[swift-evolution] [Pitch] Add toplevel keyword for protocols

[Chris Lattner]

> On May 13, 2016, at 12:12 AM, Patrick Pijnappel via swift-evolution <swift-evolution at swift.org> wrote:
> 
> For some protocols we'd like to require top-level (free) functions, e.g. for many math functions such as abs() or sin(). We already do this implicitly for operators. 

Hi Patrick,

FYI, we’re very likely to remove this special behavior for operators, by making operator requirements only find operators declared in a conforming type.  This would eliminate the special case for operators that exists now, and has other advantages as well.  Check out this proposal for more information:
https://github.com/apple/swift-evolution/blob/master/proposals/0091-improving-operators-in-protocols.md

-Chris

> 
> Proposal
> Allow top-level function/property requirements in protocols, e.g.:
> 
> public protocol AbsoluteValuable : SignedNumber {
>   /// Returns the absolute value of `x`.
>   @warn_unused_result
>   toplevel func abs(_ x: Self) -> Self
> }
> 
> We'd probably want to require this for operators. This also opens up syntax if we ever get dynamically dispatched operators.
> 
> public protocol SignedNumber : Comparable, IntegerLiteralConvertible {
>   /// Returns the result of negating `x`.
>   @warn_unused_result
>   toplevel prefix func - (x: Self) -> Self
> }
> 
> Currently this is done using the combination of a static method and a top-level generic function on that protocol. As I understand that approach does have some benefits in terms of type-checker performance, though I'm not sure whether that is likely to stay relevant in the future.
> 
> Advantages
> Cleaner than current approach (esp. floating point types have tons of top-level functions)
> Makes operators less of a special case
> Opens up syntax for member operators
> Could also apply to top-level properties (esp. useful if we get generic properties, for e.g. π<...>)
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