perlmeditation Aristotle <h2>I feel like a broken record</h2> Are you like me, and often find yourself wanting to include an element in a list conditionally (often depending on whether it is defined)? F.ex., I may want to construct a URI which has a number of moving parts. <ul> <li>In the simplest case, it may look like <tt>http://example.net/app/doc</tt>, where <tt>doc</tt> is a controller in the web app.</li> <li>To link a specific item, the URI might be <tt>http://example.net/app/doc/42</tt></li> <li>Some controllers are broken down further, giving things like <tt>http://example.net/app/doc/42/notes</tt></li> <li>The app supports multiple sites, so I might also have something like <tt>http://example.net/app/joebob/doc</tt></li> <li>To get the editing interface, you need to prefix the whole shebang accordingly, ie. for the <tt>joebob</tt> subsite: <tt>http://example.net/app/admin/joebob/doc</tt></li> </ul> There are two dead obvious approaches to write this in Perl: <readmore> <ol> <li> The ugly [doc://push]-based version: <c> my @part = ( 'http://example.net/app' ); push @part, 'admin' if $is_admin_link; push @part, $subsite if defined $subsite; push @part, $mode; push @part, $id if defined $id; push @part, $submode if defined $submode; my $uri = join '/', @parts; </c> </li> <li> The nicer approach with a ternary: <c> # [updated: originally had `'admin' ? $is_admin_link : ()`] my @part = ( 'http://example.net/app', ( $is_admin_link ? 'admin' : () ), ( defined $subsite ? $subsite : () ), $mode, ( defined $id ? $id : () ), ( defined $submode ? $submode : () ), ); my $uri = join '/', @part; </c> </li> </ol> But both approaches require you to repeat yourself unncessarily. In the [doc://push] case, you have to repeat the <tt>push @part</tt> bit, and in the ternary case, you have to supply <tt>: ()</tt> as an else clause for every case. <h2>Enter <tt>x!!</tt></h2> Obviously, this is a composite operator, consisting of <tt>x</tt> and two <tt>!</tt> negations. The double negation is there to forcibly convert the right side to a boolean value with the same truthness as the original value: <c> $a = undef; print $a ? 'true' : 'false'; # prints 'false' print $a; # prints '' and warns $a = 0; print $a ? 'true' : 'false'; # prints 'false' print $a; # prints '0' $a = 'abc'; print $a ? 'true' : 'false'; # prints 'true' print $a; # prints 'abc' # whereas $a = undef; print !!$a ? 'true' : 'false'; # prints 'false' print !!$a; # prints '' without warning $a = 0; print !!$a ? 'true' : 'false'; # prints 'false' print !!$a; # prints '' $a = 'abc'; print !!$a ? 'true' : 'false'; # prints 'true' print !!$a; # prints 1 </c> So the <tt>!!</tt> will force anything that shows up on the right side to be either <tt>1</tt> or <tt>!1</tt>. The <tt>x</tt> is, of course, Perl’s repetition operator: <c> @_ = ( 'a' ) x 4; print join ':', @_; # prints 'a:a:a:a' print scalar @_; # prints '4' </c> Of interest to us are the cases where the number of repetitions is 1 or 0: <c> @_ = ( 'a' ) x 1; print join ':', @_; # prints 'a' print scalar @_; # prints '1' @_ = ( 'a' ) x 0; print join ':', @_; # prints '' print scalar @_; # prints '0' </c> Since boolean values in Perl are either 1 or !1, that means <tt>( $something ) x $boolean</tt> will do exactly what we want. And we can force everything to be a boolean using the <tt>!!</tt> double negative. That is how we get <tt>x!!</tt>. <ins> Update: caveat codor – as [ikegami] [id://564827|emphasises], the parens are required! <tt>( $something ) x $boolean</tt> does a very different thing from <tt>$something x $boolean</tt>. I knew this, but forgot to harp on it. (Yes, this difference is too subtle. In Perl 6, there will therefore be two different operators, <tt>x</tt> for strings and <tt>xx</tt> for lists.) </ins> Bottom line: the example I gave can be written like this: <c> my @part = ( 'http://example.net/app', ( 'admin' ) x!! $is_admin_link, ( $subsite ) x!! defined $subsite, $mode, ( $id ) x!! defined $id, ( $submode ) x!! defined $submode, ); my $uri = join '/', @parts; </c> Neato! Not only is it nicer, but I also find that <tt>x!!</tt> has a beautifully evocative quality. <tt>:-)</tt> <ins> Update: but note that <tt>( $x ) x!! $cond</tt> differs from <tt>$cond ? $x : ()</tt> in shortcircuiting behaviour: the ternary will avoid evaluating <tt>$x</tt> if <tt>$cond</tt> is false, but <tt>x!!</tt> will always evaluate it. </ins> <h2>Conventional notes</h2> You’ll notice that [doc://defined] always returns a boolean anyway, so the <tt>!!</tt> isn’t actually necessary in three of our cases, and might not be necessary in the first one either. However, I find that one should use <tt>x!!</tt> anyway, for two reasons: <ol> <li>It documents intent. Using <tt>x!!</tt> clearly signals that this is not just a repetition.</li> <li>It protects you from errors. If the value of <tt>$is_admin_link</tt> is 2 for some reason, it is still true according to Perl, but in that case <tt>( 'admin' ) x $is_admin_link</tt> (without <tt>!!</tt>) will yield a very different result from <tt>( 'admin' ) x!! $is_admin_link</tt> (with <tt>!!</tt>).</li> </ol> </readmore> <h2>Conclusion</h2> So there you have it. If you want to conditionally include a sub-list within a larger list, you can use the composite <tt>x!!</tt> operator express exactly that. Makeshifts last the longest.