GNU bug report logs - #18224
Shared arrays are incompatible with bytevector accessors

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Panicz Maciej Godek <godek.maciek <at> gmail.com>
To: bug-guile <at> gnu.org
Subject: Shared arrays are incompatible with bytevector accessors
Date: Fri, 8 Aug 2014 16:48:08 +0200

Creating a shared array based on a bytevector results with an object
whose print representation is identical with a print-representation of
a bytevector:

(define bv #vu8(0 1 2 3))

(define bv/shared
  (make-shared-array bv (lambda(i)(list (+ i 2))) '(0 1)))
bv/shared
====> #vu8(2 3)

However, accessing these "shared bytevectors" using the bytevector api
is impossible:

(bytevector-u8-ref bv/shared 0)
====> error: wrong type argument in position 1 (expecting bytevector): #vu8(2 3)

Since the print-representation is identical, this behaviour is
unexpected. Furthermore, the lack of capability of creating shared
bytevectors limits their usefullness. On the other hand, if the shared
bytevector remains contiguous, there should be no performance
concerns.

Message #10 received at 18224-done <at> debbugs.gnu.org (full text, mbox):

From: Daniel Llorens <daniel.llorens <at> bluewin.ch>
To: 18224-done <at> debbugs.gnu.org
Subject: Re: bug-guile Digest, Vol 137, Issue 2
Date: Fri, 30 Oct 2015 17:15:21 +0100

http://git.savannah.gnu.org/gitweb/?p=guile.git;a=commitdiff;h=eb3d623da57e6d31a58d95f932345fb761f9b701 on master fixes printing of scm_tc7_array (shared array) to be #1(...) or #1typetag(...) so they aren't confused with actual vectors or bytevectors.

With respect to the lack of capability to create shared bytevectors, use the array procedures instead. If you need the arrays to be contiguous, you can check this at run time.

The fundamental distinction between shared arrays on one side and bytevectors/vectors/bitvectors on the other side is not of contiguity, rank, element type, etc. but that the latter types are containers, while the shared arrays are just views. That is why I don't think indirection fields should be added to the bytevectors, this would complicate the lookup functions and overlap in functionality with arrays.

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