Re: Guile: What's wrong with this?

[Bruce Korb]

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!!

> 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,
the string must have been copied off somewhere.  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.

> Guile 1.8's permissivity allowed Bruce to unwittingly create a large
> body of code that was inherently buggy.  IMHO, it would have been much
> better to nip that in the bud and alert him to the fact that he was
> doing something that was almost certainly unwise.

Fail early and fail hard.  Yes.  But after all these discussions, I
now doubt I have too many places where I am expecting to change a
static value.  Most of the strings that I wind up  altering are
created with a scm_from_locale_string() C function call.  Very few
strings are ever actually initialized with (define foo "something"),
other than when creating placeholders because you cannot define
within a nested collection of functions.  e.g.
  (if (whatever)
      (define foo (get "this"))
      (define foo (get "that"))  )
  (string-upcase! foo)

====

Anyway, I did compile and build my toy and guile with CFLAGS='-g -O0'.
The error message did not show.  Instead it seg faulted while trying
to make this call:  scm_from_locale_string("");

There must be a corruption somewhere.  It is either asymptomatic with
Guile 1.8 (viz. my fault) or it is introduced with Guile 2.0 (meaning
a Guile code issue).  More in a few days.

Thank you.