[swift-evolution] [Proposal idea] Improved interop for ErrorType->NSError

[Brent Royal-Gordon]

> Whatever the solution is, it shouldn’t be invisible to developers. We need a good way to get non-NSError ErrorTypes to turn into NSErrors that have decently propagated userInfo dictionaries, so that they can be presented to the user in a form that isn’t just a cryptic “<domain> error <code>”.

No, my point is that the developer should not have to do anything special to get this bridging, or explicitly create an _NSSwiftError—it should happen automatically. Swift should take care of preserving your associated values through casts to NSError and back, and fetching your userInfo as needed; all you should need to do is tell it how to express your error's userInfo.

Actually, now that I try it, it looks like Swift *does* do this much...

  1> import Foundation
  2> enum MyError: ErrorType { case FileNotFound (url: NSURL) }
  3> let url = NSURL(string: "missing.txt")!
url: NSURL = "missing.txt"
  4> MyError.FileNotFound(url: url)
$R0: MyError = FileNotFound {
  FileNotFound = {
    url = "missing.txt"
  }
}
  5> MyError.FileNotFound(url: url) as! NSError
$R1: NSError = domain: "MyError" - code: 0 {
  ObjectiveC.NSObject = {
    NSObject = {
      isa = _SwiftNativeNSError
    }
    _reserved =
    _code = 0
    _domain = "MyError"
    _userInfo = nil
  }
}
  6> (MyError.FileNotFound(url: url) as! NSError) as! ErrorType
$R2: MyError = FileNotFound {
  FileNotFound = {
    url = "missing.txt"
  }
}
  7> (MyError.FileNotFound(url: url) as! NSError) as! ErrorType as! MyError
$R3: MyError = FileNotFound {
  FileNotFound = {
    url = "missing.txt"
  }
}

Funny, I'm sure that didn't work last time I tried it...

Anyway, my point remains: this _SwiftNativeNSError should use a userInfo property on your ErrorType to populate NSError.userInfo. There should be no need to go through the full rigamarole of calling NSError's initializer yourself—just return a dictionary at the appropriate moment.

-- 
Brent Royal-Gordon
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