[swift-users] [swift-evolution] What about a VBA style with Statement?

[Sean Heber]

> On Apr 13, 2016, at 10:08 AM, Erica Sadun <erica at ericasadun.com> wrote:
> 
> 
>> On Apr 13, 2016, at 8:25 AM, Sean Heber via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> This pair works pretty well, too, if you don’t mind free functions:
>> 
>> func with<T>(inout this: T, @noescape using: inout T->Void) { using(&this) }
>> func with<T>(this: T, @noescape using: T->Void) { using(this) }
>> 
>> It works either with classes or mutable structs if you call it correctly and the type doesn’t matter.
>> 
>> l8r
>> Sean
> 
> My current version is this:
> 
> func with<T>(item: T, @noescape update: (inout T) -> Void) -> T {
>     var this = item; update(&this); return this
> }
> 
> Used, for example:
> 
> struct Foo { var (a, b, c) = ("a", "b", "c") }
> class Bar: DefaultReflectable { var (a, b, c) = ("a", "b", 2.5) }
> var f = with(Foo()) { $0.a = "X" }
> var b = with(Bar()) { $0.a = "X" }
> print(f) // Foo(a: "X", b: "b", c: "c")
> print(b) // Bar(a=X, b=b, c=c)
> 
> Is there an advantage to having a pair of functions?

I don’t know if it’s a huge advantage or not, but with warnings on unused results, using a with() that has a return with a class instance would mean you’d have to discard the return result explicitly or pointlessly reassign the results to your instance (thus meaning not using a let) just to avoid the warning. If you annotated the with() to allow discarding the result, then it’d be error-prone for structs. It seemed “safer” to me to have a pair.

l8r
Sean