Re: How to exit out of a function ? what is try-catch-throw in terms of Program Counter

["Malcolm McLean" <regniztar@btinternet.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