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From: Mark H Weaver <mhw@netris.org>
To: Bruce Korb <bkorb@gnu.org>
Cc: guile-devel@gnu.org
Subject: Re: Guile: What's wrong with this?
Date: Wed, 04 Jan 2012 18:59:39 -0500	[thread overview]
Message-ID: <8762grf28k.fsf@netris.org> (raw)
In-Reply-To: <4F04D01D.5050801@gnu.org> (Bruce Korb's message of "Wed, 04 Jan 2012 14:18:05 -0800")

Bruce Korb <bkorb@gnu.org> writes:
> You have to go to some extra trouble to be certain that a string
> value that you have assigned to an SCM is not read only.

If you're going to mutate a string, you'd better be safe and make a copy
before mutating it, unless you know very clearly where it came from.
Otherwise, you might be mutating a string that some other data structure
still references, and it might not take kindly to having its string
mutated behind its back.

The fact that some string (whose origin you don't know about) might be
read-only is the least of your problems.  At least that problem will now
be flagged immediately, which is far better than the subtle and
hard-to-debug damage might be caused by mutating a string that other
data structures may reference.

All mutable values in Scheme are pointers.  In the case of strings, that
means that they're like "char *", not "char []".  A great deal of code
freely makes copies of these pointers instead of copying the underlying
string itself.  That's a very old tradition, because it is rare to
mutate strings in Scheme.

> If Guile were to implement copy on write, then the user would not have
> to care whether a string were shared read only or not.  It would be
> easier to use.

Guile already implements copy-on-write strings, but only in the sense of
postponing the copy done by `string-copy', `substring', etc.

Implementing copy-on-write transparently without the user explicitly
making a copy (that is postponed) is _impossible_.  The problem is that
although we could make a new copy of the string, we have no way to know
which pointers to the old object should be changed to point to the new
one.  We cannot read the user's mind.

    Mark



  parent reply	other threads:[~2012-01-04 23:59 UTC|newest]

Thread overview: 117+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-01-03  4:08 What's wrong with this? Bruce Korb
2012-01-03 15:03 ` Mike Gran
2012-01-03 16:26   ` Guile: " Bruce Korb
2012-01-03 16:30     ` Mike Gran
2012-01-03 22:24     ` Ludovic Courtès
2012-01-03 23:15       ` Bruce Korb
2012-01-03 23:33         ` Ludovic Courtès
2012-01-04  0:55           ` Bruce Korb
2012-01-04  3:12             ` Noah Lavine
2012-01-04 17:37               ` bytevector -- was: " Bruce Korb
2012-01-04 21:17             ` Ludovic Courtès
2012-01-04 22:36               ` Bruce Korb
2012-01-05  0:01                 ` Ludovic Courtès
2012-01-05 18:36                   ` non-reproduction of initial issue -- was: " Bruce Korb
2012-01-05 18:50                     ` Mark H Weaver
2012-01-04 12:19         ` Ian Price
2012-01-04 17:16           ` Bruce Korb
2012-01-04 17:21             ` Andy Wingo
2012-01-04 17:39             ` David Kastrup
2012-01-04 21:52             ` Ian Price
2012-01-04 22:18               ` Bruce Korb
2012-01-04 23:22                 ` Mike Gran
2012-01-04 23:59                 ` Mark H Weaver [this message]
2012-01-05 17:22                   ` Bruce Korb
2012-01-05 18:13                     ` Mark H Weaver
2012-01-05 19:02                       ` Mark H Weaver
2012-01-05 20:24                     ` David Kastrup
2012-01-05 22:42                     ` Mark H Weaver
2012-01-06  1:02                       ` Mike Gran
2012-01-06  1:41                         ` Mark H Weaver
2012-01-06  2:38                           ` Noah Lavine
2012-01-06 13:37                           ` Mike Gran
2012-01-06 14:11                             ` David Kastrup
2012-01-06 18:13                             ` Mark H Weaver
2012-01-06 19:06                               ` Bruce Korb
2012-01-06 19:19                                 ` David Kastrup
2012-01-06 20:03                                   ` Mark H Weaver
2012-01-07 16:13                                 ` Mark H Weaver
2012-01-07 17:35                                   ` mutable interfaces - was: " Bruce Korb
2012-01-07 17:47                                     ` David Kastrup
2012-01-07 18:30                                     ` Mark H Weaver
2012-01-07 18:55                                       ` Mark H Weaver
2012-01-06 22:23                               ` Guile BUG: " Bruce Korb
2012-01-06 23:11                                 ` Mark H Weaver
2012-01-06 23:35                                   ` Andy Wingo
2012-01-06 23:41                                   ` Bruce Korb
2012-01-07 15:00                                     ` Mark H Weaver
2012-01-07 15:27                                       ` Bruce Korb
2012-01-07 16:38                                         ` Mark H Weaver
2012-01-07 17:39                                           ` Bruce Korb
2012-01-09 15:41                                             ` Mark H Weaver
2012-01-09 17:27                                               ` Bruce Korb
2012-01-09 18:32                                                 ` Andy Wingo
2012-01-09 19:48                                                   ` Bruce Korb
2012-01-07 15:47                                       ` David Kastrup
2012-01-07 17:07                                         ` Mark H Weaver
2012-01-07 14:35                                   ` Mark H Weaver
2012-01-07 15:20                                     ` Mike Gran
2012-01-07 22:25                                     ` Ludovic Courtès
2012-01-10  9:13                                     ` The empty string and other empty strings Ludovic Courtès
2012-01-10 11:28                                       ` Mike Gran
2012-01-10 13:03                                         ` Mark H Weaver
2012-01-10 13:09                                           ` Mike Gran
2012-01-10 15:41                                           ` Mark H Weaver
2012-01-10 15:48                                             ` David Kastrup
2012-01-10 16:15                                               ` Mark H Weaver
2012-01-12 22:33                                                 ` Ludovic Courtès
2012-01-13  9:27                                                   ` David Kastrup
2012-01-13 16:39                                                     ` Mark H Weaver
2012-01-13 17:36                                                       ` David Kastrup
2012-01-16  8:26                                                       ` Marijn
2012-01-16  8:47                                                         ` David Kastrup
2012-01-20 21:31                                                       ` Andy Wingo
2012-01-10 14:10                                         ` David Kastrup
2012-01-10 12:21                                       ` Mike Gran
2012-01-10 12:27                                       ` Mark H Weaver
2012-01-10 16:34                                         ` Ludovic Courtès
2012-01-10 17:04                                           ` David Kastrup
2012-01-06 23:28                                 ` Guile BUG: What's wrong with this? Bruce Korb
2012-01-07 20:57                           ` Guile: " Ian Price
2012-01-08  5:05                             ` Mark H Weaver
2012-01-06  9:23                         ` David Kastrup
2012-01-05  7:22                 ` David Kastrup
2012-01-04 22:46             ` Ludovic Courtès
2012-01-04  3:04       ` Mike Gran
2012-01-04  9:35         ` nalaginrut
2012-01-04  9:41         ` David Kastrup
2012-01-04 21:07         ` Ludovic Courtès
2012-01-04 10:03     ` Mark H Weaver
2012-01-04 14:29       ` Mike Gran
2012-01-04 14:45         ` David Kastrup
2012-01-04 16:47         ` Andy Wingo
2012-01-04 17:14           ` David Kastrup
2012-01-04 17:32             ` Andy Wingo
2012-01-04 17:49               ` David Kastrup
2012-01-04 18:09                 ` Andy Wingo
2012-01-04 17:30           ` Bruce Korb
2012-01-04 17:44             ` David Kastrup
2012-01-04 18:26             ` Ian Price
2012-01-04 18:48               ` Mark H Weaver
2012-01-04 19:29               ` Bruce Korb
2012-01-04 20:20                 ` David Kastrup
2012-01-04 23:19                 ` Mark H Weaver
2012-01-04 23:28                   ` Bruce Korb
2012-01-07 15:43                   ` Fixed string corruption bugs (was Guile: What's wrong with this?) Mark H Weaver
2012-01-07 16:19                     ` Fixed string corruption bugs Andy Wingo
2012-01-04 18:31           ` Guile: What's wrong with this? Mark H Weaver
2012-01-04 18:43             ` Andy Wingo
2012-01-04 19:29               ` Mark H Weaver
2012-01-04 19:43                 ` Andy Wingo
2012-01-04 20:08                   ` Bruce Korb
2012-01-04 20:14                     ` David Kastrup
2012-01-04 20:56                     ` Andy Wingo
2012-01-04 21:30                       ` Bruce Korb
2012-01-04 17:19         ` Mark H Weaver
2012-01-05  4:24           ` Mark H Weaver
2012-01-04 22:37       ` Ludovic Courtès

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