Re: Guile: What's wrong with this?

[Bruce Korb <bkorb@gnu.org>]

On 01/04/12 15:59, Mark H Weaver wrote:
> 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.

So because it might be the case that one reference might want to
see changes made via another reference then the whole concept is
trashed?  "all or nothing"?  Anyway, such a concept should be kept
very simple:  functions that modify their argument make copies of
any input argument that is read only.  Any other SCM's lying about
that refer to the unmodified object continue referring to that
same unmodified object.  No mind reading required.

    (define a "hello")
    (define b a)
    (string-upcase! a)
    b

yields "hello", not "HELLO".  Simple, comprehensible and, of course,
not the problem I was having.  :)

"it goes without saying (but I'll say it anyway)":

    (define a (string-copy "hello"))
    (define b a)
    (string-upcase! a)
    b

*does* yield "HELLO" and not "hello".  Why the inconsistency?

   Because it is better to do what is almost certainly expected
   rather than throw errors.

It is an ease of use over language purity thing.