CowArray feature

[andrei-papou]

No description provided.

[nilgoyette]

Any reason why you can't use the good old assert_eq!? Your first usage looks like

assert!(is_content_identical(&arr_cow, &expected_arr));

and I don't understand why assert_eq! can't do the job.

[nilgoyette]

It's only a suggestion, but I would merge test_is_view and test_is_owned because one is simply the reverse of the other,

[andrei-papou]

@nilgoyette thanks for your notes, I've updated the PR.

[andrei-papou]

@jturner314 any feedback on this? Once it's merged, I will be able to update #616 .

[jturner314]

I'm sorry for the delay. Life has been busy recently. I've created andrei-papou#7 with the following:

• Apply the tests to arrays with non-standard layouts.
• Add/improve documentation.
• Minor implementation changes.
• Rename ArrayCow to CowArray. The name CowArray seems a little more natural to me, and it seems to fit better with ArcArray/RawArrayView/RawArrayViewMut. It's also nice that alphabetic sorting moves CowArray after e.g. ArrayView.

Once that's merged, I think we're good-to-go on this PR.

[jturner314]

Yay! I've been wanting this feature for quite a while. Thanks for working on it!

[SuperFluffy]

Excellent, this brings me one step closer to being able to pass in slices containing arrays, like &[&ArrayBase<S, D>], where S: Data, D: Dim.

I believe that right now, due to monomorphisation, I wouldn't actually be able to pass in a mixture of Array and ArrayView, e.g. &[&Array2<f64>, &ArrayView2<f64>, &Array2<f64>].

I have to play around with it, but I don't think this feature will completely fix this either, because the trait bound fn f<T: Into<CowArray>>(myarrays: &[&T]) won't work for mixed T, again due to monomorphization.

Might a Deref<Target=ArrayView> for Array help?

[andrei-papou]

@SuperFluffy I'm not sure &[&Array2<f64>, &ArrayView2<f64>, &Array2<f64>] will work with Deref<Target=ArrayView> for Array because you still need all the members of a slice to be of the same type. &Array2<f64> and &ArrayView2<f64> are completely different types from the compiler's point of view.
IMHO what you need in your case is dynamic dispatch, but it is not available for ArrayBase. You cannot write ArrayBase<dyn Data, dyn Dim> just because S and D type arguments of ArrayBase are Sized by default.

[jturner314]

Right, all the elements in a slice need to have the same type. @SuperFluffy It seems like you want to be able to do something like this, but for ArrayView and Array instead of &[A] and Vec<A>?

let slice = &[1, 2, 3][..];
let vec = vec![4, 5, 6];
let collection = &[slice, &vec];

Note that this doesn't work with

let collection = &[&slice, &vec];

which is the statement analogous to let collection = &[&view, &owned];.

An implementation of Deref<Target = ArrayView> for ArrayBase would be pretty dangerous because we'd effectively have to bitwise re-interpret a portion of the ArrayBase as an ArrayView. (I think it would be possible to do correctly if we make ArrayBase #[repr(C)], but it's pretty tricky.)

For arrays, why not just call .view() on all the arrays and create a slice of ArrayViews (all the same type)? Like this:

let collection = &[arr1.view(), arr2.view()];

If you need mutable views, then use .view_mut() instead.