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[RFC] Operator ?! for conditional on exceptions

From: Jose E. Marchesi
Subject: [RFC] Operator ?! for conditional on exceptions
Date: Fri, 24 Dec 2021 18:44:06 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

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:




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:

    int alt1 : ... ;
    int alt2;
    method _print =
      if (alt1 ?! E_elem)

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?

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