impl Borrow<&ArrayBase<ViewRepr<&T>, _>> for ArrayBase<OwnedRepr<T>, _>

[garro95]

If possible, the trait specified in the title would make it easier to store owned arrays in a (Hash)Map and retrieve them without the need to build a new owned array (performing allocations).

[garro95]

Probably the implementation of borrow would be a simple self.slice(s![..]) with .. repeated for every axes

[jturner314]

Unfortunately, this is not possible in current Rust. (Who would own the ArrayView that borrow would return a reference to?) For the same reason, we can't return an &ArrayView from our implementation of Index.

To do something like this, we need the language to support custom dynamically-sized types, i.e. custom "fat pointers." (The shape/strides/etc. would be stored in the fat pointer itself.) It's worth noting that current proposals, e.g. rust-lang/rfcs#2594, would make it possible to return &ArrayView for constant-ndim cases (e.g. Ix1, Ix2, etc.), but they still wouldn't allow returning an &ArrayView for the IxDyn case.

[bluss]

The crate indexmap implements a more mechanism for this (instead of Borrow) - its Equivalence trait should make it possible to implement this, if you also use indexmap's hash table implementation.

[jturner314]

I realized that my initial response was misleading. It's not possible to implement Borrow<ArrayView<'a, A, D>> for ArrayBase<S, D> where S: Data since it's not possible to correctly express the lifetime 'a. However, it is possible for us to introduce a new "storage" type RefRepr such that &'a ArrayBase<RefRepr<A>, D> is analogous to ArrayView<'a, A, D>, and we can implement Borrow<ArrayBase<RefRepr<A>, D>> and Deref<Target = ArrayBase<RefRepr<A>, D>> for ArrayBase<S, D> where S: Data. I'll write up my thoughts in a separate issue.