"I wonder if we’re pushing down the road of convenience at the expense of
truth. The if/guard let syntax is clear that you’re getting a separate
reference or copy, but what you’re suggesting is hiding the reality from the
user for what I see as relatively little convenience."
It just might be me trying to avoid using !, and especially avoid implicit
unwrapped options. While there is nothing wrong with the following code it
makes me very uncomfortable from a defensive programming point of view:
today = today ?? NSDate()
let timeInterval = today!.timeIntervalSinceNow
Some developer coming along and changing the code could easily introduce a
crash, such as by removing the default value. In the above example, such a
change wouldn’t introduce a compiler warning/error and the bug might not reveal
itself until a much later.
Also using if-let or guard also doesn’t seem right, in this case, as it should
never fail:
today = today ?? NSDate() // self.today changed!
if let today = today {
let timeInterval: NSTimeInterval = today!.timeIntervalSinceNow
} else {
assertFailure()
}
Same issue with guard:
today = today ?? NSDate() // self.today changed!
guard let today = today else {
assertFailure()
return // that should never happen
}
let timeInterval: NSTimeInterval = today!.timeIntervalSinceNow
This introduces code that just gets in the way of the code’s meaning for cases
that should never happen. Yuck, there has to be a better way!
- Tod
From:
<swift-evolution-boun...@swift.org<mailto:swift-evolution-boun...@swift.org>>
on behalf of Rod Brown via swift-evolution
<swift-evolution@swift.org<mailto:swift-evolution@swift.org>>
Reply-To: Rod Brown <rodney.bro...@icloud.com<mailto:rodney.bro...@icloud.com>>
Date: Sunday, May 1, 2016 at 1:25 AM
To: David Sweeris <daveswee...@mac.com<mailto:daveswee...@mac.com>>
Cc: Erica Sadun via swift-evolution
<swift-evolution@swift.org<mailto:swift-evolution@swift.org>>
Subject: Re: [swift-evolution] Auto Unwrapping Of Optionals
On 1 May 2016, at 3:00 PM, David Sweeris
<daveswee...@mac.com<mailto:daveswee...@mac.com>> wrote:
On Apr 30, 2016, at 5:42 PM, Rod Brown
<rodney.bro...@icloud.com<mailto:rodney.bro...@icloud.com>> wrote:
Re-sent for Swift Evolution. Response at end.
On 1 May 2016, at 6:31 AM, David Sweeris
<daveswee...@mac.com<mailto:daveswee...@mac.com>> wrote:
I think your idea makes a lot more sense in respect to ensuring we don't have
as much magic.
That said, I still wonder about the implications for thread safety etc. While
it isn't a focus of Swift 3, it's something to think about whether this
promotes a paradigm that cannot be supported in a threaded environment,
specifically accessing properties.
The if-let paradigm is a lot stronger for this set of actions. It gains a
separate reference or copy to the internal value, and allows you to action it
safely. Should the property change in the meantime, it isn't relevant, because
you have you own reference/copy, and then you have the right to re-set the
property as required.
This, however, would theoretically add in an invisible ! for you. This leaves
you unable to handle the situation should the variable have been changed by
another thread between your check and your subsequent action.
Unless I'm missing something, I worry about the behaviour of such a "feature"
in a multithreaded environment. I think the previous "inout" idea actually held
a lot more weight in this regard - at least then you can act on the copy, and
have the change propagate to the main declaration, and overwrite any changes
made on another thread.
I think it would have the same resiliency as if-let, since I was envisioning
this to just be syntactic sugar for a switch statement. That is, this:
if foo is .Result { //`foo` refers to foo's associated or raw value within the
following code block
//code block
}
would get rewritten to this, for enums with associated values:
switchfoo {
case .Result(let foo): //we get a local copy of `foo` (the associated value)
for the following code block
//code block
default: break
}
or this, for enums with raw values:
switchfoo {
case .Result:
let _foo = foo.rawValue //the compiler substitutes `_foo` for `foo`
//code block
default: break
}
There’d have to be some more auto-generated code to copy assigned values back
into the original `foo`, but I don’t think it’d be hard to do.
- Dave Sweeris
Ah yes, that makes sense. So how do you see the compiler dealing with the
assignment/access problem on structs? If you assign to foo, the compiler
assigns to both “_foo” and “foo”?
I wonder if we’re pushing down the road of convenience at the expense of truth.
The if/guard let syntax is clear that you’re getting a separate reference or
copy, but what you’re suggesting is hiding the reality from the user for what I
see as relatively little convenience.
This is not to say I don’t see the problem, or the convenience… I just wonder
if this might be going a little too far.
- Rod
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