[swift-evolution] Obsoleting `if let`
Taras Zakharko
taras.zakharko at uzh.ch
Wed Feb 3 22:41:40 CST 2016
> On 04 Feb 2016, at 05:03, Jonathan Tang <jonathan.d.tang at gmail.com> wrote:
>
> FWIW, I don't see any problems with the current "if let x = ... " syntax or behavior. I view it as a mainstream language finally implementing anaphoric-if (as described in Paul Graham's _On Lisp_), and the unwrapping seems like a natural consequence of the semantics, because what else *could* it do? The if-statement needs to test on something; it seems natural to me that the if tests for optional == nil, and then the let binds the payload of the optional if it matches.
To be honest, I agree. But the bind threads reveals that many consider the current if let less transparent, so it makes sense to discuss alternatives.
One problem I can see with privileging optionals binding in conditional statements however, is the inability to bind non-optionals. That would be quite useful sometimes. E.g. in conditionals:
if let x = optional_value, y = some_fun_returning_non_optional() where x > y {}
forcing one to write something like this instead (or a nested if)
myif: if x = optional_value {
let y = some_fun_returning_non_optional()
guard x > y else {break myif}
...
}
Same goes for guard, it would be nice to do something like
guard let
x = optional1,
y = x.optional2,
z = x.non_optional,
w = z.optional3
else {
// failed to initialise
}
If I remember correctly, I encountered this problem when playing with Metal, where I wanted to initialise a bunch of relevant variables in one go, but had to break up my guards into a number of blocks. This breaks the program logic.
— Taras
>
> I wouldn't rule out there being something better, but I'm -1 on all the proposals I've seen so far to change it.
>
> On Wed, Feb 3, 2016 at 7:49 PM, Taras Zakharko via swift-evolution <swift-evolution at swift.org <mailto: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
>>>>
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