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

[Malcolm McLean]

<gnuist006@gmail.com> wrote in message 
1192913158.922454.108100@k35g2000prh.googlegroups.com">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.

--
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http://www.personal.leeds.ac.uk/~bgy1mm