[swift-evolution] [Idea] Use optionals for non-optional parameters

[Xiaodi Wu]

Top-replying because Google is forcing me to:

If you want to print an error for all early exits, declare a variable to
keep track of exit status such as `var isEarlyExit = false`, then use a
defer block that prints `error` only after checking `isEarlyExit` (or, you
know, design your code so that `error` itself would be nil if you're
exiting without an error).

It is not "really bad" if your code "fails" unless the lines of code are
executed in the explicitly written order. There are no tricks hidden in
that behavior: lines of code are *supposed* to be executed from top to
bottom in the absence of a control flow statement, because Swift is a
procedural programming language. Proceeding from one line to the next is
the absolute most primitive flow of control.

`guard` and `defer` were introduced in a later version of Swift to solve a
practical problem encountered in daily use, the nested pyramid of doom from
too many `if let` blocks. The point is that `guard` and `defer` together
constitute an ingenious and *complete* solution to that problem; you have
not shown me any scenario that cannot be trivially refactored to avoid
nested blocks using these two language constructs. So more sugar is not
necessary to solve this problem.

"This is not explicit enough" *is* an argument against almost any sugar you
can propose. I think you are seeing why the core team is actively
discouraging sugar proposals on this list. Unless something comes along
that totally blows the alternative out of the water, I'm inclined to agree
that more sugar is almost a non-goal for Swift.

(What would be something that could change my mind? Here would be my
criteria:

* The non-sugared version is extremely painful to write (>>5 LOC, maybe
>>20), difficult to write correctly, and even if correctly written, does
not express the intended solution clearly to the reader.

* There is a single, overwhelmingly obvious, universally or nearly
universally appropriate solution, and the proposed sugar would always be a
shorthand for that one solution.

Something like a copy-on-write attribute would fit the bill, because good
luck implementing that by hand over and over again, and if you're a reader
of code, good luck verifying that all that code does what you think.)

On Mon, Aug 15, 2016 at 23:56 Justin Jia <justin.jia.developer at gmail.com>
wrote:

> On Aug 16, 2016, at 1:51 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> On Mon, Aug 15, 2016 at 12:31 PM, Justin Jia <
> justin.jia.developer at gmail.com> wrote:
>
>>
>> Since you mentioned do and defer:
>>
>> ```
>> func foo(wantsToBreak: Bool) {
>>     out: do {
>>         defer { print("Hello, world!") }
>>         guard wantsToBreak else { break out }
>>     }
>>     print("End of function.")
>> }
>>
>> foo(wantsToBreak: true) // Output: Hello, world!\nEnd of function.
>> foo(wantsToBreak: false) // Output: Hello, world!\nEnd of function.
>> ```
>>
>> Do you think this is confusing?
>>
>
> No, I don't. But I also don't see why you would put `defer` inside `do`
> like that. `defer` and `guard` can be used profitably without nesting
> inside blocks.
>
>
>
> Because I don’t want `defer` to execute outside do block. Let me give you
> a simplified example: I wanted to print error for all early exits except
> normal return (reaches last line). I would like to use defer otherwise I
> need to write `else { print(error); return }` for all guards. The intuitive
> way of achieving this for me was to nest defer inside do blocks. But it
> turned out that defer will be executed even if you choose to break a block.
> I’m not arguing this is a bad design decision. My point is: sometimes
> non-intuitive design decisions are non-avoidable.
>
>
> At least it confused me in the fast. However, defer is still very useful.
>>
>> Even if I choose to use guard, defer and do, it will still look like the
>> one with `if let`. Lots of blocks. The code should be straightforward
>> without any brackets.
>>
>
> Huh? I don't buy this argument at all. You don't like the look of `{ }`,
> so you are proposing new sugar using `?`--is that what you're claiming?
> This sounds to me like the same motivation as that behind early suggestions
> to move to a Python-like syntax.
>
>
>> See this example (since it’s a lot of code I rendered a PDF).
>>
>
> I don't see the motivation in this example. Why wouldn't you just move the
> code to update `cell.heading` right after you guard that `imageName` is not
> nil?
>
>
>
> I already explained why. It was just a naive example. In real life methods
> can be a lot more complicated than my example. It’s really bad if your code
> will fail unless it follows the same exact order. We need to modify our
> code everyday, and most of the time we are working on code that is not even
> written by ourselves. If you scan through methods with name like
> updateCell, intuitively, you will think the order of the code will not
> matter. And it shouldn’t! It is really easy to make mistakes with guard
> statement because the order matters here. IMO, guard is only useful if we
> place it at the beginning of the function—for all or nothing.
>
> Why we chose to use brackets and indentation? Because they can warn us
> that the behavior of the code will change. Either the outcome will vary
> (if) or the code will be executed for more than one time (for). Checking an
> object if is nil doesn’t always belong here. Using `if let` is not being
> explicit. It’s boilerplate. A not-so-good fix for the side effect of
> optionals. Most of the time, we want the flow to be “flat”. That’s why
> swift supports `guard` and `object?.method`. If you think `foo(x?)` is not
> important, do you think `guard` and `object?.method` are also not important?
>
> I understand that Swift is designed to be explicit. I also agree with it.
> But I saw an unhappy trend in the mailing list: "this is not explicit
> enough" can be used to argue against anything. Shall we remove
> @autoclosure? Shall we remove trailing closures? Shall we remove
> `object?.method`?
>
> On Aug 16, 2016, at 1:16 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>> On Mon, Aug 15, 2016 at 12:07 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>>> On Mon, Aug 15, 2016 at 11:43 AM, Justin Jia <
>>> justin.jia.developer at gmail.com> wrote:
>>>
>>>> I believe the core team has considered 99% of the ideas in the mailing
>>>> list in the past, but it doesn’t mean we can’t discuss it, right?
>>>>
>>>
>>> No, it certainly doesn't! I'm saying that you haven't come up with a
>>> solution to a known problem with the idea.
>>>
>>>
>>>>
>>>> Assuming we have the following declaration:
>>>>
>>>> ```
>>>> func foo(a: Int, b: Int?, c: Int, d: Int?) -> Int
>>>> ```
>>>>
>>>> For this:
>>>>
>>>> ```
>>>> let z = foo(a: f1(), b: f2()?, c: f3(), d: f4()?) // z becomes optional
>>>> ```
>>>>
>>>> We have a few different “possible solutions”:
>>>>
>>>> 1. Short-circuiting from left to right. This is equivalent to:
>>>>
>>>> ```
>>>> var z: Int? = nil
>>>> let a = f1()
>>>> guard let b = f2() else { return }
>>>> let c = f3()
>>>> guard let d = f4() else { return }
>>>> z = foo(a: a, b: b, c: c, d: d)
>>>> ```
>>>>
>>>> 2. Short-circuiting from left to right for optionals. Then evaluate
>>>> non-optional parameters. This is equivalent to:
>>>>
>>>> ```
>>>> var z: Int? = nil
>>>> guard let b = f2() else { return }
>>>> guard let d = f4() else { return }
>>>> let a = f1()
>>>> let c = f3()
>>>> z = foo(a: a, b: b, c: c, d: d)
>>>> ```
>>>>
>>>> 3. Do not short-circuiting.
>>>>
>>>> ```
>>>> var z: Int? = nil
>>>> let a = f1()
>>>> let optionalB = f2()
>>>> let c = f3()
>>>> let optionalD = f4()
>>>> guard let b = optionalB else { return }
>>>> guard let d = optionalD else { return }
>>>> z = foo(a: a, b: b, c: c, d: d)
>>>> ```
>>>>
>>>> Like I said before, I agree that there is no intuitive solution to this
>>>> problem. However, I'm still not convinced that this feature is *not
>>>> important*.
>>>>
>>>> Thank you for pointing out the problem to me. I didn't notice it at the
>>>> time I wrote my first email. I really appreciate that. However, instead of
>>>> saying I don't know which is the best solution so let's assume the core
>>>> team made the right decision, we should discuss whether 1, 2, 3 is the best
>>>> solution. Or you can convince me we don't *need* this feature.
>>>>
>>>
>>> I'm going to convince you that 1, 2, and 3 are all bad solutions. Thus,
>>> this feature won't fly.
>>> The fundamental issue is that having this sugar means that I can no
>>> longer reason about the order in which code is executed. An innocuous
>>> statement such as `print(a(), b(), c(), d())`, once you mix in your
>>> proposed `?` syntax with some but not all of these function calls, might
>>> have d() executed before a(), after a(), or not at all. This is greatly
>>> damaging to the goal of writing clear, understandable code.
>>>
>>>
>>>>
>>>> Back to the original topic.
>>>>
>>>> I spent some time thinking and changed my mind again. I think solution
>>>> 1 is most reasonable. It is consistent with if statements. Instead of
>>>> treating it as sugar for `if let`, we can treat it as sugar for `guard`,
>>>> which is much easy to understand and remember.
>>>>
>>>> -
>>>>
>>>> Below is the reason why I think this feature is important (quoted from
>>>> another email).
>>>>
>>>> The problem with `if let` is you need to call the function inside { }.
>>>>
>>>> ```
>>>> /* code 1 */
>>>> if let x = x, let y = y {
>>>>     /* code 2, depends on x and y to be non-optional */
>>>>     let z = foo(x, y)
>>>>     if let z = z {
>>>>         bar(z)
>>>>     }
>>>>     /* code 3, depends on x and y to be non-optional */
>>>> }
>>>> /* code 4 */
>>>> ```
>>>>
>>>> I can't use `guard` for this situation because guard will force me to
>>>> leave the entire function.
>>>>
>>>
>>>> ```
>>>> /* code 1 */
>>>> guard let x = x, y = y else { return }
>>>> /* code 2, depends on x and y to be non-optional */
>>>> guard let z = foo(x, y) else { return }
>>>> bar(z)
>>>> /* code 3, depends on x and y to be non-optional */ <- This won't
>>>> execute if z is nil
>>>> /* code 4 */ <- This won't execute if x, y or z is nil
>>>> ```
>>>>
>>>
>>> Then surround it with a do block.
>>>
>>> ```
>>> out: do {
>>>   guard foo else { break out }
>>>   guard bar else { break out }
>>>   /* other code */
>>> }
>>> ```
>>>
>>
>> Or, more idiomatically, since your use case is that you want /* code 4 */
>> to be executed no matter what, while everything else depends on x and y not
>> being nil:
>>
>> ```
>> defer { /* code 4 */ }
>> guard let x = x, let y = y else { return }
>> /* code 2 */
>> /* code 3 */
>> ```
>>
>>
>>>> What I really want is some like this:
>>>>
>>>> ```
>>>> / * code 1 */
>>>> let z = foo(x?, y?)
>>>> /* code 2, depends on x and y to be non-optional, use x? and y? */
>>>> bar(z?)
>>>> /* code 3, depends on x and y to be non-optional, use x? and y? */
>>>> /* code 4 */
>>>> ```
>>>> This is much easier to read. Sometimes people choose to use `guard` to
>>>> avoid `{ }`, which usually lead to code could easily get wrong (like the
>>>> second example).
>>>>
>>>> Sincerely,
>>>> Justin
>>>>
>>>>
>>>>
>>>>
>>>> On Aug 15, 2016, at 11:41 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>>
>>>> What do you mean, limited to variables? What about a computed property?
>>>> You will have the same problem.
>>>>
>>>> I'm not sure where you want to go with this, given that the core team
>>>> has considered the same idea in the past and found these issues to have no
>>>> good solution.
>>>>
>>>> On Mon, Aug 15, 2016 at 04:56 Justin Jia <
>>>> justin.jia.developer at gmail.com> wrote:
>>>>
>>>>> IMO I don't this bar should be evaluated unless we decide if let can
>>>>> accept non-optional values.
>>>>>
>>>>> Actually, what if we allow if let to accept non-optional values?
>>>>>
>>>>> I agree this is confusing at the beginning. But people who are not
>>>>> familiar with the detail design can avoid this situation easily. People who
>>>>> are familiar with the design can adopt it quickly. Sometimes, this is
>>>>> unavoidable.
>>>>>
>>>>> Btw, do you think this is still something nice to have if we limit
>>>>> this syntax to only variables?
>>>>>
>>>>> On Aug 15, 2016, at 4:59 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>>>
>>>>> On Mon, Aug 15, 2016 at 3:55 AM, Xiaodi Wu <xiaodi.wu at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> On Mon, Aug 15, 2016 at 3:25 AM, Justin Jia via swift-evolution <
>>>>>> swift-evolution at swift.org> wrote:
>>>>>>
>>>>>>>
>>>>>>> On Aug 15, 2016, at 4:09 PM, Charlie Monroe <
>>>>>>> charlie at charliemonroe.net> wrote:
>>>>>>>
>>>>>>> The example above was to better demonstrate the problem with *when*
>>>>>>> to evaluate the latter argument. Why should both arguments be evaluated
>>>>>>> *before* the if statement? If both calls return Optionals,
>>>>>>>
>>>>>>> if let x = bar(42), y = baz(42) { ... }
>>>>>>>
>>>>>>> is how would I write it without the suggested syntax - baz(42) will
>>>>>>> *not* be evaluated if bar(42) returns nil. Which bears a question why would
>>>>>>>
>>>>>>> foo(bar(42)?, baz(42)?)
>>>>>>>
>>>>>>> evaluate both arguments even if the first one is nil, making it
>>>>>>> incosistent with the rest of the language?
>>>>>>>
>>>>>>>
>>>>>>> I see your point. I understand that maybe 1/2 of the people think we
>>>>>>> should evaluate both arguments and 1/2 of the people think we should only
>>>>>>> evaluate the first argument.
>>>>>>>
>>>>>>> I changed my idea a little bit. Now I think you are right. We should
>>>>>>> only evaluate the first argument in your example. It’s not only because of
>>>>>>> inconsistent, but also because the language should at least provide a way
>>>>>>> to “short-circuit” to rest of the arguments.
>>>>>>>
>>>>>>> If they want to opt-out this behavior, they can always write:
>>>>>>>
>>>>>>> ```
>>>>>>> let x = bar(42)
>>>>>>> let y = baz(42)
>>>>>>> foo(x?, y?)
>>>>>>> ```
>>>>>>>
>>>>>>
>>>>>> Well, that was just the easy part. Now, suppose bar is the function
>>>>>> that isn't optional.
>>>>>>
>>>>>> ```
>>>>>> foo(bar(42), baz(42)?)
>>>>>> ```
>>>>>>
>>>>>> Is bar evaluated if baz returns nil? If you want this syntax to be
>>>>>> sugar for if let, then the answer is yes.
>>>>>>
>>>>>
>>>>> s/yes/no/
>>>>>
>>>>>
>>>>>> If short-circuiting works left-to-right, then the answer is no.
>>>>>>
>>>>>
>>>>> s/no/yes/
>>>>>
>>>>> (See? Confusing.)
>>>>>
>>>>>
>>>>>> This is very confusing, and there is no good intuitive answer.
>>>>>>
>>>>>>
>>>>>>> _______________________________________________
>>>>>>> swift-evolution mailing list
>>>>>>> swift-evolution at swift.org
>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>
>>>>>>
>>
>>
>
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