[swift-evolution] Obsoleting `if let`

[Chris Lattner]

> On Feb 3, 2016, at 3:47 PM, Jordan Rose via swift-evolution <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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