[swift-evolution] A use for semi-colons: alternating between if and if let

[Ross O'Brien]

I don't know if this will help clarify things, so take or leave as you like:

I wasn't aware before this discussion that the first of the comma-separated
expressions of an if let could be just an 'if'.

Now I have learned it, it just seems to be an odd construction: the first
clause of an if can be a boolean expression, a let binding, a var binding
or a case let/var binding, and then after the first comma every boolean
expression must be preceded by 'where'. There's no obligation for the
boolean expression to relate to the constant or variable just bound.

My original premise was simply to allow boolean expressions and binding
expressions to alternate within the if; boolean operators like & and |
would separate boolean expressions, commas would separate binding
expressions, and semi-colons would allow alternating from one to the other.
I hadn't anticipated that this would deprecate any existing code; a let
followed by a var in the if would still be comma-separated binding
expressions - but I'm not objecting to deprecating something if it makes
the code clearer.

So (and I apologise for the trivialised example) my proposed change would
be that this:

if a == 5, let b = c where array.count > 5, let d = c, var e = array[0], f
= array[1] where e == f, case .Some(let g) = c where g.characters.count > 2
{ }

would become

if a == 5; let b = c; array.count > 5; let d = c, var e = array[0], f =
array[1] where e == f; case .Some(let g) = c where g.characters.count > 2 {
}

So, mostly the same, but 'where' can be used only where it clarifies to the
reader that it's qualifying a binding, rather than being used because it's
the only way to follow a binding expression with a boolean expression.


On Fri, Feb 5, 2016 at 12:04 AM, James Hillhouse IV via swift-evolution <
swift-evolution at swift.org> wrote:

> Here’s a question that Drew raised as he was educating me on this
> discussion. Given that the following works, are you proposing to deprecate,
>
> if let a = 2, var b = 3, let c = 4, var d = 5{ } // 1
>
> in favor of?
>
> if let a = 2; var b = 3; let c = 4; var d = 5{ } // 2
>
> If semi-colon is still being considered, can Joe or Jordon clarify?
>
> On Feb 2, 2016, at 7:40 PM, Joe Groff via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> On Feb 2, 2016, at 5:13 PM, Jordan Rose via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> On Feb 2, 2016, at 16:53, Ross O'Brien via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> For example: getting the first element from an array of optionals. Here's
> my sample trivial example:
> var array : [Int?] = []
> if let x = array[0] where array.count > 0
> {
> print(x)
> }
> This code doesn't work. The array index is out of range. But there's no
> way I know of to rearrange the 'if' to ensure the array isn't empty before
> binding to the first element - the developer has to write a nested if
> statement.
>
>
> We already allow this, with commas:
>
> var array: [Int?] = []
> if !array.isEmpty, let x = array[0] {
>   print(x)
> }
>
>
> That's true, but there are some problematic ambiguities with our current
> syntax, since 'let' and 'case' conditions both admit multiple
> sub-conditions also separated by commas, e.g. 'if let x = y, y = z { }' or
> 'if case let x = y, let y = z'. Using ';' for this purpose would be less
> ambiguous.
>
>
>
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>
>
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