[swift-evolution] [Pitch] Moving where Clauses Out Of Parameter Lists

[Howard Lovatt]

I like the idea, I find the beginning of a declaration too cluttered and it
obscures the main purpose. Like Brent I am wary of the design by contract
(DBC) bit, `lhs <= rhs`. DBC is probably for a separate discussion, but
moving the where clause would probably help.

  -- Howard.

On 7 April 2016 at 10:36, Matthew Johnson via swift-evolution <
swift-evolution at swift.org> wrote:

>
>
> Sent from my iPad
>
> On Apr 6, 2016, at 7:13 PM, Brent Royal-Gordon via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> >> I would actually move them as far as after everything else, and right
> before the definition body. For the above function that would mean:
> >>
> >> func anyCommonElements<T, U>(lhs: T, _ rhs: U) -> Bool
> >>    where T : SequenceType,
> >>          U : SequenceType,
> >>          T.Generator.Element: Equatable,
> >>          T.Generator.Element == U.Generator.Element
> >> {
> >>    ...
> >> }
> >>
> >> That would make the definition look closer to what the call site looks
> like.
> >>
> >> The same would work for generic types  too:
> >>
> >> public struct Dictionary<Key, Value>
> >>    where Key : Hashable
> >> {
> >>   ...
> >> }
> >
> > Another nice thing about this style is that, in principle, I think it
> could be extended to specify requirements on non-type parameter values.
> >
> >    func ..< <Element: Incrementable>(lhs: Element, rhs: Element) ->
> Range<Element>
> >        where Element: Comparable, lhs <= rhs {
> >        …
> >    }
> >
> > I'm not saying we must or even should include that feature, merely that
> it gives us a nice syntactic slot to use if we choose to do so later.
>
> I know you're not proposing that feature right now, but want to understand
> how you expect it to work.  It looks to me like this starts to enter design
> by contract territory.  Would the predicate behave as if it was part of a
> precondition?
>
> > --
> > Brent Royal-Gordon
> > Architechies
> >
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