xclosure

[SylvainCorlay]

For expressions:

• The closure type for an lvalue reference is a const reference.
• The closure type for anything else is a value.

For scalars:

• The closure type is an xscalar

This should solve #75.

[SylvainCorlay]

TODO: re-add -> get_xfunction_type?

[SylvainCorlay]

Heads up to @wolfv since this is going to change how one writes ufuncs.

[wolfv]

Thanks!
Out of curiosity, would you mind to elaborate a tiny bit on what the xclosure is and why it's needed?

[SylvainCorlay]

The current mechanism may result in dangling references.

Closure types are how expressions hold their different components (xfunction, xbroadcast). In x + y, the members of the expression (x and y) are stored by the xfunction in a tuple of "closure types".

In the old mechanism, when using an xfunction on an rvalue xarray (e.g built by a function), the closure type was always a const reference, the xfunction is then holding a const reference to a temporary, resulting in a dangling reference when composing it in another expression.

The new mechanism is different. An expression built with a rvalue will hold a value, and move the rvalue to it. An expression built with an lvalue reference will hold a const reference to it.

[wolfv]

Thanks, good to know! I can also confirm that this PR is fixing the view iterator error I was constantly seeing 🎉

[SylvainCorlay]

Cool, that is good to hear.

[SylvainCorlay]

@JohanMabille yay!

[JohanMabille]

Let's rock !

[SylvainCorlay]

Ping @juanalday. You might be interested in this fix. #101 also somplifies this.

[SylvainCorlay]

@JohanMabille this also fixed the issue when replacing xscalar by 0-D xtensors or xarrays in the code base. So it is a good sanity check.