[swift-evolution] Nil-rejection operator

[Jean-Daniel]

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 <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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