From: "Malcolm McLean" <regniztar@btinternet.com>
To: help-gnu-emacs@gnu.org
Subject: Re: How to exit out of a function ? what is try-catch-throw in terms of Program Counter
Date: Sun, 21 Oct 2007 09:03:17 +0100 [thread overview]
Message-ID: <mtqdnWxfgK9ZmobanZ2dnUVZ8tyqnZ2d@bt.com> (raw)
In-Reply-To: <1192913158.922454.108100@k35g2000prh.googlegroups.com>
<gnuist006@gmail.com> wrote in message
news:1192913158.922454.108100@k35g2000prh.googlegroups.com...
>I have some code like this:
>
> (if (test)
> (exit)
> (do something))
>
>
> or
>
> (if (test)
> ( do something)
> (exit))
>
>
> Various levels of nestings.
>
> I have several questions, basic to sophisticated.
>
> (1) What is the lisp equivalent idiom for (exit) as in bash or
> in C.
> (2) What is the best practice to handle this kind of problems?
>
> (3) What is the intermediate practice to handle this kind of
> problems.
>
> NOTE: I am really afraid of try-catch-throw. I have never been
> able to understand it since it does not exist in C and I cant
> really visualize the construct in terms of C. That is what my
> brain can process. If you understand it so well, you can show
> me how one would really implement that kind of construct in
> C and then by extension I can see that kind of program flow
> in LISP. Whether its imperative programming or functional,
> beneath there is program counter and assembly. C is close
> to machine so much that it is almost assembly. So understanding try-c-
> t in C is equivalent to understanding at
> the level of machine language.
>
> I therefore take the liberty to crosspost in C and C++ groups.
>
C++ compilers that compile to C will produce C code something like this
C++
double safesqrt(double x)
{
if(x < 0)
throw "imaginary root";
return sqrt(x);
}
double foo()
{
if( pow( sqrt(-1), sqrt(-1)) != exp(-M_PI/2) )
printf("Stupid computer can't do basic maths\n");
}
int main(void)
{
try
{
foo();
}
catch(char * err)
{
printf("Sorry %s\n", err);
}
}
C
void * safesqsrt(int *type, double *ret, double x)
{
if(x < 0)
{
*type = CHARSTAR;
return "imaginary root";
}
* ret = sqrt(x);
return 0;
}
void *foo(int *type)
{
double temp;
void *throw;
int throwtype;
throw = safesqrt(&throwtype, &temp, -1.0);
if(throw)
{
*type - throwtype;
return throw;
}
/* etc */
}
int main(void)
{
char *throw;
int throwtype;
char *err;
throw = foo(&throwtype);
if(throw)
goto catch;
return 0;
catch:
switch(throwtype)
{
case CHARSTAR:
err = throw;
printf("Sorry %s\n", err);
break;
}
}
As you see it is totally impractical to try to do this in handwritten C
code for very long, though a compiler will happily chug through it. There
are in fact more subtleties - local objects in foo() and safesqrt() have to
be destroyed.
--
Free games and programming goodies.
http://www.personal.leeds.ac.uk/~bgy1mm
next prev parent reply other threads:[~2007-10-21 8:03 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-10-20 20:45 How to exit out of a function ? what is try-catch-throw in terms of Program Counter gnuist006
2007-10-20 20:57 ` gnuist006
2007-10-20 22:55 ` Alf P. Steinbach
2007-10-23 18:06 ` gnuist006
2007-10-23 19:53 ` Alf P. Steinbach
2007-11-04 23:02 ` David Thompson
2007-11-05 5:07 ` Alf P. Steinbach
2007-11-05 5:59 ` Alf P. Steinbach
2007-12-02 23:20 ` David Thompson
2007-12-02 23:51 ` Alf P. Steinbach
2007-10-21 1:08 ` Jim Langston
2007-10-21 2:30 ` Keith Thompson
2007-10-21 10:44 ` Kenny McCormack
2007-10-21 10:55 ` santosh
2007-10-21 12:26 ` Richard Heathfield
2007-10-21 22:35 ` Mark McIntyre
2007-10-21 12:09 ` Malcolm McLean
2007-10-21 15:23 ` Kenny McCormack
2007-10-21 15:54 ` santosh
2007-10-21 17:39 ` Malcolm McLean
2007-10-21 21:53 ` Kenny McCormack
2007-10-21 8:03 ` Malcolm McLean [this message]
2007-10-21 16:07 ` abhy
2007-10-21 17:43 ` santosh
2007-10-23 9:04 ` Joel Yliluoma
2007-10-23 16:33 ` Stefan Monnier
2007-10-23 16:44 ` gnuist006
2007-10-23 16:45 ` Victor Bazarov
2007-10-24 1:06 ` Stefan Monnier
2007-10-23 16:54 ` gnuist006
2007-10-23 17:14 ` Alf P. Steinbach
2007-10-24 1:09 ` Stefan Monnier
2007-10-24 0:02 ` Joel Yliluoma
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