover the regression for the moment. It seems odd that the test would pass in CI when an error is thrown with a try! but fail on OSX
> 

Task is certainly one of the cases where the underlying stuff that we’re abstracting is significantly different, so I’m not too surprised.

We should try to get something in place so we’re not failing on OS X in the short term for sure.

- Tony

> Sent from my iPhone
> 
>> On 13 May 2016, at 20:48, Tony Parker <anthony.parker at apple.com> wrote:
>> 
>> Hi James,
>> 
>>> On May 13, 2016, at 12:25 PM, James Lee via swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:
>>> 
>>> Following on from a previous discussion with Tests failing on OSX. I have been looking into the failures. It seems that one of the earliest failures is due to an error from a try! within NSTask.launch(). This came in with this commit: https://github.com/apple/swift-corelibs-foundation/commit/4c6f04cfcad3d4b06688558021595d06751fc66a
>>> 
>>> Going by the docs for Foundation - The launch function apparently "Raises an NSInvalidArgumentException if the launch path has not been set or is invalid or if it fails to create a process."
>>> 
>>> https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSTask_Class/#//apple_ref/occ/instm/NSTask/launch
>>> 
>>> My question is, should this be built into the Swift Foundation API? The documentation for Swift doesn't state that the launch function throws.
>>> 
>>> With the test that is failing expecting an error, it feels more Swift-y to have any errors throw explicitly, rather than looking at what the lower level fills the data with.
>>> 
>>> But before jumping into doing this, I would rather put it out there and see what the community feels about this?
>> 
>> Unfortunately the ‘throws’ syntax in Swift often causes a mixup between two different things, because it flipped the terminology from what all of our documentation and header comments use.
>> 
>> 1. Cocoa uses exceptions (@throw in ObjC) to indicate programmer errors and they are generally not intended to be recoverable.  Example: passing nil where not expected, passing an invalid argument, failing to meet a precondition of an API.
>> 2. Cocoa uses NSError ** to indicate runtime errors that are recoverable or at least presentable to user. Example: out of disk space, name of file already exists.
>> 
>> The ‘throws’ syntax in Swift is actually for case #2, not #1. In Swift, #1 is fatalError or preconditionFailure. #2 is ‘throw Error’.
>> 
>> In the case of NSTask, when the documentation says “raises an NSInvalidArgumentException” (#1) then in Swift, that should translate to fatalError or preconditionFailure.
>> 
>> Hope this helps,
>> - Tony
>> 
>>> Cheers
>>> 
>>> James
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>> 
>