# exceptions.pl
# tchrist@convex.com
# 
# Here's a little code I use for exception handling.  It's really just
# glorfied eval/die.  The way to use use it is when you might otherwise
# exit, use &throw to raise an exception.  The first enclosing &catch
# handler looks at the exception and decides whether it can catch this kind
# (catch takes a list of regexps to catch), and if so, it returns the one it
# caught.  If it *can't* catch it, then it will reraise the exception
# for someone else to possibly see, or to die otherwise.
# 
# I use oddly named variables in order to make darn sure I don't conflict 
# with my caller.  I also hide in my own package, and eval the code in his.
# 
# The EXCEPTION: prefix is so you can tell whether it's a user-raised
# exception or a perl-raised one (eval error).
# 
# --tom
#
# examples:
#       if (&catch('/$user_input/', 'regexp', 'syntax error') {
#               warn "oops try again";
#               redo;
#       }
#
#       if ($error = &catch('&subroutine()')) { # catches anything
#
#       &throw('bad input') if /^$/;
sub catch {
    package exception;
    local($__code__, @__exceptions__) = @_;
    local($__package__) = caller;
    local($__exception__);
    eval "package $__package__; $__code__";
    if ($__exception__ = &'thrown) {
        for (@__exceptions__) {
            return $__exception__ if /$__exception__/;
        } 
        &'throw($__exception__);
    } 
} 
sub throw {
    local($exception) = @_;
    die "EXCEPTION: $exception\n";
} 
sub thrown {
    $@ =~ /^(EXCEPTION: )+(.+)/ && $2;
} 
1;