[swift-evolution] Implicit Optionals for Tuples & Tuples as Function Parameter

[John McCall]

> On Nov 11, 2016, at 11:54 AM, Muhammad Tahir Vali via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Im curious to know the reasoning behind why implicit-wrapped optionals aren't allowed when creating tuples. Below is an example with type aliases and anonymous tuples.
> 
> ex. 
> typealias Name = (first: String!, middle: String?, last: String!)
> typealias Name = (first: String!, middle: String?, last: String!)!
> var name : (first: String!, middle: String?, last: String!)
> var name : (first: String!, middle: String?, last: String!)!
> 
> error from all 4 examples above:
> Implicitly unwrapped optionals are only allowed at top level and as function results

This is intentionally-designed behavior.  Preventing IUO at arbitrary locations was SE-0054:
  https://github.com/apple/swift-evolution/blob/master/proposals/0054-abolish-iuo.md <https://github.com/apple/swift-evolution/blob/master/proposals/0054-abolish-iuo.md>

> I also noticed that I can modify tuples using functions ONLY if I use inout. 
> 
> func name(Person : inout (first: String?, middle: String?, last: String?)) {
>     Person.first = "John"
>     Person.last = "Doe"
> }
> 
> OR 
> 
> func name(Person : inout Name) {
>     Person.first = "John"
>     Person.last = "Doe"
> }
> 
> This is because tuples are passed into functions as 'let' constants. Why not add the ability to modify a copy of the tuple inside the function ?

We used to allow "var" parameters, but they were removed in SE-0003:
  https://github.com/apple/swift-evolution/blob/master/proposals/0003-remove-var-parameters.md <https://github.com/apple/swift-evolution/blob/master/proposals/0003-remove-var-parameters.md>

Tuples are values like any other; I don't know why they would warrant a special exception to that rule.

John.
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