whyfields - or Modern use of fields.pm and %FIELDS.


Here I try to explain why fields is useful still and also propose alternative way of use %FIELDS.

fields.pm - old story.

fields allows you to extend use strict style typo-check to fields (or slots, properties, attributes, instance variables, member variables.. as you like).

    use strict;
    package Cat {

       use fields qw/name birth_year/; # field declaration

       sub new {
          my Cat $self = fields::new(shift); #  my TYPE $var.
          $self->{name}       = shift; # Checked statically!
          $self->{birth_year} = shift  # Checked statically!
             // $self->_this_year;

       sub age {
          my Cat $self = shift;
          return $self->_this_year
                    - $self->{birth_year}; # Checked statically!

       sub _this_year {
          (localtime)[5] + 1900;

    my @cats = map {Cat->new($_, 2010)} qw/Tuxie Petunia Daisy/;

    foreach my Cat $cat (@cats) {

       print $cat->{name}, ": ", $cat->age, "\n";

       # print $cat->{namae}, "\n"; # Compile-time error!

Above program defines a class Cat, with members {name} and {birth_year}. It also defines constructor new and method age, which computes cat's age from birth_year.

In above program, variables $self, $cat are declared with type annotation Cat like my Cat $self so that detect typos about members at compile time (eg. perl -wc).

Since this typo-check can be applied as soon as program became syntactically correct, you can check it very early stage of development (even when you have no unit tests!). And if you integrate this check to editor's file-save-hook (using flycheck and/or App::perlminlint), you can detect typos just after every file savings.

Why most people do not use fields.pm?

Today, use strict is well known best practice for perl programming. If fields is useful too, why is it rarely used?

Here is my guess list.

Type annotation becomes too long for real world apps

In real world application, most classnames are very long like MyProject::SomeModule::SomeClass. But you wouldn't love to write following:

   my MyProject::SomeModule::SomeClass $obj = ...;

I saw some codes uses __PACKAGE__ like following, but it is still long.

  my __PACKAGE__ $obj = ...;
fields.pm doesn't generate accessors/constructor

In OOP, encapsulation (of internal) is important topic. Directly accessing $cat->{birth_year} outside from Cat is simply violation of OO philosophy.

So we need accessors and constructor. But fields.pm does nothing about them. So you need to use other accessor generator like Class::Accessor, anyway.

fiels.pm was limited to single inheritance.

fields.pm was introduced when perl was 5.005. At that time, it was actually based on ARRAY, so it was single inheritance only.

Then after release of perl5.009, fields::new returns real HASH. But above restriction kept in fields.pm for backward compatibility.

A few tips you should know about fields.

So, you might think Class::Accessor::Fast or Moo... is final answer. But wait, are they check fields typos for you at compile time? I don't think so. (If I'm wrong, please let me know). Also, use of accessor in internal code can slowdown your code (remember perl's sub call is not so fast than simple hash access).

In my humble opinion, compile time typo checking is still strong point of perl5 over other LL like ruby, python and php. I hope more perl mongers cares about this.

Anyway, I want to introduce some facts about fields so that you could stop worrying and start using %FIELDS.

fields works even for unblessed HASH!

Since typo check by fields and type annotation (my Cat $cat) is executed at compile time, actual value in the variable is not limited to instances of annotated class (Cat). Even unblessed HASH can be used.

For example, you can check PSGI $env statically like following (this is a shorthand version of MOP4Import::PSGIEnv):

   use strict;
   use 5.012;
      package Env;
      use fields qw/REQUEST_METHOD psgi.version/; # and so on...

   return sub {
      (my Env $env) = @_;

      if ($env->{REQUEST_METHOD} eq 'GET') { # Checked!
         return [200, header(), ["You used 'GET'"]];
      elsif ($env->{REQUEST_METHOD} eq 'POST') { # Checked!
         return [200, header(), ["You used 'POST'"]];
      else {
         return [200, header()
                , ["Unsupported method $_\n", "psgi.version="
                   , join(" ", $env->{'psgi.version'})]]; # Checked too!

   sub header {
      ["Content-type", "text/plain"]

constant sub can be used as shorthand(type-alias) for my TYPE slot.

In fact, you can shorten type annotation using constant sub which returns actual long class name. So, you can rewrite following:

   # OLD:
   my MyProject::SomeModule::Purchase $obj = ...;

   # NEW:
   sub Purchase () {'MyProject::SomeModule::Purchase'}
   my Purchase $obj = ...;

Some of you may feel above acceptable to write.

And bonus point of having classname sub is that you can use it to abstract-out classname for future overriding in subclass.

  # OLD: Class name is hardcoded.
  sub create_purchase {
    my $self = shift;
    my Purchase $obj = MyProject::SomeModule::Purchase->new(...);
    return $obj

  # NEW: Class name becomes overridable.
  sub create_purchase {
    my $self = shift;
    my Purchase $obj = $self->Purchase->new(...);
    return $obj

Note: Unlike my Purchase $obj, $self->Purchase is method invocation. So later doesn't become constant and subclass can override it.

values of %FIELDS can be anything (at least for now).

Actually fields is an abstraction interface of perl's core interface %FIELDS. For example:

  package Cat;
  use fields qw/name birth_year/;

Above code briefly does following:

  package Cat;
    our %FIELDS;
    $FIELDS{name} = 1;
    $FIELDS{birth_year} = 2;

After the BEGIN {...} block is successfully executed, perl continues compiling rest of the code with knowledge of package Cat and its %FIELDS relation.

When my variable declaration has type alias like my Cat $cat, it is marked with the type. And then compiler find field access like $cat->{name}, it looks up %CAT::FIELDS and checks if $CAT::FIELDS{name} exists. If it exists, field access is valid. If it doesn't, you will get compilation error like following:

  No such class field "namae" in variable $cat of type main::Cat

Interestingly, above story tells nothing about the value of $CAT::FIELDS{name}. Actually, the values are only used in fields.pm to achieve single inheritance restriction. Perl's core itself does not care its content.

I think this means we are able to write our own alternative to fields.pm using %FIELDS, at our own risks. So, let's start experimenting!

(Proposed) Modern use of fields and strict.

Based on the above discussion, here I propose alternative style use of fields (actually %FIELDS) to obtain more typo checking at compile time like use strict. In short:

  • Limited use of direct field access for method implementations.

  • my MY $obj is enough short.

  • generate accessors from %FIELDS and have generic constructor.

Private use of direct field access.

First, we should divide and conquer our problem. In this case, I want to divide it over the boundary of encapsulation. That is "public interfaces" and "private implementations".

To isolate the private implementations from the public interfaces, we must use some kind of accessor (and constructor) subs anyway.

Having such public interfaces, use of direct HASH member access in each method implementations no more harms public interface. It is totally internal matter.

   # Direct HASH access from user code is evil.
   package main;
   my $foo = Foo->new(...);
   print $foo->{width} * $foo->{height}; # The evils.

   # In implementation code, direct HASH access has no problem.
   package Foo {
     use fields qw/width height/;
     sub area {
       my Foo $self = shift;
       $self->{width} * $self->{height};
   # Users of `Foo->area` doesn't care
   # whether it is direct access or accessor call.
   print $foo->area;

For example, suppose you once provided an accessor method width() and later want to add a read-logging for it, you can rename the field width to _width and change accessor width() to provide the logging. This change is totally invisible to class users.

Also note when you change the field width to _width, perl will tell you which line should be changed compile-time!

my MY $obj

Second, I propose shorthand name MY as default type alias. Then every method argument declaration starts like (my MY $self, ...) = @_. I hope this will be short enough style change to adapt, especially if you are already familiar with my (it's only 3 chars increase!). To define this alias, just write sub MY () {__PACKAGE__} at the beginning of your package.

  package MyApp::Model::Company::LongLongProductName {
    sub MY () {__PACKAGE__}; # This!
    use fields qw/price/;

    sub add_price {
      (my MY $self, my $val) = @_;
      $self->{price} += $val;

I propose this because IMHO spending time for a shorthand type name of merely $self doesn't make sense and having compile-time checking for $self is far more important than it. (Do you really want to change variable name of the $self for every classes? Life is short, isn't it?)

Of course if you already reached a good type-alias naming for your current code, just use it. This tip is about time-saving.

accessor generator + generic constructor

Third, let's generate accessors from %FIELDS. As noted before, compile-time field checking doesn't rely on the value of $FIELDS{$field_name}. So we can put our nice field specifications (like readonly, default value, type ...) there, at own risks;-).

Also, to make our accessors really useful, we should also provide a base constructor which is designed to work well with the accessors. Because accessors are about object states(fields/properties/attributes...) and constructor defines initial state of the object.

In this document, I will show you a tiny getter generator, a base updater method (named configure) and a base constructor. It is rooted in "Perl/Tk" and tcl/tk widget API. Such object is used like followings:

   # key => value list
   my $obj = Foo->new(width => 8, height => 3);

   # HASH is ok too.
   $obj = Foo->new({width => 8, height => 3});

Once object is created, its properties(fields) are fetched by name.

   print $obj->width * $obj->height; # => 24

To update its properties, invoke configure with key-value pair list.

   $obj->configure(height => 3, width => 3);

   print $obj->width * $obj->height; # => 9
  • Getters are generated only for fields which have name starting with [A-Za-z]. Other fields are private.

  • In this style, I generate only getters from fields declaration.

  • If you want to have complex getter, change the field name starting with '_' and write down your getter by hand.

       sub dbh {
         (my Foo $foo) = @_;
         $foo->{_dbh} //= do {
            DBI->connect($foo->{user}, $foo->{password}, ...);
  • For updating, I define general purpose updater configure in base class. And it eventually calls onconfigure_... hooks if it exists.

       sub onconfigure_file {
         (my Foo $foo, my $fn) = @_;
         $foo->{string} = read_file($fn);
  • To set default values, define after_new in your class. (There would be better name though :-<).

       sub after_new {
         (my Foo $foo) = @_;
         $foo->{name}       //= "(A cat not yet named)";
         $foo->{birth_year} //= $foo->default_birth_year;
       sub default_birth_year {

Here is a sample implementation of above behavior.

Note: below doesn't care about subclassing. To achieve it, you must merge %FIELDS from base class. Easiest way to do it is using base. Alternatively, you may implement some kind of equiv of "declare_as_base" in MOP4Import::Declare.

    use strict;
    use 5.009;
    package MyProject::Object { sub MY () {__PACKAGE__}
       use Carp;
       use fields qw//; # Note. No fields could cause a problem.
       sub new {
         my MY $self = fields::new(shift);
         $self->configure(@_) if @_;
       sub after_new {}
       sub configure {
          my MY $self = shift;
          my (@task);
          my $fields = _fields_hash($self);
          my @params = @_ == 1 && ref $_[0] eq 'HASH' ? %{$_[0]} : @_;
          while (my ($name, $value) = splice @params, 0, 2) {
            unless (defined $name) {
              croak "Undefined key for configure";
            unless ($name =~ /^[A-Za-z]\w*$/) {
              croak "Invalid key for configure $name";
            if (my $sub = $self->can("onconfigure_$name")) {
              push @task, [$sub, $value];
            } elsif (not exists $fields->{$name}) {
              confess "Unknown configure key: $name";
            } else {
              $self->{$name} = $value;
          $$_[0]->($self, $$_[1]) for @task;
       sub _fields_hash {
         my ($obj) = @_;
         my $sym = _globref($obj, 'FIELDS');
         unless (*{$sym}{HASH}) {
           *$sym = {};
       sub _globref {
         my ($thing, $name) = @_;
         my $class = ref $thing || $thing;
         no strict 'refs';
         \*{join("::", $class, defined $name ? $name : ())};
       # Poorman's MOP4Import::Declare.
       sub import {
          my ($myPack, @decls) = @_;
          my $callpack = caller;
          *{_globref($callpack, 'ISA')} = [$myPack];
          foreach my $decl (@decls) {
             my ($pragma, @args) = @$decl;
             $myPack->can("declare_$pragma")->($myPack, $callpack, @args);
       sub declare_fields {
         my ($myPack, $callpack, @names) = @_;
         my $fields = _fields_hash($callpack);
         foreach my $name (@names) {
           $fields->{$name} = 1; # or something more informative.
           *{_globref($callpack, $name)} = sub { $_[0]->{$name} };

Here is user code of above base class.

    package MyProject::Product; sub MY () {__PACKAGE__}
    use MyProject::Object [fields => qw/name price/];