[swift-evolution] [Pitch] 'Double modulo' operator

[Adam Nemecek]

Would you want to make this a function? Or an operator but a different one?

On Mon, May 23, 2016 at 7:30 AM, Stephen Canon <scanon at apple.com> wrote:

> I’m not really sold on the `%%` spelling, but I think the operation itself
> is worth exposing.  This is the remainder of a “flooring” division (as
> opposed to the C-family “truncating” division[1]).  If we do provide it, we
> should also provide the accompanying divide operation.
>
> – Steve
>
> [1] there are several other ways to define division beyond these two:
> remainder is always positive, remainder is closest to zero, etc.
> Truncating and flooring division are the most common by a wide margin,
> however.
>
> On May 21, 2016, at 4:22 PM, Adam Nemecek via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Hello,
>
> I think that Swift could use the 'double modulo' operator which is for
> example in CoffeeScript (some discussion can be found here
> https://github.com/jashkenas/coffeescript/issues/1971).
>
> This operator, unlike normal modulo, takes sign from the divisor, not the
> dividend e.g. -10 % 3 == -1, but -10 %% 3 == 2.
>
> In practice, this operator is useful for 'cyclical' indexing. For example,
> it would be useful for calculating the real index into a collection when we
> are using an index outside of the range of valid indices and could be used
> to index into a collection using a negative index à la Python and Ruby
> (where [1,2,3,4][-1] == 4).
>
>
> The implementation would probably be something along these lines:
>
> infix operator %% {
>   associativity left
>   precedence 150
> }
>
> func %%<T: IntegerArithmeticType>(lhs:T, rhs:T) -> T {
>   return (lhs % rhs + rhs) % rhs
> }
>
> If accepted, this could be later incorporated into a method or operator
> that works directly with collections using their count property.
> Maybe the syntax could be something like [1,2,3,4] %% -1 == 4.
>
> Ideas, suggestions?
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