[swift-evolution] Proposal Draft: Optional Upgrading Assignment.

[Dave Abrahams]

on Thu Feb 04 2016, Paul Ossenbruggen <swift-evolution at swift.org> wrote:

> Draft Proposal feedback welcome.
>
> ## Introduction
>
> Currently to make a optional from a value the type name must be repeated:
>
> 	let x = “Some Value”
> 	let y : String? = x
>
> This takes away some of the advantage of type inference because the
> very next line I must specify the type.

These work:

let y: Optional = x
let z = Optional(x)


> ## Proposal
>
> I propose the following syntax:
>
> 	let y? = x
>
> This binds a new optional y which wraps the same value of x. It
> follows the same conventions as var and let that currently exist in
> the language.
>
> The advantages: this is easier to read, more compact, and lets type
> inference determine the type.
>
> If let and other conditionals would not support this syntax and
> optionals of optionals is not supported. So let x?? = y would not
> result in an optional optional. Also if y was an optional it would not
> let you upgrade that.
>
> ## Detailed Design
>
> The grammar: 
>
> 	let optionalValue? = value
>
> This would not make sense in the if let, (guard let) context because
> if let unwraps the value, so this would be an error.
>
> This should not conflict with pattern matching as that requires the case keyword. 
>
> Wrapping optional in an optional is an error. 
>
> ## Impact on existing code
>
> The current syntax will continue to exist so it will have no impact on existing code. 
>
> ## Alternatives Considered
>
> None. 
>
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-- 
-Dave