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

[Alf P. Steinbach]

* gnuist006@gmail.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.

C++ does not have a built-in 'exit' command.  There is a library 
function 'exit' which exits the process.  One must assume that's not 
what you mean, and that you're not asking C and C++ programmers to teach 
you Lisp.

Therefore, assuming you want to exit the function or the block.


> (2) What is the best practice to handle this kind of problems?

It's not a general class of problem.

Appropriate solutions depend on the problem at hand.

E.g., in C++,

  // (if (test) (exit) (do something))

  void foo()
  {
      if( !test )
      {
          doSomething();
      }
  }

  void bar()
  {
      if( test ) { return; }
      doSomething();
  }



> (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.

The closest equivalent in C would be a 'longjmp'.  However, a C++ 
exception is more limited, in that it will only jump up the call chain, 
and it's more powerful, in that it will destroy local objects as it does 
so.  Also, if you use 'longjmp' in C++ you're practically doomed (unless 
you use it to jump between co-routines with their own stacks), because 
'longjmp' doesn't destroy local objects.



> I therefore take the liberty to crosspost in C and C++ groups.

Uh.

Cheers, & hth.,

- Alf

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