Re: [RFC] Operator ?! for conditional on exceptions

[Mohammad-Reza Nabipoor]

Hi, Jose.

On Fri, Dec 24, 2021 at 06:44:06PM +0100, Jose E. Marchesi wrote:
> 
> Hi people!
> 
> I have written an implementation of an "excond" operator ?!, that can be
> found in the branch jemarch/excond in poke.git.
> 
> This operator has two forms:
> 
>   EXPRESSION ?! EXCEPTION
> 
> and
> 
>   COMP_STMT ?! EXCEPTION
> 
> i.e. as the first operator it can get an arbitrary expression or a
> compound statement.
> 
> In both cases, the ?! operator always evaluates to a boolean value.  If
> the execution of the expression or statement raises EXCEPTION, the
> result of the operation is 0 (false).  Otherwise it is 1 (true).  The
> value to which EXPRESSION evaluates is discarded.
> 
> Simple example:
> 
>  if (1/zero ?! E_div_by_zero)
>    print "division by zero\n";
> 
> A motivation for this feature is, given an union, an easy way to
> determine whether a given alternative is the chosen one without having
> to resort to a clumsy try-catch:
> 
>   union
>   {
>     int alt1 : ... ;
>     int alt2;
>   
>     method _print =
>     {
>       if (alt1 ?! E_elem)
>         pretty_print_alt1;
>       else
>         pretty_print_alt2;
>     }
>   }
> 
> An example with a statement:
> 
>   if ({ do_whatever_io_operation; } ?! E_eof)
>     printf "EOF happened\n";
> 
> Using the operator in a statement-expression is also useful.  Consider
> this idiom we use in pkl-rt-1.pk and other places:
> 
>   /* Add the directories in POKE_LOAD_PATH, if defined.  */
>   try load_path = getenv ("POKE_LOAD_PATH") + ":" + load_path;
>   catch if E_inval { }
> 
> This can now be rewritten as:
> 
>   /* Add the directories in POKE_LOAD_PATH, if defined.  */
>   { load_path = getenv ("POKE_LOAD_PATH") + ":" + load_path } ?! E_inval
> 
> What do you people think?  Should we put this support in master?  Is the
> syntax ok?
> 
> 


I like it.
The COMP_STMT version is a little bit unusal, but it's useful. Because
otherwise one should write something like this:

```poke
if (lambda int: { do_whatever_io_operation; return 0; } ?! E_eof)
  print "EOF happend\n";
```

Interestingly enough, Poke accepts this, too (note the `void`):

```poke
if (lambda void: { do_whatever_io_operation; } ?! E_eof)
  print "EOF happend\n";
```

What about supporting more exception codes?

```poke
var not_ok = do_something ?! [E_inval, E_eof];
```