[swift-evolution] Proposal: Remove the "fallthrough" keyword
Jacopo Andrea Giola
swift-evolution at jacopo.giola.org
Sun Dec 6 14:06:37 CST 2015
Hi Colin,
the initial proposal was indeed to remove entirely the `fallthrough` keyword but many people expressed your similar concern and from that point the discussion was steered through an "enhancement" and better refinement of the keyword.
The new idea is to substitute the old keyword with "reswitch" passing the desired new value on which the switch is applied.
So something like this:
switch (enum) {
case .One:
// do something
reswitch .Two
case .Two:
// do something else
default:
// and so one
}
This new behaviour, IMO, is better suited for Swift because is more declarative of the developer intent and doesn't carry over unintentional misbehaviour.
Is more declarative because you are forced to state in which case you want to go, and even if the order of the switch’ cases will change in the future, you don't fall in the wrong case by mistake.
switch (enum) {
case .One:
// do something
reswitch .Two
case .OneAndAHalf
// maybe this change is not made by you but by a messed up merge
case .Two:
// do something else
default:
// and so one
}
In this case if you are using the fallthrough keyboard your code is now broken by accident, and depending on what are you trying to do inside the cases you can have a hidden bug that your tests are not seeing right away.
Another advantage is that in this way you can made more cases fallthrough in the same one even if they are not one over each other
switch (enum) {
case .One:
// do something
reswitch .Two
case .OneAndAHalf
// so something that you don’t want to do for .One
reswitch .Two
case .Two:
// do something else that you may want to do for .One and .Two
default:
// and so one
}
I must say that this is a side effect that can be used to messed up the code flow in a way that is not intended, but is a new behaviour that gives more power to the switch statement.
The reswitch keyword in addition is not a mere fallthrough on the new case without doing the optional checking attached to it, but is intended to be a new call and all the check are executed.
switch (enum) {
case .One:
// do something
x = 0;
reswitch .Two
case .OneAndAHalf
// so something that you don’t want to do for .One
reswitch .Two
case .Two where x > 0:
// do something else that you may want to do for .One and .Two
element = array[x]
default:
// and so one
}
(I’m going by memory and by writing this snippets in the mail app directly, so the code must be incorrect in the syntax and for this I’m sorry).
In this case if enum is .One the only case that is executed is case .One and the code doesn’t fallthrough in the .Two case because we are made the where invalid by changing the x to a value less than 1.
Now I don’t remember who was the first one who mede this proposal, and I don’t know if he is working on a first draft to lay down the things better, but for me this can be a nice improvement and a neat break with the C-switch behaviour that Swift has trying to change from the very first beta disallowing the implicit fallthrough.
I can be completely wrong but I see the `fallthrough`keyword as a “temporary” implementation for ease the transition from Obj-C to Swift and is time to improve it and made the switch statement even more powerful.
- Jacopo
Sent from my iPad
On 06 Dec 2015, at 19:57, Colin Barrett via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> tl;dr The fallthrough keyword, as far as I am aware, isn’t costing us anything; and has at least minimal utility, as I try to demonstrate.
>
> Apologies for jumping into this thread at an awkward point, but I’ve only just now subscribed to this list.
>
> I think the fallthrough keyword is useful in certain circumstances. I’ve also yet to see an example of where it creates a negative impact, either in code, optimization, or what have you. Other than “It’s like something in C, and C is old and busted” I’m unsure of the rationale for removing it. (Feel free to point me in the right direction.)
>
> Consider the Planet enum from the documentation. One of the simplest way to define the number of a planet (i.e. its 1-based index in the ordering of planets wrt. distance from the sun) is using a switch and fall-through:
>
> https://gist.github.com/cbarrett/23b24a9fe76efdf006df <https://gist.github.com/cbarrett/23b24a9fe76efdf006df>
>
> This technique is very extensible — for instance imagine computing the force induced by the gravity of the other planets on a particular planet. All that would need to change is the case statements.
>
> Yes, you could write this by putting the planets into a list and mapping or folding (or looping) over that, but unless the compiler can “unroll” that construct, you’re paying for an allocation simply bc of your choice of control flow. But in fact, you could imagine generalizing this construct into the implementation of fold for the Planet type — low-overhead folds for monomorphic types seems like a pretty compelling an natural use case for fallthrough to me.
>
> Thanks,
> -Colin
>
>> On Dec 6, 2015, at 4:52 AM, Jacopo Andrea Giola via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>> +1 for this idea, but I will prefer the reswitch keyword instead of overloading continue with a new syntax.
>>
>> If this proposal is accepted, it must be coupled with a compiler check that the reswitch statements don't introduce an infinite "switch" loop.
>>
>> Sent from my iPad
>>
>> On 06 Dec 2015, at 00:23, Steve Canon via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>>> Very much thinking out loud and not really the implications, I wonder if we might just use "continue" instead of "reswitch".
>>>
>>> I very much like specifying what case to fall through into, no matter how we spell it.
>>>
>>> - Steve
>>>
>>> On Dec 5, 2015, at 4:45 PM, John McCall via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>
>>>>> On Dec 5, 2015, at 1:31 PM, John McCall via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>> On Dec 4, 2015, at 11:37 PM, John Calsbeek <john.calsbeek+lists at gmail.com <mailto:john.calsbeek+lists at gmail.com>> wrote:
>>>>>> `fallthrough` is conceptually similar to `goto` in that both allow natural expression of concepts that exist at the instruction level but are otherwise difficult to express with nested control structures. `fallthrough` is perhaps slightly less objectionable because control flow remains local, but it has a similar role.
>>>>>>
>>>>>> It is not particularly natural to write `switch` statements with `fallthrough` in the reverse order that can be seen in Duff’s Device and similar constructs (case 7 falls through to 6 which falls through to 5, etc.). It’s just because you know for certain that all the code in case 6 would be duplicated in case 7, so 7 can transfer into 6 without a jump instruction. Communicating that to the compiler without `fallthrough` requires deeply nested `if`s.
>>>>>
>>>>> Right. One idea that I’ve always had for “fallthrough” is that we might parameterize it in the future; parameterized it would mean “repeat the switch with this new value”, so that unparameterized fallthrough would mean “repeat the switch with a notional value that ends up in the next case”. There’s a very common pattern in switches of deferring to another case that I’ve always found very awkward to write in C, and while sometimes there’s no choice but to extract a helper function, there’s a still-fairly-structural code pattern here that I think we can sensibly support.
>>>>>
>>>>> On the other hand, there’s an argument that this is an inappropriate extension for “fallthrough” specifically, which is one reason we’ve never pursued it.
>>>>
>>>> Oh, I see that Joe already brought this up, spelled “reswitch”.
>>>>
>>>> John.
>>>>
>>>>>
>>>>> John.
>>>>>
>>>>>>
>>>>>> One defense comes to mind: there is talk of Swift aiming at systems programming. Is writing a threaded interpreter loop within the potential scope of Swift? That’s a use case that could make use of both `fallthrough` and `goto` (computed goto, really).
>>>>>>
>>>>>> switch op {
>>>>>> case LOAD_INDIRECT:
>>>>>> in0 = memory[in1]
>>>>>> fallthrough
>>>>>> case LOAD:
>>>>>> out0 = memory[in0]
>>>>>> //...
>>>>>> }
>>>>>>
>>>>>> I am personally interested in the prospect of a language that can scale up to high-level concepts and down to “portable assembler,” but I don’t know if that is the right direction for Swift’s evolution.
>>>>>>
>>>>>> Cheers,
>>>>>> John
>>>>>>
>>>>>>> On Dec 4, 2015, at 2:42 PM, John McCall <rjmccall at apple.com <mailto:rjmccall at apple.com>> wrote:
>>>>>>>
>>>>>>>> On Dec 4, 2015, at 2:33 PM, Kevin Ballard <kevin at sb.org <mailto:kevin at sb.org>> wrote:
>>>>>>>> It's not actually Duff's Device. Duff's Device relies on the fact that C switch statements don't actually introduce a new scope, and so it overlaps a switch with a do-while loop. This lets it only test the number of bytes once, to jump into the middle of the loop, and then it switches over to a while loop that decrements a counter every 8 instructions. Basically, it's a trick for manual loop unrolling that deals with non-multiple-of-8 counts efficiently.
>>>>>>>
>>>>>>> To be pedantic, C switch statements do introduce a new scope. What Duff’s Device exploits is that switch is allowed to jump into (almost) arbitrary scopes, and cases can appear anywhere recursively inside a switch.
>>>>>>>
>>>>>>> But your point that Swift’s switch requires cases to be at the top level within a switch and thus prevents the use of Duff’s Device is 100% correct.
>>>>>>>
>>>>>>> John.
>>>>>>>
>>>>>>>>
>>>>>>>> Steve's code is also an example of manual loop unrolling that deals with non-multiple-of-8 counts, but it has calculate the number of bytes on every iteration instead of once. It's a good example of one of the uses of `fallthrough`, it's just not Duff's Device. It's impossible to use Duff's Device in Swift.
>>>>>>>>
>>>>>>>> -Kevin Ballard
>>>>>>>>
>>>>>>>> On Fri, Dec 4, 2015, at 02:16 PM, Greg Titus wrote:
>>>>>>>>> Streza’s source code is an example of Duff’s Device, which is a big place where switch fallthrough is arguably the cleanest way to do things and the reason why I’d personally prefer to keep it as part of the language.
>>>>>>>>>
>>>>>>>>>> On Dec 4, 2015, at 2:12 PM, Erica Sadun <erica at ericasadun.com <mailto:erica at ericasadun.com>> wrote:
>>>>>>>>>>
>>>>>>>>>> Oh let it die, let it die. Any time I use fallthrough I find myself re-factoring to stop using it.
>>>>>>>>>>
>>>>>>>>>> True fact: On all of gist.github.com <http://gist.github.com/>, there are only 22 gist results for "fallthrough language:swift".
>>>>>>>>>> Half of those are people just testing out the feature. Most of the remaining ones are just complex cases:
>>>>>>>>>> case .Enum1, .Enum2:
>>>>>>>>>> expressed as
>>>>>>>>>> case .Enum1: fallthrough
>>>>>>>>>> case .Enum2:
>>>>>>>>>>
>>>>>>>>>> And then there's streza: https://gist.github.com/stevestreza/2557dc5ec9e7c694d7ea <https://gist.github.com/stevestreza/2557dc5ec9e7c694d7ea> I'm pretty sure that ponies were harmed in the production of whatever that last bit is.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> On Dec 4, 2015, at 3:05 PM, jalkut at red-sweater.com <mailto:jalkut at red-sweater.com> wrote:
>>>>>>>>>>>
>>>>>>>>>>> In the spirit of some other proposals that remove C or C++ style artifacts, what do folks think about the possibility of removing the "fallthrough" keyword from the language?
>>>>>>>>>>>
>>>>>>>>>>> My understanding is this keyword is only used for the archaic seeming purpose of perpetuating C-style fallthrough from one switch statement to the subsequent one. The documentation hedges the use of this keyword in forbidding terms that make it clear its use is not encouraged. The presence of the keyword, while an improvement over C’s implicit fallthrough, is a mark of inelegance on an otherwise well-designed, opinionated implementation of swtich statements.
>>>>>>>>>>>
>>>>>>>>>>> The ugliness of fallthrough’s C-style behavior even demands a caveat in the documentation:
>>>>>>>>>>>
>>>>>>>>>>> "The fallthrough keyword does not check the case conditions for the switch case that it causes execution to fall into. The fallthrough keyword simply causes code execution to move directly to the statements inside the next case (or default case) block, as in C’s standard switch statement behavior."
>>>>>>>>>>>
>>>>>>>>>>> To my mind, the caveat explains just what is wrong with fallthrough, both in C or Swift: coded that is clearly labeled with deliberate conditions can nonetheless be reached.
>>>>>>>>>>>
>>>>>>>>>>> I quipped about this on Twitter, and the most common pushback I got seemed to be from people who either did not know about Swift’s support for comma-separated case statements, or harbored an aesthetic preference for clustering such cases together with fallthrough statements.
>>>>>>>>>>>
>>>>>>>>>>> In my opinion, unless somebody can think of a strong defense for supporting intentional fallthrough in Swift, removing the keyword would be a move in the direction of minimizing the language’s complexity while also discouraging poor coding style in switch statements.
>>>>>>>>>>>
>>>>>>>>>>> Thoughts?
>>>>>>>>>>>
>>>>>>>>>>> Daniel
>>>>>>>>>>>
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>>>>>>>>>>
>>>>>>>>>>
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