[swift-evolution] Proposal: Remove the "fallthrough" keyword

Sun Dec 6 03:52:09 CST 2015

+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> 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> wrote:
>> 
>>>> On Dec 5, 2015, at 1:31 PM, John McCall via swift-evolution <swift-evolution at swift.org> wrote:
>>>> On Dec 4, 2015, at 11:37 PM, John Calsbeek <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> wrote:
>>>>>> 
>>>>>> On Dec 4, 2015, at 2:33 PM, Kevin Ballard <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> 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, 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   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 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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