<gnuist006@gmail.com> wrote in message
news:1192913158.922454.108100@k35g2000prh.googlegr oups.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