[FR] Optional channels about einops

[arogozhnikov]

@ehknight thanks for coming up with suggestion.

I've been previously considering this feature, however it is quite difficult to generalize beyond a single optional dimension.
The way your example should be implemented in existing einops syntax is:

rearrange(x, "b ... -> (b ...)")

where ellipsis can be 0 or 1 dimensions (or more). However, right now that's not yet supported and is subject of #43

[arogozhnikov]

@ehknight that's implemented now and will become a part of the next release

[shoyer]

It looks like transformations like "b ... -> (b ...)" work currently, but not "(b ...) -> b ..."?

[arogozhnikov]

@shoyer that's right, those expressions are disallowed.

Let's analyze your example:

(b ...) 

left part implies there is one dimension which should be decomposed in b and undefined number of dimensions.
To make this a valid operation one needs to provide number of dimensions in ... and their sizes.
In this scenario user can write ... as separate axes like (b b1 b2) -> b b1 b2.

[arogozhnikov]

Ellipsis collapsing is now available as a part of einops 0.3 release.