Interesting, but I’m unsure if all of it is significantly better than just
using the guard that is effectively inside of the operator/func that is being
proposed:
guard let value = Int("NotANumber") else { throw InitializerError.invalidString
}
It is only a couple of characters longer and already works (it’s what I use
currently). If guard allowed for a special single-expression variation so that
you didn’t need to specify the ugly braces or something, it’d look prettier and
be nice for a lot of other situations, too:
guard let value = Int("NotANumber") else: throw InitializerError.invalidString
guard someVal < 10 else: return false
guard mustBeTrue() else: return
// etc
Not to derail this, but I sort of want this ability anywhere as a shorthand for
a single-expression block.
if something < 42: doThing()
for a in list: print(a)
But I imagine that’ll never fly. :P
l8r
Sean
> On Apr 6, 2016, at 9:46 AM, Erica Sadun via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> Pyry Jahkola and I have been plugging away on the following which is
> preliminary enough not to qualify as an actual draft. He prefers the Mike Ash
> approach. I prefer the operator approach. So we have not actually settled on
> which one we would actually propose despite how I've written this up.
>
> I'm putting this out there to try to gain a consensus on:
>
> * Would this be a viable proposal?
> * If so, which of the options would work best within Swift's design and
> philosophy
>
> Thanks for your feedback.
>
> -- Erica
> Introduction
>
> Swift's try? keyword transforms error-throwing operations into optional
> values. We propose adding an error-throwing nil-coalescing operator to the
> Swift standard library. This operator will coerce optional results into
> Swift's error-handling system.
>
> This proposal was discussed on the Swift Evolution list in the name thread.
>
> Motivation
>
> Any decision to expand Swift's set of standard operators should be taken
> thoughtfully and judiciously. Moving unaudited or deliberately
> non-error-handling nil-returning methods and failable initializers into
> Swift's error system should be a common enough use case to justify
> introducing a new operator.
>
> Detail Design
>
> We propose adding a new operator that works along the following lines:
>
> infix operator ??? {}
>
> func ???<T>(lhs: T?, @autoclosure error: () -> ErrorType) throws -> T {
> guard case let value? = lhs else { throw error() }
> return value
> }
>
> The use-case would look like this:
>
> do {
> let error = Error(reason: "Invalid string passed to Integer initializer")
> let value = try Int("NotANumber") ??? InitializerError.invalidString
> print("Value", value)
> } catch { print(error) }
>
> Note
>
> SE-0047 (warn unused result by default) and SE-0049 (move autoclosure) both
> affect many of the snippets in this proposal
>
> Disadvantages to this approach:
>
> • It consumes a new operator, which developers must be trained to use
> • Unlike many other operators and specifically ??, this cannot be
> chained. There's no equivalent to a ?? b ?? c ?? dor a ?? (b ?? (c ?? d)).
> Alternatives Considered
>
> Extending Optional
>
> The MikeAsh approach extends Optional to add an orThrow(ErrorType) method
>
> extension Optional {
> func orThrow(@autoclosure error: () -> ErrorType) throws -> Wrapped {
> guard case let value? = self else { throw error() }
> return value
> }
> }
>
> Usage looks like this:
>
> do {
> let value = try Int("NotANumber")
> .orThrow(InitializerError.invalidString)
> print("Value", value)
> } catch { print(error) }
>
> An alternative version of this call looks like this: optionalValue.or(throw:
> error). I am not a fan of using a verb as a first statement label.
>
> Disadvantages:
>
> • Wordier than the operator, verging on claustrophobic, even using
> Swift's newline dot continuation.
> • Reading the code can be confusing. This requires chaining rather than
> separating error throwing into a clear separate component.
> Advantages:
>
> • No new operator, which maintains Swift operator parsimony and avoids
> the introduction and training issues associated with new operators.
> • Implicit Optional promotion cannot take place. You avoid mistaken
> usage like nonOptional ??? error and nonOptional ?? raise(error).
> • As a StdLib method, autocompletion support is baked in.
> Introducing a StdLib implementation of raise(ErrorType)
>
> Swift could introduce a raise(ErrorType) -> T global function:
>
> func raise<T>(error: ErrorType) throws -> T { throw error }
>
> do {
> let value = try Int("NotANumber") ?? raise(InitializerError.invalidString)
> print("Value", value)
> } catch { print(error) }
>
> This is less than ideal:
>
> • This approach is similar to using && as an if-true condition where an
> operator is abused for its side-effects.
> • It is wordier than the operator approach.
> • The error raising function promises to return a type but never will,
> which seems hackish.
> Overriding ??
>
> We also considered overriding ?? to accept an error as a RHS argument. This
> introduces a new way to interpret ?? as meaning, "throw this error instead of
> substituting this value".
>
> func ??<T>(lhs: T?, @autoclosure error: () -> ErrorType) throws -> T {
> guard case let value? = lhs else { throw error() }
> return value
> }
>
> Usage:
>
> let value = try Int("NotANumber") ?? Error(reason: "Invalid string passed to
> Integer initializer")
>
> This approach overloads the semantics as well as the syntax of the coalescing
> operator. Instead of falling back to a RHS value, it raises the RHS error.
> The code remains simple and readable although the developer must take care to
> clarify through comments and naming which version of the operator is being
> used.
>
> • While using try in the ?? statement signals that a throwing call is
> in use, it is insufficient (especially when used in a throwing scope) to
> distinguish between the normal coalescing and new error-throwing behaviors.
> • Error types need not use the word "Error" in their construction or
> use. For example try value ?? e may not be immediately clear as an
> error-throwing intent.
> • Overloading ?? dilutes the impact and meaning of the original
> operator intent.
> Future Directions
>
> We briefly considered something along the lines of perl's die as an
> alternative to raise using fatalError.
>
> Acknowledgements
>
> Thanks Mike Ash, Jido, Dave Delong
> _______________________________________________
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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