[RFC] Operator ?! for conditional on exceptions

[Jose E. Marchesi]

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?