The function in question is std.algorithm.searching's until [1].

Here are the definitions:


Until!(pred, Range, Sentinel)
until(alias pred = "a == b", Range, Sentinel)
(Range range, Sentinel sentinel, OpenRight openRight = OpenRight.yes)
if (!is(Sentinel == OpenRight));


and:


Until!(pred, Range, void)
until(alias pred, Range)
(Range range, OpenRight openRight = OpenRight.yes);


Now the first thing that came to mind was that it seemed odd to have openRight be a runtime flag instead of a compile-time flag. But that didn't hinder my usage of it, so I went ahead and called the function like so:

range.until!(e => e > f)(No.openRight)

and I got an error saying the function could not be deduced, so I went looking in the source code and found:


enum OpenRight
{
    no,
    yes
}


But I had assumed OpenRight was an alias for a Flag type [2], so I had called it wrong.

Does anyone have any idea why the function is defined this way?

1. Why is openRight a runtime flag? Is there really a use case for this?

2. Why is openRight not a Flag type?

here is the definition I was expecting:


alias OpenRight = Flag!"openRight";

auto until(alias pred = "a == b", OpenRight openRight = Yes.openRight, Range, Sentinel)
(Range range, Sentinel sentinel);

auto until(alias pred, OpenRight openRight = Yes.openRight)(Range range);

[1] https://dlang.org/phobos/std_algorithm_searching.html#until
[2] https://dlang.org/phobos/std_typecons.html#.Flag

On Thursday, 22 September 2016 at 20:35:13 UTC, e-y-e wrote:
> [...]

Disclaimer: my answers are just early guesses.

> 1. Why is openRight a runtime flag? Is there really a use case for this?

Runtime evaluation is more flexible. The reason to have compile-time evaluation is to allowed aggressive specialization, highly tuned codegen and different attribute inference. If that flag is going to add a single very cheap branch (compared to the total cost), then it may not be worth to have it as a compile-time argument, as this would increase the number of instantiations and slightly reduce the flexibility.

But there is a second possible explanation, see below.

> 2. Why is openRight not a Flag type?

It may be that until is quite old, predating the extensive usage of Flag, and maybe also the extensive use of compile-time flags.

On Thursday, 22 September 2016 at 21:41:38 UTC, Lodovico Giaretta wrote:
>> 2. Why is openRight not a Flag type?
>
> It may be that until is quite old, predating the extensive usage of Flag, and maybe also the extensive use of compile-time flags.

Ok, I suspected this might be the case. I might work on a pull request for the enum -> Flag item, as I can't think of any reason not to. Off the top of my head I don't think it would break any existing uses.

Thanks,

On Thursday, September 22, 2016 21:41:38 Lodovico Giaretta via Digitalmars-d- learn wrote:
> On Thursday, 22 September 2016 at 20:35:13 UTC, e-y-e wrote:
> > 2. Why is openRight not a Flag type?
>
> It may be that until is quite old, predating the extensive usage of Flag, and maybe also the extensive use of compile-time flags.

until predates Flag by a considerabl margin. And even after Flag was introduced, it wasn't used much for quite a while.

- Jonathan M Davis