[swift-evolution] [Proposal] Shorthand Argument Renaming

[Frédéric Blondiau]

Thanks for your feedback.

I personally never found that this “$n” convention was “Swift-like”... but it’s true that a “$n” is easier to spot than a “.n”, and I got used to also.

However, I realised how much this was disturbing for newcomers, when explaining closure shorthand argument names to a classroom of computer science students (aged 21) discovering Swift.

The feedback some gave me, during the course, was quite surprisingly strong and negative about this “$n” convention. This convinced me to write this proposal : for newcomers, the “$n” zero-based is something wrong.

I understand that association between tuples and function parameters in Swift is to be removed, but, in this case, as we have no parameters at all, I thought this was a distinct enough situation.

As suggested, considering this is a kind of compiler magic, using #0, #1 instead, may indeed be a better alternative.

I’m still waiting some feedback before writing an official proposal.

> On 30 May 2016, at 19:12, Leonardo Pessoa <me at lmpessoa.com> wrote:
> 
> Fréderic, the idea and reasoning are good and do make sense but I'm
> not 100% go on this. When I started using this syntax I felt like I
> was back in PHP but once I got used it's ok. I don't see anything
> wrong that justifies the change in syntax. Looking at your examples I
> even think it's easier to spot "$n" in my code than ".n".
> 
> 
> On 30 May 2016 at 13:44, Frédéric Blondiau <swift-evolution at swift.org> wrote:
>> Hello,
>> 
>> I was thinking about this, and would like to get some feedback before making my first proposal.
>> 
>> Best regards,
>> 
>> 
>> Fred.
>> ---
>> 
>> Shorthand Argument Renaming
>> 
>> 
>> Introduction
>> 
>> Swift automatically provides shorthand argument names to inline closures which cleverly allows us to write
>> 
>>    reversed = names.sort( { $0 > $1 } )
>> 
>> I would suggest to use another syntax, using these new “names”
>> 
>>    reversed = names.sort( { .0 > .1 } )
>> 
>> 
>> Motivation
>> 
>> The $n notation is generally used with positional parameters using one-based numbering, $1 referring to argument 1; $2, to argument 2... with a special meaning for $0 (could be the name of the function, or the full list of parameters).
>> 
>> This $n notation is often handy, but feels strange in Swift... like imported from UNIX scripting (but here zero-based, anyway).
>> 
>> 
>> Proposed solution
>> 
>> The .n notation is more Swift-like — as used to access Tuple members, for example.
>> 
>> 
>> Detailed design
>> 
>> Today, .0 or .1 (as any .n's) are refused by the compiler, as being not valid floating point literals.
>> 
>> I’m not a compiler expert, but eventually fetching this error inside a closure body could easily be translated into accepting this new syntax.
>> 
>> There can’t be conflict with other shorthands (like accessing static members using dot notation) as members can’t consist of only digits characters.
>> 
>> 
>> Impact on existing code
>> 
>> $n need to be rewritten .n
>> 
>> 
>> Alternatives considered
>> 
>> Create a default argument named “arguments” (like “error” in catch, “newValue” in setters or “oldValue” in a a didSet observer) accessed like a Tuple
>> 
>>    reversed = names.sort( { arguments.0 > arguments.1 } )
>> 
>> but this is (of course) much less convenient.
>> 
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