[swift-evolution] Obsoleting `if let`

[David Waite]

-1 , as it changes the meaning of ‘!’ from ‘fault program on error’ to mere flow control. 

I’m surprised nobody has beat me to suggesting the interrobang, though.

-DW

> On Feb 3, 2016, at 8:49 PM, Taras Zakharko via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I already suggested this in the bind thread but I guess it was either not interesting or people missed it, so here it goes again :)
> 
> What about changing the syntax of optional binding such that the optional unwrapping becomes explicit? I.e. instead of 
> 
>   if let x = some_optional { }
> 
> one writes
> 
>   if let x = some_optional! { }
> 
> Essentially, the point of this suggestion is that the runtime error generated by unwrapping an empty Optional is instead treated as condition failure in a conditional statement. While there is some typing overhead over the current syntax, I see a number of potential benefits of this approach:
> 
>  1. It is in line with the current semantics and syntax of optional unwrapping (does not introduce any new syntagm)
>  2. It makes the unwrapping operation explicit (thus addressing the basic criticism from the bind discussion)
>  3. It frees variable declaration of the contextual polisemy (i.e. let and var have the same semantics as nowhere else, there is no ‘unwrapping’ magic)
>  4. The change is minimal compare to what we have now and can be easily ported automatically
> 
> Potential issues:
> 
>  1. One character typing overhead — but I dot think that should matter. I always had the impression that Swift favours clarity over compactness (which is a good thing IMO)
>  2. it allows syntactic ambiguity with optional chaining. E.g. if let x = a.b?.c! { } and if let x = a.b!.c would have the same meaning. Then again, this ambiguity already exits in the language to begin with. 
> 
> — Taras
> 
>> On 04 Feb 2016, at 01:25, Chris Lattner via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> 
>>> On Feb 3, 2016, at 3:47 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> Data point (which Chris brought up already, I think?): We tried this* and got a lot of negative feedback. Optionals are unwrapped too often for people to be comfortable writing "if let name? = optionalCondition”.
>> 
>> Yes, I even implemented this and it was in the compiler for awhile, then later ripped it back out.  You can find the history in git.  I would guess that this all happened in ~March 2015.
>> 
>> -Chris
>> 
>>> 
>>> It may be more uniform and even more pedantically correct, but our users hated it.
>>> 
>>> Jordan
>>> 
>>> * The actual thing we tried only allowed patterns that began with 'let', but that's close enough.
>>> 
>>> 
>>>> On Feb 3, 2016, at 15:36, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> This is a continuation of and alternative proposal to "The bind thread", which seems to have petered out without consensus.
>>>> 
>>>> Currently there are three forms of `if` statement (and `guard` and `while`, but for simplicity I'll just say `if` throughout this discussion):
>>>> 
>>>> 	if booleanCondition
>>>> 	if let name = optionalCondition
>>>> 	if case pattern = expression
>>>> 
>>>> The boolean condition form is fine, but there are flaws in the other two. `if let` is unprincipled and doesn't really say what it does; `if case` is bulky and rarely used.* 
>>>> 
>>>> One very interesting thing about `if case`, too, is that it can actually do optional unwrapping:
>>>> 
>>>> 	if case let name? = optionalCondition
>>>> 
>>>> This avoids the problems with `if let`—it's principled (it comes from a larger language feature) and it explicitly says it's handling optionality—but it still runs up against `if case`'s rarity and wordiness.
>>>> 
>>>> So what I suggest is that we drop the `if let` form entirely and then drop the `case` keyword from `if case`. Pattern-matching conditions can still be distinguished from boolean conditions because boolean conditions can't contain an `=` operator. This, there would now only be two forms of if:
>>>> 
>>>> 	if booleanCondition
>>>> 	if pattern = expression
>>>> 
>>>> And the current `if let` is handled elegantly and clearly by existing pattern-matching shorthand, with only one additional character needed:
>>>> 
>>>> 	if let name? = optionalCondition
>>>> 
>>>> I see two complications with this.
>>>> 
>>>> The first is that, naively, `if let foo = bar` would still be valid, but would have different and vacuous behavior, since the pattern cannot fail to match. The compiler should probably emit an error or at least a warning when this happens.
>>>> 
>>>> The second is our other weird use of the `case` keyword, `for case`, which is now an orphan in the language. I see several ways this could be handled:
>>>> 
>>>> 1. Drop the `for case` functionality entirely; if you want that behavior, use a pattern-matching `if`.
>>>> 2. Replace the loop variable slot in the `for` statement with a pattern. This would force you to put `let` on all simple `for` statements.
>>>> 3. Try to automatically distinguish between simple variables/tuples and patterns in this slot. What could possibly go wrong?
>>>> 4. Require an equals sign before the `in`, like `for let foo? = in optionalFoos`. Looks a little gross, but it's unambiguous.
>>>> 5. Replace `for case` with `for if`, like `for if let foo? in optionalFoos`. This helps flag the unusual conditional behavior of this form of `for`.
>>>> 6. Just keep `for case` and don't worry about the fact that it's not parallel to the other statements anymore.
>>>> 
>>>> Thoughts on any of this?
>>>> 
>>>> 
>>>> 
>>>> * `if case` also has the problem that the `=` isn't appropriate unless you happen to bind some of the data matched by the pattern, but I don't know how to address that. A prior version of this proposal suggested saying `:=` instead of `=`, with the idea that `:=` could become a general pattern-matching operator, but the people I talked over this post with hated that.
>>>> 
>>>> -- 
>>>> Brent Royal-Gordon
>>>> Architechies
>>>> 
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>> 
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>> 
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160203/a0ae1f8e/attachment.html>