list assignment for matrix objects

[ThomasBreuer]

The following happens in GAP 4.11.1 as well as in the master branch.

gap> m:= NewZeroMatrix( IsPlistMatrixRep, Integers, 2, 3 );
<2x3-matrix over Integers>
gap> m{[1,2]}:= m{[2,1]};
Error, List Assignments: <rhss> must be a dense list (not a positional object)
[...]

The point is that the operation {}:= (ASSS_LIST) calls AsssListCheck, and this calls RequireDenseList for the third argument (the right hand side of the assignment). However, the matrix objects in question aren't lists, in particular they are not dense lists.
We want {}:= for manipulation matrix objects that aren't lists, thus the checks must be changed.

[fingolfin]

Really? I thought the agreement was the opposite, i.e., that {} is only for lists. After all, it can not be efficiently implemented for general matrix objects...

See #4356 for proposed syntax extension that might make sense for matrix objects.

[ThomasBreuer]

O.k., it is more complicated.

The problem in #4517 is about IsRowListMatrix, and currently {}:= is defined for this kind of matrix objects.
Thus the question is what we want: forbid {}:= for IsRowListMatrix (and explain this, analogous to the situation with IsVectorObj, in the section about element access) or change the kernel code that expects dense lists on the right hand side.

[fingolfin]

Discussed this with Thomas today. One thing I wasn't aware is that IsRowListMatrix does not imply IsList (this is useful in so far as we may not want to require IsRowListMatrix to really implement the full API for a list type (whatever that means exactly).

But the GAP kernel currently has some checks in the code implementing {} resp. {}:= which checks that the right hand side is a dense list. We check this in a bunch of places in the kernel, both with assertions (e.g. in ASSS_LIST) and explicit checks like CheckIsDenseList("List Assignments", "rhss", objs); in AsssListDefault. We could relax those, but then we also need to go through several kernel functions related to these operations, and carefully change ELMW_LIST invocations to ELM_LIST, to deal with "holes" in the lists....

A maybe easier approach would be change the checks to test for "IsDenseList OR IsRowListMatrix" (here I am assuming that any matrix with IsRowListMatrix(matrix) = true is guaranteed to behave like a dense list in a few key aspects; e.g. matrix[i] is bound for all i in [1 .. Length(matrix)].

[fingolfin]

My view on this is that this operation is not to be supported; though a more helpful error would be nice, perhaps pointing out that sometimes CopySubMatrix or SwapMatrixRows can be good alternatives...

[ThomasBreuer]

Fine.

In order to give a better error message, it would make sense to install an additional method for the GAP operation ASSS_LIST with first argument a matrix object.

The technical problem is that currently, the GAP kernel calls AsssListCheck, which checks the arguments and throws the error before the method selection for ASSS_LIST is called.
I think AsssListCheck is not needed at all:
The checks are not wanted if AsssListObject is called later on, and AsssListDefault does already all the checks which are done by AsssListCheck.
Once we eliminate AsssListCheck, it should be possible to install the better error message.

I can provide a pull request for that.