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

From: Jim Langston
Subject: Re: How to exit out of a function ? what is try-catch-throw in terms of Program Counter
Date: Sat, 20 Oct 2007 18:08:11 -0700

<address@hidden> wrote in message 
>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.

Your message post is asking "How to exit out of a function..." yet you are 
showing code that almost exits out of the program.  exit() would do that.

If you wish to exit out of the function then the correct keyword is return. 
Such as:

if (condition)
(do something)

if ( condition )
   (do something )

return can return a value if the function returns a value, such as
return 1;
return foo;
return bar();

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

It depends on coding style.  There is some coding style that states that any 
function will only have one return point.  In which case you would do:

if ( condition )
   ( do something )

Personally, I chose to return early if it's what I would consider an error 

if ( condition )
   return NULL;
( do something )
return Value;

There really is no best way, it depends on coding standard and how easy the 
code is to write, read and maintain.

> (3) What is the intermediate practice to handle this kind of
> problems.

Again, it depends on the problems.  At different times in my career I have 
gone with returning whenever I've wanted to to returning only at the very 
I find that huge if statments, however, make my code harder to write, read 
and maintain, so I will check for conditions early and return early if I 
can.  I try not to return in the middle of a function, but only at the top 
or bottom.  But it depends on the situation.

> 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.

As for try...catch blocks, I try to reserve those only for actual errors. 
One reason I have found I need to use them when a function is supposed to 
return a reference to an object, and that object doesn't exist.  I have to 
return something, and something is not good enough.  So I get around it by 
throwing out of the function.

(Untested code)

Player& FindPlayer( const std::string& Name, std::map<std::string, Player> )
   std::map<std::string, Player>::iterator it = Player.find( Name );
   if ( it == Player.end() )
      // Player was not found in map.  Have to return something.  Lets just 
      throw std::exception("Player not found");
   return (*it).second;

int main()
   // ...
       Player ThisPlayer& = FindPlayer( SomePlayer );
       // use ThisPlayer
    castch ( std::exception e )
       std::cout << "Player " << SomePlayer << " not found.\n";

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