2012 twenty-four merry days of Perl Feed

Creating Your Own Perl

Syntax::Collector - 2012-12-16
use List::Util qw( reduce );

my @numbers = 1 .. 10;
my $even_sum = reduce { $a + $b } grep { $_ % 2 == 0 } @numbers;

See what I did there? Unlike some functional programming languages, Perl doesn't have a built-in fold or reduce keyword, so I cleverly imported the reduce function from List::Util. (Of course, if I'd been really clever, I'd have noticed List::Util also has a sum function available.)

Due to some trickery with sub prototypes and manipulating its caller's symbol table, List::Util manages to make its reduce function feel just like a built-in language feature. It uses the same codeblock syntax as grep and map, and the same magic $a and $b variables as sort.

Via tricks like these, plus ties, overloads, custom import functions, source filters, Devel::Declare, %^H, and (in newer versions of Perl) the pluggable keyword API, Perl modules have the power to affect their caller in ways far beyond the mechanisms that other programming languages make available. When you use a module that does this, you're not just loading a library and using it at arm's length; you're changing the very syntax of Perl - lexically, within your module.

When starting a new script, or a new module, this is what we do. We add a bunch of use statements to the top of the file to tweak Perl's flavour to our liking. We make Perl a more suitable language for getting the job done; we turn a general purpose programming language into a domain-specific language suitable for our exact task. This will often begin with something like:

use v5.14;
use strict;
use warnings;

but if you're writing anything non-trivial, it's likely that a bunch of other use statements will join them.

(Of course, some modules are plain old object-oriented code that make no attempt to alter their caller's syntax. Different approaches are appropriate for different tasks.)

Twelve Lords A Leaping

Here are some of my favourite syntax-bending modules:

List::Util / List::MoreUtils

List::Util is a core Perl module with a small collection of array munging functions; List::MoreUtils is a collection of extras that didn't quite make the shortlist.

Many of these make creative use of sub prototypes to look and act like Perl's built-in list manipulation functions. The first, uniq and reduce functions are especially useful, and should be in every Perl programmer's toolkit.


PerlX::Maybe provides a tiny function making it easier to work with optional named parameters, a la:

my $santa = Person->new(
        title => "Saint",
        name => "Nicholas",
  maybe telephone => $phone, # $phone might be undef
  maybe email => $email, # $email might be undef


Syntax::Keyword::Junction implements support for something approaching the Perl 6 concept of junctions; that is, variables which have multiple values at once.

my $reindeer = any(qw/
  Dasher Dancer Prancer Vixen Comet Cupid Donder Blitzen
$reindeer eq "Dasher"; # true
$reindeer eq "Prancer"; # true
$reindeer eq "Rudolf"; # false

It achieves this with nothing more than careful use of overloading.


aliased provides short aliases for long class names.

use aliased "Rangifer::Tarandus" => "Reindeer";

my $rudolf = Reindeer->new;

The short alias is just a constant that returns the class name as a string. Simple idea, but useful.


Ever get the Can't call method "isa" on an undefined value blues? Safe::Isa gives you a way to call methods like isa and can on scalars without checking that they are defined and blessed.

# Might return undef if there are no cheap desserts
my $pudding = $menu->find_food(max_price => 5, category => DESSERT);

if ($pudding->$_isa('Plum::Pudding')) {
  say "Yum!";

It takes advantage of the fact that coderefs may be called as methods even on unblessed or undefined invocants.


While eval and $@ can be used as a try-catch mechanism in Perl, there are numerous gotchas. Try::Tiny works around them for you, giving you a nice syntax for exception catching.

try {
catch {
  when (/^Can't call method "give"/) { } # ignore
  default { die $_ }

There are even nicer modules like TryCatch available, but Try::Tiny's zero-dependency approach - it uses nothing more than prototypes and guards (dummy objects with just a destructor) - is perfect for even small projects.


NEXT adds a SUPER-like pseudo-class to your module, but with more control of method redispatch than SUPER gives you. Good if you're programming with multiple inheritance.

These days you should probably use mro instead, but NEXT deserves a mention for its clever use of AUTOLOAD and capitalised package names to create the illusion of new syntax.


This Plack-based web app framework uses a sub prototype hack for dispatching.

sub (POST + /naughty_list/person+ ?name=&*) {
  my ($self, $name, $misc_params) = @_;


Perl's autovivification feature can sometimes be counterintuitive.

my $menu = undef;
exists $menu->{plum}{pudding}; # false
exists $menu->{plum}; # true !!!

The autovivification module can selectively disable autovivification for particular scopes, or get Perl to issue a warning or fatal error when autovivification occurs. Very handy.

Lots of deep XS magic in this module.


Perl has various built-in quote-like operators. qw() constructs arrays; qr() quotes regular expressions and qx() acts like backticks. PerlX::QuoteOperator allows you to define your own.

use PerlX::QuoteOperator qdeer => {
  -with => sub ($) { Reindeer->new(name => $_[0]) },

my $rudolf = qdeer(Rudolf);

PerlX::QuoteOperator uses Devel::Declare to rewrite qdeer(...) to qdeer qq(...) while Perl is compiling your code.


Function::Parameters provides parameter lists for Perl subs. Instead of:

sub give {
  my ($gift, $recipient) = @_;

You can write:

fun give ($gift, $recipient) {

It supports named and positional parameters, optional parameters and methods with invocants. It provides an introspection API, and if you're using Moose, then it can hook into the Moose type constraint system to validate parameter types. Such fun!!

Function::Parameters uses Perl's new(ish) pluggaable keyword API, so is only available for Perl 5.14 and above.


Where to start? MooseX::Declare gives you class and role keywords for declaring Moose classes and Moose roles; extends, with and is for inhertitance, role composition and meta traits; method for declaring methods with signatures; before, after, around, override and augment for method modifiers; and clean for scrubbing away helper functions so that outside code can't call them.

role Flight
  method fly (DateTime $when, Location $where) {

class MagicReindeer extends Reindeer with Flight
  before fly (DateTime $when, Location $where) {
    TimingException->throw("not Christmas Eve")
      unless $when->month == 12 && $when->day == 24;

It uses Devel::Declare. Extensively. And a partridge in a pear tree.

Bundle Up!

If you're working on a large project with many modules, you may find that you are repeating the same set of imports at the top of almost every file. Perhaps something like:

use v5.14;
use strict;
use warnings;
use Try::Tiny;
use Scalar::Util qw( blessed );
use List::Util qw( first reduce );
use List::MoreUtils qw( uniq );
use Path::Class qw( file dir );

OK, so you can copy and paste, but copy-paste code is the enemy. Don't repeat yourself. Wouldn't it be nice to bundle up all the above functionality into a single module?

use My::Syntax;

Well, here's an example of how you could write that module:

package My::Syntax;

use v5.14;
use strict;
use warnings;
use Try::Tiny qw();
use Scalar::Util qw();
use List::Util qw();
use List::MoreUtils qw();
use Path::Class qw();
use import::into;

sub import {
  my $caller = caller;
  feature->import::into($caller, ':5.14');
  Scalar::Util->import::into($caller, 'blessed');
  List::Util->import::into($caller, 'first', 'reduce');
  List::MoreUtils->import::into($caller, 'uniq');
  Path::Class->import::into($caller, 'file', 'dir');


Alternatively, Syntax::Collector makes it a little neater:

package My::Syntax;

use v5.14;
use Syntax::Collector -collect => q/
use strict 1.00 ;
use warnings 1.00 ;
use feature 1.00 qw( :5.14 );
use Try::Tiny 0.11 ;
use Scalar::Util 1.23 qw( blessed );
use List::Util 1.23 qw( first reduce );
use List::MoreUtils 0.33 qw( uniq );
use Path::Class 0.26 qw( file dir );


Yes, that's a big quoted string (q/.../), but no, it's not just evaled.

Bundling up imports into a single module makes it easier to encourage project-wide coding standards. You can't "forget" to enable warnings any more (but of course you can explicitly unimport it). You no longer have any excuse for using ref when you mean blessed, or grep when you want first.

Bundling up imports allows you to consider ideas like true.pm which would seem ridiculous if you needed to repeat them at the top of every file, but become more appealing if they are included as part of an import collection.

And bundling up imports allows you to manage your project's dependencies from a single place. Don't want to depend on List::MoreUtils any more? Then write your own replacement for uniq and get My::Syntax to export that instead. (The Syntax::Collector documentation includes examples of how to write a syntax collection that also acts as an exporter.)

So go on; create your own Perl. Make it your gift to yourself.


Gravatar Image This article contributed by: Toby Inkster <tobyink@cpan.org>