Day 22 – Features of Perl 6.d

So there we are. Two years after the first official release of Rakudo Perl 6. Or 6.c to be more precise. Since Matt Oates already touched on the performance improvements since then, Santa thought to counterpoint this with a description of the new features for 6.d that have been implemented since then. Because there have been many, Santa had to make a selection.

Tweaking objects at creation

Any class that you create can now have a TWEAK method. This method will be called after all other initializations of a new instance of the class have been done, just before it is being returned by .new. A simple, bit contrived example in which a class A has one attribute, of which the default value is 42, but which should change the value if the default is specified at object creation:

class A {
    has $.value = 42;
    method TWEAK(:$value = 0) { # default prevents warning
        # change the attribute if the default value is specified
        $!value = 666 if $value == $!value;
    }
}
# no value specified, it gets the default attribute value
dd A.new;              # A.new(value => 42)

# value specified, but it is not the default
dd A.new(value => 77); # A.new(value => 77)

# value specified, and it is the default 
dd A.new(value => 42); # A.new(value => 666)

Concurrency Improvements

The concurrency features of Rakudo Perl 6 saw many improvements under the hood. Some of these were exposed as new features. Most prominent are Lock::Async (a non-blocking lock that returns a Promise) and atomic operators.

In most cases, you will not need to use these directly, but it is probably good that you know about atomic operators if you’re engaged in writing programs that use concurrency features. An often occurring logic error, especially if you’ve been using threads in Pumpking Perl 5, is that there is no implicit locking on shared variables in Rakudo Perl 6. For example:

   my int $a;
    await (^5).map: {
        start { ++$a for ^100000 }
    }
    say $a; # something like 419318

So why doesn’t that show 500000? The reason for this is that we had 5 threads that were incrementing the same variable at the same time. And since incrementing consists of a read step, an increment step and write step, it became very easy for one thread to do the read step at the same time as another thread. And thus losing an increment. Before we had atomic operators, the correct way of doing the above code would be:

   my int $a;
    my $l = Lock.new;
    await (^5).map: {
       start {
           for ^100000 {
               $l.protect( { ++$a } )
           }
       }
    }
    say $a; # 500000

This would give you the correct answer, but would be at least 20x as slow.

Now that we have atomic variables, the above code becomes:

   my atomicint $a;
    await (^5).map: {
        start { ++⚛$a for ^100000 }
    }
    say $a; # 500000

Which is very much like the original (incorrect) code. And this is at least 6x as fast as the correct code using Lock.protect.

Unicode goodies

So many, so many. For instance, you can now use , , as Unicode versions of <=, >= and != (complete list).

You can now also create a grapheme by specifying the Unicode name of the grapheme, e.g.:

say "BUTTERFLY".parse-names; # 🦋

or create the Unicode name string at runtime:

my $t = "THUMBS UP SIGN, EMOJI MODIFIER FITZPATRICK TYPE";
print "$t-$_".parse-names for 3..6; # 👍🏼👍🏽👍🏾👍🏿

Or collate instead of just sort:

# sort by codepoint value
say <ä a o ö>.sort; # (a o ä ö)
# sort using Unicode Collation Algorithm
say <ä a o ö>.collate; # (a ä o ö)

Or use unicmp instead of cmp:

say "a" cmp "Z"; # More
 say "a" unicmp "Z"; # Less

Or that you can now use any Unicode digits Match variables ( for $1), negative numbers ( for -1), and radix bases (:۳("22") for :3("22")).

It’s not for nothing that Santa considers Rakudo Perl 6 to have the best Unicode support of any programming language in the world!

Skipping values

You can now call .skip on Seq and Supply to skip a number of values that were being produced. Together with .head and .tail this gives you ample manipulexity with Iterables and Supplies.

By the way, .head now also takes a WhateverCode so you can indicate you want all values except the last N (e.g. .head(*-3) would give you all values except the last three). The same goes for .tail (e.g. .tail(*-3) would give you all values except the first three).

Some additions to the Iterator role make it possible for iterators to support the .skip functionality even better. If an iterator can be more efficient in skipping a value than to actually produce it, it should implement the skip-one method. Derived from this are the skip-at-least and skip-at-least-pull-one methods that can be provided by an iterator.

An example of the usage of .skip to find out the 1000th prime number:

say (^Inf).grep(*.is-prime)[999]; # 7919

Versus:

say (^Inf).grep(*.is-prime).skip(999).head; # 7919

The latter is slightly more CPU efficient, but more importantly much more memory efficient, as it doesn’t need to keep the first 999 prime numbers in memory.

Of Bufs and Blobs

Buf has become much more like an Array, as it now supports .push, .append, .pop, .unshift, .prepend, .shift and .splice. It also has become more like Str with the addition of a subbuf-rw (analogous with .substr-rw), e.g.:

my $b = Buf.new(100..105);
$b.subbuf-rw(2,3) = Blob.new(^5);
say $b.perl; # Buf.new(100,101,0,1,2,3,4,105)

You can now also .allocate a Buf or Blob with a given number of elements and a pattern. Or change the size of a Buf with .reallocate:

my $b = Buf.allocate(10,(1,2,3));
say $b.perl; # Buf.new(1,2,3,1,2,3,1,2,3,1)
$b.reallocate(5);
say $b.perl; # Buf.new(1,2,3,1,2)

Testing, Testing, Testing!

The plan subroutine of Test.pm now also takes an optional :skip-all parameter to indicate that all tests in the file should be skipped. Or you can call bail-out to abort the test run marking it as failed. Or set the PERL6_TEST_DIE_ON_FAIL environment variable to a true value to indicate you want the test to end as soon as the first test has failed.

What’s Going On

You can now introspect the number of CPU cores in your computer by calling Kernel.cpu-cores. The amount of CPU used since the start of the program is available in Kernel.cpu-usage, while you can easily check the name of the Operating System with VM.osname.

And as if that is not enough, there is a new Telemetry module which you need to load when needed, just like the Test module. The Telemetry module provides a number of primitives that you can use directly, such as:

use Telemetry;
say T<wallclock cpu max-rss>; # (138771 280670 82360)

This shows the number of microseconds since the start of the program, the number of microseconds of CPU used, and the number of Kilobytes of memory that were in use at the time of call.

If you want get to a report of what has been going on in your program, you can use snap and have a report appear when your program is done. For instance:

use Telemetry;
snap;
Nil for ^10000000;  # something that takes a bit of time

The result will appear on STDERR:

Telemetry Report of Process #60076
Number of Snapshots: 2
Initial/Final Size: 82596 / 83832 Kbytes
Total Time:           0.55 seconds
Total CPU Usage:      0.56 seconds
No supervisor thread has been running

wallclock  util%  max-rss
   549639  12.72     1236
--------- ------ --------
   549639  12.72     1236

Legend:
wallclock  Number of microseconds elapsed
    util%  Percentage of CPU utilization (0..100%)
  max-rss  Maximum resident set size (in Kbytes)

If you want a state of your program every .1 of a second, you can use the snapper:

use Telemetry;
snapper;
Nil for ^10000000;  # something that takes a bit of time

The result:

Telemetry Report of Process #60722
Number of Snapshots: 7
Initial/Final Size: 87324 / 87484 Kbytes
Total Time:           0.56 seconds
Total CPU Usage:      0.57 seconds
No supervisor thread has been running

wallclock  util%  max-rss
   103969  13.21      152
   101175  12.48
   101155  12.48
   104097  12.51
   105242  12.51
    44225  12.51        8
--------- ------ --------
   559863  12.63      160

Legend:
wallclock  Number of microseconds elapsed
    util%  Percentage of CPU utilization (0..100%)
  max-rss  Maximum resident set size (in Kbytes)

And many more options are available here, such as getting the output in .csv format.

The MAIN thing

You can now modify the way MAIN parameters are handled by setting options in %*SUB-MAIN-OPTS. The default USAGE message is now available inside the MAIN as the $*USAGE dynamic variable, so you can change it if you want to.

Embedding Perl 6

Two new features make embedding Rakudo Perl 6 easier to handle:
the &*EXIT dynamic variable now can be set to specify the action to be taken when exit() is called.

Setting the environment variable RAKUDO_EXCEPTIONS_HANDLER to "JSON" will throw Exceptions in JSON, rather than text, e.g.:

$ RAKUDO_EXCEPTIONS_HANDLER=JSON perl6 -e '42 = 666'
{
  "X::Assignment::RO" : {
    "value" : 42,
    "message" : "Cannot modify an immutable Int (42)"
  }
}

Bottom of the Gift Bag

While rummaging through the still quite full gift bag, Santa found the following smaller prezzies:

  • Native string arrays are now implemented (my str @a)
  • IO::CatHandle allows you to abstract multiple data sources into a single virtual IO::Handle
  • parse-base() performs the opposite action of base()

Time to catch a Sleigh

Santa would like to stay around to tell you more about what’s been added, but there simply is not enough time to do that. If you really want to keep up-to-date on new features, you should check out the Additions sections in the ChangeLog that is updated with each Rakudo compiler release.

So, catch you again next year!

Best wishes from

🎅🏾

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