[swift-evolution] [Returned for revision] SE-0089: Renaming String.init<T>(_: T)

[Patrick Smith]

These are some great points. I like the sound of ValuePreservingStringConvertible. A few questions to add:

- If its conformers produce a value-preserving representation, would it make sense for it to also have an initializer accepting the value? What specifically makes it value preserving otherwise?

- What is the difference between CustomStringConvertible and CustomDebugStringConvertible? Are most implementations of description and debugDescription identical? It says CustomStringConvertible is for writing to an output stream. Is a ‘value preserving string’ going to be a better fit for that all the time?

- To the question of ‘is CustomStringConvertible enough?’, what about replacing it with ValuePreservingStringConvertible? Then there are two very distinct protocols: ValuePreservingStringConvertible and CustomDebugStringConvertible, one obviously safely value preserving and one obviously just for inspecting.

- Could `.description` be renamed to something more specific and clear? For example, `preservedValue` or `.valuePreservingDescription`. If the recommended way is to use `init<T: ValuePreservingStringConvertible>(_ v: T)`, will anybody be using `.description` directly anyway? I always found NSObject’s seize of the ‘description’ property annoying, as on models it’s a perfect valid property to want as a member, so I was a little disappointed to see it in Swift too. If there’s an alternative, more clear name for this property then it won’t clash with anything else.

I like this clear separation of ‘value preserving’ and ‘just show me something’ representations.

Patrick


> On 26 May 2016, at 3:08 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Proposal Link: https://github.com/apple/swift-evolution/blob/master/proposals/0089-rename-string-reflection-init.md
> 
> The review of "SE-0089: Renaming String.init<T>(_: T)" ran from May 17…23, 2016. The proposal has been *returned for revision* and another round of discussion - the core team would love to see the revised proposal make it into Swift 3.
> 
> The community and core team both want to remove this “footgun” from the standard library, where someone could write "String(x)” with the intention of getting a value-preserving conversion to String, but may instead get a potentially lossy and potentially expensive reflection-based conversion to a String.  After extensive discussion, the core team recommends that the community consider a somewhat more elaborate design:
> 
> - Rename the existing reflection-based "String.init<T>(_: T)” initializer to "String.init<T>(describing: T)” as recommend by the community.  This initializer would rarely be invoked in user code directly.
> 
> - Introduce a new protocol (for sake of discussion, call it “ValuePreservingStringConvertible") that refines CustomStringConvertible but that adds no new requirements.  Conformance to this protocol indicates that the “description” requirement produces a value-preserving representation in String form.
> 
> - Introduce a new unlabeled initializer on String: "init<T: ValuePreservingStringConvertible>(_ v: T) { return v.description }".  This permits the “String(x)” syntax to be used on all values of types that can be converted to string in a value-preserving way.
> 
> - Audit important standard library types (e.g. the integer and floating point types), and make them explicitly conform to ValuePreservingStringConvertible with an explicitly implemented “description” property.
> 
> - As a performance optimization, change the implementation of the string literal interpolation syntax to prefer the unlabeled initializer when interpolating a type that is ValuePreservingStringConvertible or that has otherwise has an unlabeled String initializer, but use the "String.init<T>(describing: T)” initializer if not.
> 
> 
> The expected advantages of this design are:
> 
> - Swift encourages the T(x) syntax for value preserving conversions, and this design ensures that String(x) continues to work for the value preserving cases.
> 
> - This ensures that the String(x) syntax does not accidentally fall off a performance cliff by using the extremely-dynamic reflection mechanism unintentionally.
> 
> - The preferred “I don’t care how you do it, just convert this value to a string somehow” syntax remains string interpolation syntax.  This syntax is efficient in the cases where the String(x) syntax is allowed, but fully general to the other cases where custom convert-to-string code has not been provided.
> 
> 
> Some remaining open questions:
> 
> - Exactly what types should conform to ValuePreservingStringConvertible.  It seems clear that integer, floating point types, and Character can and should conform.  What other types should?
> 
> - Do we need the ValuePreservingStringConvertible at all, or is the existing CustomStringConvertible enough?  We already have a few protocols for handling string convertibility, it would be great to avoid adding another one.
> 
> Thank you to Austin Zheng for driving this proposal forward!
> 
> -Chris Lattner
> Review Manager
> 
> 
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