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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