[swift-evolution] Nil-rejection operator

[Chris Lattner]

> On Feb 20, 2017, at 10:22 AM, Jack Newcombe via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hi all,
> 
> Now that phase 2 has begun, am I able to submit a proposal for this?

Hi Jack,

Stage 2 has limited scope, proposals are on topic if they fit the criteria Ted laid out in his email:
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170213/032116.html <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170213/032116.html>

-Chris

> 
> Best regards,
> 
> Jack
> 
>> On 8 Feb 2017, at 20:00, Jack Newcombe <jack at newcombe.io <mailto:jack at newcombe.io>> wrote:
>> 
>> 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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