[Numpy-discussion] adding a .M attribute to the array.

[Travis Oliphant]

> > Other suggestions?
>
> Here is one suggestion that is based on the observation that all we need
> is an easy way to tell that the following operation should be applied
> as a matrix operation. So, the suggestion is to provide an attribute or a
> member function that returns an array (note, not a Matrix instance) that
> has a bit, called it asmatrix, set true but _only_ temporally. The bit is
> cleaned on every operation. And before applying an operation, the
> corresponding method (currently there seem to be only four relevant
> methods: __mul__, __pow__ and their r-versions) checks if either of
> the operants has asmatrix bit true, then performs the corresponding matrix
> operation, otherwise the default element-wise operation. And before
> returning, it cleans up asmatrix bit.
>

Frankly, I like this kind of proposal.  I disagree with Konrad about the
separation between arrays and matrices.   From my discussions with other
people, it sounds like this is actually a point of disagreement for many
in the broader community.

To me, matrices are just arrays of rank <=2  which should be interpreted
with their specific algebra.


> For the sake of an example, let .m be Numeric array attribute that when
> accessed sets asmatrix=1 and returns the array.
> Examples:
>
>   a * b       - is element-wise multiplication
>   a.m * b, a * b.m   - are matrix multiplications, the resulting
>                        array, as well a and b, have asmatrix=0
>   a.m ** -1   - is matrix inverse
>   sin(a)      - element-wise sin
>   sin(a.m)    - matrix sin
>
> To summarize the main ideas:
>  * array has asmatrix bit that most of the time is false.
>  * there is a way to set the asmatrix bit true, either by .m or .M
>    attributes or .m(), .M(), .. methods that return the same array.
>  * __mul__, __pow__, etc. methods check if either operant has asmatrix
>    true, then performs the corresponding matrix operation, otherwise
>    the corresponding element-wise operation.
>  * all operations clean asmatrix bit.
>


Again, I wouldn't mind it, but I suspect the more aesthetically critical
on the list will dislike it because it blurs the (currently clumsy)
distinction between arrays and Matrices that I'm beginning to see people
actually like.

-Travis