[swift-evolution] [Review] SE-0056: Allow trailing closures in `guard` conditions

[Jeremy Pereira]

> On 4 Apr 2016, at 18:18, Haravikk <swift-evolution at haravikk.me> wrote:
> 
>> 
>> On 4 Apr 2016, at 15:49, Jeremy Pereira <jeremy.j.pereira at googlemail.com> wrote:
>> 
>>> On 3 Apr 2016, at 17:20, Haravikk via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> Although I use trailing closures a lot less now, I think I’m a +1 anyway for consistency’s sake.
>>> 
>>> I actually really like the idea of having trailing keywords in loops and if statements, these needn’t be required (except where a trailing closure is used) but for example it means I could do a fully natural language loop like:
>>> 
>>> 	for eachValue in theValues do { … }
>> 
>> This is actually kind of bizarre. Here we are trying to invent new syntax so that the trailing closure can be used in if/while conditions and for sequences. However, there is already a perfectly good syntax for putting closures in these positions: put the closure in the parentheses of the function call. Are people really so desperate to use trailing closures everywhere that we have to add new keywords to the language? I don’t think they are.
> 
> While I kind of agree (and personally prefer the use of parenthesis in most places anyway) it’s an inconsistency to be unable to use them I think. While It’s understandable from a parsing/ambiguity perspective, it’s not really intuitive.

But the resolution would be another inconsistency i.e. a separate keyword that is only required if the condition has a trailing closure. Furthermore, that is an inconsistency that adds extra complexity to the language.

> 
>>> I like the consistency of every block having a kind of type (do, else, defer, catch etc.). 
>> 
>> That is a rabbit hole down which you probably shouldn't go. If we go down the route of blocks having a “type”, the current situation in Swift becomes somewhat inconsistent. I would argue that the `else` block on a `guard` is of a different type to the `else` block on an `if`. If anything, the `else` block of an `if` is closer to the `then` block. Also, would you allow the `do` block in a `for` or `while` to have a `catch` block following it? If not, then these blocks are different to the  existing bare `do` block. 
> 
> Actually that’s not quite what I meant by “type”; while there is a case to be made for unifying these more (else and catch on loops for example) I just meant more along the lines that “do” would always group the main branch, “else” indicates an alternative path if a condition isn’t met and so-on. For the short term however this would just be a case of allowing do on the end to eliminate ambiguity and thus allow trailing closures, but in the long term it could be explored further.

I don’t view the `else` on an `if` as being some kind of second class citizen, it’s simply one of two alternate execution paths, whereas the `else` on a `guard` really is a second class citizen - it is even restricted what you can put in the block i.e. it must cause the enclosing scope to be exited. 

The reason why I called it a rabbit hole is because regarding the keyword in front of a block as denoting the kind of block it is, leads us to having to change a lot in order to make it meaningful and consistent.