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

[Russ Bishop]

> On Jan 22, 2017, at 4:53 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Sent from my iPad
> 
> On Jan 22, 2017, at 3:15 PM, Chris Lattner <clattner at nondot.org <mailto:clattner at nondot.org>> wrote:
> 
>> 
>>> On Jan 20, 2017, at 12:22 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> 
>>>> Yeah, maybe there's a more general language feature that could replace 'fallthrough' here. Instead
>>>> of labelling cases, we could support a 'reswitch' statement that redispatches the switch to the case
>>>> matching the operand:
>>>> 
>>>>    switch some_value {
>>>>    case .REFINED:
>>>>        if !validate(some_value) { return NULL }
>>>>        reswitch .BASE
>>>> 
>>>>    case .BASE:
>>>>        handle_enum_value();
>>>>    }
>>> 
>>> We should just call a spade a spade and spell that "goto" ;-)
>> 
>> Goto has traditionally been used for unstructured control flow, but this form is still fully structured.  You cannot produce irreducible loops, jump into scopes, over declarations, etc.
> 
> It would be useful to know the technical definition of "structured." The fact is that there's a large category of sensible goto-like jumps that we probably want to allow and I see no advantage to giving them all distinct names.  I don't believe users will distinguish them.  We already have "break looplabel" and, aside from the label being in the wrong place, it's effectively a goto.  It seems to me we should just have goto and either prohibit the troublesome uses or give them reasonable semantics.
> 
> I could live with "break" as well as "goto", but it feels a bit like it's pretending to be something it isn't, and there's the source compatibility problem with the label position.
> 
> -Dave

I always assumed structured control flow means control only jumps to approved points, not arbitrary statements. Implicitly this is jumping out of a scope to a less-nested scope but never into a more-nested scope.

Swift takes this even further: you are limited to early exit of control flow. That’s a lot more restrictive than arbitrary goto.


Russ

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