your approach really physicalizes arrays from 2D to 4D
No, it does not. It uses clump and slice, both of which are affine transformations (as shown by the P2Child=>1 in https://github.com/PDLPorters/pdl/blob/master/Basic/Slices/slices.pd for the relevant operations). Physicalising only happens on output or use in calculations, and when loop fusion comes in, not even then. I'd love to hear of an approach that Kronecker-products m1,n1 and m2,n2 matrices without producing a m1*m2,n1*n2 in-memory object. While you're pondering that, please show some numbers demonstrating these "grave consequences" :-)
Is this a bug in PDL::NiceSlice, documentation flaw, or just me reading it incorrectly?
You're going to need to show some code that you're saying is behaving unexpectedly. This worked as described (with output): 
pdl> p sequence(2)->(*1)
[
 [0]
 [1]
]
pdl> p sequence(2)->(*)
[
 [0]
 [1]
]
pdl> p sequence(2)->(*,*)
[
 [
  [0]
 ]
 [
  [1]
 ]
]

[download]
Naturally, you need to make sure a use PDL::NiceSlice is in scope. You will have noticed that generally I avoid using it in core PDL code, because it both slows down module-loading, and has limitations; see PDL::LinearAlgebra, where I had to remove a lot of it in favour of building arguments to slice that took account of adding optional parts for PDL::Complex.

In reply to Re^4: Kronecker Product by etj
in thread Kronecker Product by choroba

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.