[swift-corelibs-dev] NSTask and try!

Tony Parker anthony.parker at apple.com
Fri May 13 14:48:29 CDT 2016

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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