[swift-evolution] Nil-rejection operator

[Xiaodi Wu]

This seems to boil down to sugar where `guard let foo = ... else { throw
... }` is spelled `let foo = try ... !! ...`.

While the analysis is interesting I don't see that this is an obvious win
sufficient for the standard library. As you show it's possible to create
for yourself.
On Wed, Feb 8, 2017 at 14:20 Jean-Daniel via swift-evolution <
swift-evolution at swift.org> wrote:

> While I find the concept interesting, I give a big -1 for using the ‘!’
> operator for something else that fatal error.
>
> Le 8 févr. 2017 à 21:00, Jack Newcombe via swift-evolution <
> swift-evolution at swift.org> a écrit :
>
> Hi all,
>
> Currently there are a number of different operators for dealing with
> optionals that cover most of the use cases. However, I think I’ve
> identified a missing complement for the existing operators for optionals.
>
> Take the following outcomes for interacting with an optional using
> existing operators (!, ?, ??). The outcomes of using these are as follows:
>
> - value? :
> if value is nil, do nothing and return nil
> if value is not nil, complete the chain by evaluating the rest of the
> expression. Return the result of the expression
> - value! :
> if value is nil, throw.a fatal error.
> If value is not nil, complete the chain by evaluating the rest of the
> expression. Return the result of the expression
> - value ?? default :
> if value is nil, return default
> if value is not nil, return value
>
> It seems to me that, if it is possible to coalesce a nil value with a
> default value, it should also be possible to reject a nil value a non-fatal
> error.
>
> I propose the introduction of a nil-rejection operator (represented here
> as !!) as a complement to the above operators.
> .
> This operator should allow an equivalent behaviour to the forced
> unwrapping of a variable, but with the provision of an error to throw in
> place of throwing a fatal error.
>
> - value !! Error :
> if value is nil, throw non-fatal error
> if value is not nil, return value
>
> Example of how this syntax might work (Where CustomError: Error):
>
> let value = try optionalValue !! CustomError.failure
>
> It is possible to implement this in Swift 3 with the following declaration:
>
> infix operator !! : NilCoalescingPrecedence
>
> func !!<UnwrappedType: Any, ErrorType: Error>(lhs: Optional<UnwrappedType>, rhs: ErrorType) throws -> UnwrappedType
> {
>     guard let unwrappedValue = lhs else {
>         throw rhs
>     }
>     return unwrappedValue
> }
>
> I’ve added further examples including composition with the nil-coalescence
> operator here:
> https://gist.github.com/jnewc/304bdd2d330131ddb8a1e615ee560d1d
>
> This would be particularly convenient in cases where a functions expects
> significant number of optional to contain non-nil values, without the need
> to repeat non-generic guard-let structures with the same else code-block.
>
> Best regards,
>
> Jack
>
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