Block syntax for Perl package definitions


Figuring out a feature that’s there since almost 12 years.

Often times I use iterators in Perl to encapsulate an iterative behaviour that might go on indefinitely, or that I’d like to consume one item at a time (as opposed to exausting the whole iteration at once). This is usually done with a closure, like this:

sub iterate_by2 ($n) {
    $n -= 2;
    return sub { return $n += 2 }

OK, OK, there’s a lot to be desired regarding corner cases but you get the idea.

I consistently found myself in a different state of mind while translating these things in Raku. I mean, they might be translated almost by the letter, but I always felt that a class was the way to go:

class IterateBy2 {
    has $!n;
    submethod TWEAK (:$n) { $!n = $n - 2 }
    method get()          { return $!n += 2 }

(I hope the code above works, I didn’t test it).

This is also what happened in previous post Portable(ish) Random(ish) Number Generator, which anyway left me with a bit of itch in the Perl implementation because I had three different behaviours scattered over three different functions:

  • the seed_to_num function to turn a string into an integer
  • the actual iterator randomish_uint32_it
  • the iterator transformer to get random bits get_bit.

The underlying reason why I have this different approach, I think, lies in the lack of a native, dedicated syntax for declaring classed and from the fact that I started dabbling seriously when 5.8 was out, i.e. at a time where a lot of the new features were not around.

In time, I tended to avoid the features in the hope of preserving compatibility with older versions, especially because often times I did not know which perl I would have found at some location. So I only adopted the defined-or // syntax (although not always) and, more recently, the signatures by requiring that my stuff runs at least in v5.24.

In this specific case, I was disregarding two facts:

  • package definitions can be scoped to an enclosing BLOCK (something I figured out consciently only recently, with A block… blocks)

  • since perl v5.14, released May 2011, we also have the package NAME { ... } syntax in our toolbox.

And yet, it’s still somehow written in my mind that I have to change package all along, which is the exact barrier that often times prevents me from using the object-oriented approach.

So, without further ado, it’s time to translate Randomish from Raku back to Perl, in true object-oriented glory:

package Randomish {
   sub new ($package, $seed = undef) {
      my $self = bless {}, $package;
      say "pre<$seed>";
      if (! defined($seed)) { $self->{s} = time() }
      elsif ($seed =~ m{\A(?: 0 | [1-9]\d* )\z}mxs) { $self->{s} = $seed }
      else {
         my $val = 0;
         $val = ($val << 8) | ord(substr($seed, $_))
            for 0 .. length($seed) - 1;
         $self->{s} = $val & 0xFFFFFFFF;
      say "pos<$self->{s}>";
      return $self;

   sub uint32 ($self) {
      $self->{s} = ($self->{s} * 1664525 + 1013904223) & 0xFFFFFFFF;

   sub bit ($self) { $self->uint32 & 0x80000000 ? 1 : 0 }

Bad habits are hard to remove because they’re hard to spot.

Stay safe!

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