RFC: scope and borrowing

[via Digitalmars-d]

On Sunday, 21 September 2014 at 11:37:19 UTC, Manu via 
Digitalmars-d wrote:
> On 21 September 2014 16:02, deadalnix via Digitalmars-d <
> digitalmars-d at puremagic.com> wrote:
>
>> On Sunday, 21 September 2014 at 03:48:36 UTC, Walter Bright 
>> wrote:
>>
>>> On 9/12/2014 6:48 PM, Manu via Digitalmars-d wrote:
>>>
>>>> What happens when a scope() thing finds it's way into 
>>>> generic code? If
>>>> the type
>>>> doesn't carry that information, then you end up in a 
>>>> situation like ref.
>>>> Have
>>>> you ever had to wrestle with ref in generic code?
>>>> ref is the biggest disaster zone in D, and I think all it's 
>>>> problems will
>>>> translate straight to scope if you do this.
>>>>
>>>
>>> I'm unaware of this disaster zone.
>>>
>>
>> Well it is very real. I had to duplicate bunch of code in my 
>> visitor
>> generator recently because of it. Getting generic code ref 
>> correct is very
>> tedious, error prone, and guarantees code duplication and/or 
>> various static
>> ifs all over the place.
>>
>
> It's also extremely hard to unittest; explodes the number of 
> static if
> paths exponentially. I'm constantly finding bugs appear a year 
> after
> writing some code because I missed some static branch paths 
> when originally
> authoring.

If I understand you right, your problems come from the fact that 
sometimes in a template you want ref, and sometimes you don't.

But I think this mostly doesn't apply to scope: either you borrow 
things, or you don't. In particular, when you do borrow 
something, you're not interested in the owner your parameter has 
inside the caller, you just take it by scope (narrowing the 
lifetime). Thus there needs to be no information about it inside 
the callee, and you don't need different instantiations depending 
on it.

One special case where scope deduction might be desirable are 
template functions that apply predicates (delegates, lambdas) to 
passed-in parameters, like map and filter. For these, the 
scope-ness of the input range can depend on whether the 
predicates are able to take their parameters as scope.