Re: Guile: What's wrong with this?

[David Kastrup <dak@gnu.org>]

Bruce Korb <bkorb@gnu.org> writes:

> 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?

Yes.  Because different references can't be distinguished, it would mean
that you'd not actually have a reference to the modified copy after
modifying it.  Which renders the modification useless.

> "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 is neither simple, nor comprehensible.

> "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.

You probably don't realize how ironic that is.

-- 
David Kastrup