Programming Perl, 4th Edition

The Ellipsis Statement

Beginning with v5.12, Perl accepts a bare ellipsis, “...”, as a stub—that is, a placeholder for code that you haven’t implemented yet. Do not confuse this ... statement with the binary flip-flop ... operator. Perl doesn’t usually confuse them because Perl can tell when it is expecting statements or operators most of the time—but see below.

When Perl parses an ellipsis statement, it accepts it silently. Later, though, if you try to execute it, Perl loudly throws an exception with the text Unimplemented:

sub unimplemented { ... }
eval { unimplemented() };
if ($@ =~ /^Unimplemented/) {
    say "Caught an Unimplemented exception!";
}

You may use the elliptical statement only as a complete statement (though a statement modifier is allowed). These examples are all legal examples of the ellipsis statement:

{ ... }
sub foo { ... }
...;
eval { ... };
... unless defined &dispatcher;
sub somemeth {
    my $self = shift;
    ...;
}
$x = do {
    my $n;
    ...;
    say "Hurrah!";
    $n;
};

However, ... cannot stand in for an expression that is part of a larger statement, since ... is also the three-dot version of the flip-flop operator (see Range Operators in Chapter 3). Hence, the following are all considered syntax errors:

print ...;                                  # WRONG
open(my $fh, ">", "/dev/passwd") or ...;    # WRONG
if ($condition && ... ) { say "Howdy" };    # WRONG

There are times when Perl can’t distinguish an expression from a statement. For example, a bare block and an anonymous hash composer look the same unless there’s something ...