On Feb 18, 2013, at 16:55, Andy Wingo wrote:

> It could make sense, yes.  What do others think?  What happens for
> array-set!?  Care to propose a patch?

Patch is attached. It looks a bit unwieldy because I am duplicating scm_array_handle_pos(), and I also had to fix the recent array_ref_1 optimization, but it shouldn't be slower for the old use.

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It was interesting to read the other replies, thank you (plural). It was disappointing that nobody seems to be using arrays, but not very surprising. I use them a fair amount, but mostly for interfacing with C/C++. I do little with them in Guile since 1) the facilities are really not there or they are inconvenient to use, 2) when I need arrays I need speed and Guile isn't there either (yet!).

More than getting this patch accepted (as it is or with different function names), my concern is that what this patch does cannot be done on the user side except through make-shared-array, which is too slow. So I think there's a good argument that scm_i_make_array(), SCM_I_ARRAY_V(), SCM_I_ARRAY_BASE() and SCM_I_ARRAY_DIMS() should be published.

Now some optional motivation:

In other languages that have arrays, rank-0 arrays are not differentiated from the scalar inside. This is the right approach if you want to treat arrays of any rank as values. Then you can say that a rank-n array is

-------------------
a rank-n array of rank-0 cells
a rank-(n-1) array of rank-1 cells
...
a rank-1 array of rank-(n-1) cells
a rank-0 array of rank-n cells = a rank-n cell.

Figure 1.
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The extended versions of array-ref and array-set! allow you to use a rank-n array as any of these.

Old Guile used to have something called ‘enclosed arrays’, they are described in the 1.6 manual. This is a mechanism inspired by APL that allows one to split an array as in Figure 1, so that if you have a function that operates on rank-k arrays, you can array-map! it on arrays of any rank >= k.

Example:

(define A #((1 2) (3 4) (5 6))
(define (sqr x) (* x x))
(define (norm v) (+ (sqr (array-ref v 0)) (sqr (array-ref v 1))))
(define out #(0 0 0))
(array-map! out norm (enclosed-array A 1))

out => #(5 25 61), hopefully (I don't have Guile 1.6 here to try).

In later versions of APL (and most obviously in J) people realized that this is a bad way to go about things, since the reason you want to split A in rank-1 cells is ‘norm’. That is, rank-1 is a property of the function ‘norm’. And you don't need to do any conversions to iterate over the rank-1 cells of A; you only need to move a pointer on each iteration, but to do this array-map! should be aware of the rank of the procedure in each of its arguments.

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Finally, to fuel the discussion of what kind of array operations Guile should provide, here is a list similar things that I have read about. I imagine there are many more. The software is all open source.

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