I would like to revisit the topic of operator overloads for
multidimensional slicing.
Bottom line: opSlice is currently limited to 1 dimension/axis
only. The cleanest workaround right now is to pass your own
"slice" structs to opIndex. It works but it's not too pretty.
----
// Suppose we have a user-defined type...
auto mat = Matrix(
[ [0,1,2],
[3,4,5],
[6,7,8] ] );
// This type of indexing can be implemented:
auto cell = mat[1, $-1];
// But multidimensional slicing cannot:
// auto submatrix = mat[0..2, 1..$];
// "Cleanest" workaround with a slice struct S taken by opIndex
// (no $ capability):
auto submatrix = mat[ S(0,2), S(1,3) ];
// With a bit more hacking, something like this could be done:
auto submatrix = mat[ S[0..2], S[1..$] ];
----
Problem with current state of affairs and rationale for a fix:
* A stated design goal of D is to "Cater to the needs of
numerical analysis programmers", and presumably HPC / scientific
computing that's heavy on linear algebra and n-dimensional
arrays. Well, it seems like the multidimensional slice/stride
syntax in Matlab, NumPy, and even Fortran has been pretty popular
with these folks. Syntactic sugar here is a clear win. I don't
think it's a niche feature.
* The limitation on slicing is inconsistent with the capabilities
of opIndex and opDollar, and workarounds are ugly.
* The approved DIP#7 briefly mentions multidimensional slicing
but it was never implemented (despite opDollar getting done).
Recap of discussions so far:
* 2009-10-10: DIP7
(http://www.prowiki.org/wiki4d/wiki.cgi?LanguageDevel/DIPs/DIP7)
* 2010-03-08: "Proposal: Multidimensional opSlice solution"
(http://forum.dlang.org/thread/hn2q9q$263e$1...@digitalmars.com)
* 2011-10-09: "Issue #6798: Integrate overloadings for
multidimensional indexing and slicing"
* 2012-06-01: "[Proposal] Additional operator overloadings for
multidimentional indexing and slicing"
(http://forum.dlang.org/thread/mailman.1202.1338515967.24740.digitalmar...@puremagic.com)
* 2012-11-19: "Multidimensional array operator overloading"
(http://forum.dlang.org/thread/mailman.2065.1353348152.5162.digitalmar...@puremagic.com)
* 2012-12-19: "Multidimensional slice"
(http://forum.dlang.org/thread/lglljlnzoathjxijo...@forum.dlang.org)
* 2013-04-06: "rationale for opSlice, opSliceAssign, vs a..b
being syntax suger for a Slice struct?"
(http://forum.dlang.org/thread/mailman.551.1365290408.4724.digitalmars-d-le...@puremagic.com)
* 2013-05-12: Andrei asks for feedback on Kenji's 2011 pull
request for #6798
* 2013-10-11: "std.linalg"
(http://forum.dlang.org/thread/rmyaglfeimzuggolu...@forum.dlang.org)
Steps forward:
So I basically want resurrect the topic and gauge support for
fixing slice overloads. Then, core committers could revisit
Kenji's 2011 proposal and pull request for issue #6798 as a very
solid near-term solution. Finally, perhaps a DIP for stride
syntax/overloads?
Looking forward to discussion.
