Re: Guile: What's wrong with this?

[David Kastrup <dak@gnu.org>]

Bruce Korb <bkorb@gnu.org> writes:

> On 01/06/12 10:13, Mark H Weaver wrote:
>> Imagine that you were evaluating Scheme by hand on paper.  You have your
>> program written on one page, and you have another scratch page used for
>> the data structures that your program creates during evaluation.
>> Suppose your program contains a very large lookup table, written as a
>> literal list.  This lookup table is on your program page.
>>
>> Now, suppose....
>
> That is where my mental model diverges!!

The mental model of the computer is what counts.

>> sprintf(buf, "(define %s \"%s\")", "foo", my_str);
>> scm_eval_string(buf);
>> sprintf(buf, "(string-upcase! %s)", "foo")
>> // the string from my_str in "buf" is now scribbled over and completely gone
>> scm_eval_string(buf);
>
> Since I know the program I initially wrote (the define) is now gone,

Why would a define be gone?

> the string must have been copied off somewhere.

I don't think you understand the concept of garbage collection.
_Everything_ in Scheme exists permanently regarding all observable
semantics (well, weak hash tables are a somewhat weird exception).
Definitions, variables, continuations.  There is no concept like a stack
of local values that would get erased.  Thanks to call/cc, there is not
even a return stack that would get erased.  Every object carries its own
lifetime with it.  It dies when nobody remembers it, not because of
being in some scope or whatever else.

> I think one's first guess is that it was copied to someplace
> modifiable.  However, that would be incorrect.  It is copied off to
> writable memory, but marked as read-only for the purposes of Guile.
> Not intuitively obvious.

Also wrong.

-- 
David Kastrup