Extending Base.stack for DimArrays

[brendanjohnharris]

Adds methods for Base.stack, and related non-exported functions from Base, that are compatible with DimArrays.

Syntax follows Base: stacking DimArrays along a given axis dims creates a new dimension. However, existing dimension data is preserved, and the new dimension becomes an AnonDim.

Optionally, a Dimension dim can be provided as the first argument to stack, in which case the new dimension is assigned as dim rather than AnonDim.

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[brendanjohnharris]

Thanks for the suggestions! Went through and cleaned up the PR.

The Base.stack(dim, X; dims) method lets you supply a Dimension to assign to the newly created array dimension; not strictly necessary, but more convenient than setting or rebuilding the resulting array to overwrite the new AnonDim (is an AnonDim a suitable way to treat a newly created dimension, as I've done here?).

Let me know if you have more thoughts on this, happy to work on it further :)

[rafaqz]

We try not to change base method signatures besides allowing dims to be named. To keep the mental model very simple.

I would just put that new Dimension in dims like cat does. If its a constructed dimension it replaces the existing one. Otherwise Type/Symbol etc just choose them. Just check d isa Dimension

You will also need to format(newdims, A) afterwards.

[brendanjohnharris]

Base.stack behaves differently than Base.cat in that it always creates a new dimension (i.e. Base.stack([A, B,...]; dims=2) inserts a dimension at position 2, increasing ndims(A) by 1); in general this new dimension is unrelated to the existing array dimensions, so we need to specify both position (dims) and a Dimension (dim).

I see your point about keeping new signatures to a minimum, though. How about allowing the keyword argument dims to be assigned as an 'Integer=>Dimension' pair, with the following behavior:

1. If dims isa Integer: The new dimension is an AnonDim at position dims
2. If dims isa Dimension: The new dimension is a DImension dims at position ndims(A)+1
3. If dims isa Pair{Integer, Dimension}: The new dimension is a Dimension last(dims) at position first(dims)

We could also, instead, have that the new dimension is always an AnonDim, leaving it up to the user to set that dimension at a later time; in that case, however, there is ambiguity if there are any existing AnonDims in the input arrays.

[rafaqz]

Using a Pair sounds like a good solution (as do points 1 and 2 as simpler cases). For 2 I guess putting it as the last dimension would be the majority use-case? (I rarely use stack)

Always AnonDim will require using set all the time with AnonDim => somedim. We may as well just put that pair in stack and skip the step.