gen_range doesn't take a Range

[cramertj]

Is there a reason https://docs.rs/rand/0.6.5/rand/trait.Rng.html#method.gen_range doesn't take an std::ops::Range? This would be more convenient for specifying inclusive/exclusive boundary behaviors (e.g. 128..=255u8).

[TheIronBorn]

gen_range has to allow types which don't implement Range.

You can use ranges with Uniform::from though:

rng.sample(Uniform::from(0..=4))

NOTE: this isn't an exact copy of gen_range. gen_range uses Uniform::sample_single which isn't possible with this syntax

[TheIronBorn]

Uniform:::new_inclusive also exists

[dhardy]

In part it's historical, and since Rust doesn't support function overloading / multi-functions, we can't allow both. Maybe rng.gen_range(1..=6) would be nice to have, but I think this is too widely used a function to bring a breaking change.

[hmble]

@TheIronBorn if Uniform::new_inclusive works for you then maybe there is no need to use rng.gen_range(1..=6). I'm a beginner I don't know if there is any performance difference between thw two methods, if that's a case then maybe we can implement rng.gen_with_range(1..=6)

[dhardy]

@hmble there is potentially some performance difference; UniformSampler::sample_single(low, high, rng) is designed to be faster (for single use) than Uniform::new(low, high).sample(rng).

But is it worth having both rng.gen_range(1, 7) and rng.gen_with_range(1..=6)? If so, should it be gen_from_range or just from_range?

[hmble]

@dhardy I guess no its not. IMO we can always use rng.gen_range(start, end+1). Is there any case where we cannot use this approach ? If we decide to implement a method then from_range would be nice method name.

[vks]

The +1 trick does not work for cases where the upper bound is the maximum possible value.

…

On Fri, Apr 5, 2019, 17:47 hmble ***@***.***> wrote: @ I guess no its not. IMO we can always use rng.gen_range(start, end+1). Is there any case where we cannot use this approach ? If we decide to implement a method then from_range would be nice method name. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#744 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AACCtEOXITGnsp23H4kJLjSa0Jog0U49ks5vd3AmgaJpZM4bLkVb> .

[hmble]

It seems a good choice to create another method from_rng

[dhardy]

While @vks has a theoretical concern, I don't know how much it is a real concern, and there are workarounds (check the upper-bound or use Uniform::new_inclusive).

We could add another method like sample_range, but this implies the need for a new function like UniformSampler::sample_single_inclusive. I'm not too keen on adding more functionality there, and we can't simply change the meaning of the bounds (the type of breaking change causing bugs).

[dhardy]

I think there are basically two options on this one:

1. We keep the status quo. This is reasonably flexible since:

   • we already support Uniform<X>: From<Range<X>>
   • unpacking values from a range is not hard: rng.gen_range(rng.start, rng.end)
   • we already support Uniform::new and Uniform::new_inclusive

2. We switch rng.gen_range and Uniform::new to support any B: RangeBounds and drop Uniform::new_inclusive. This:

   • is a major breaking change
   • is strictly more flexible and should have neat code (all relevant functions are already generic)
   • should be easy to use (Range<T> only requires that T: PartialOrd which is already satisfied by all types supported by Uniform)

If not for the breaking change, the second option might be better (though would have to see an implementation first). However, I don't think it's enough better to justify this breaking change?

There is a third option: keep Rng::gen_range as-is but add Rng::gen_bounds or similar, and either adjust Uniform::new or add Uniform::new_bounds. However, this adds redundant code and API which we prefer to avoid (we already have a large API).

[vks]

I think we should probably go with option 2 before Rand 1.0. However, for now I think we should spend our budget for breaking changes elsewhere.

[vks]

Rng::gen_range now takes an inclusive or exclusive range.