🔬70. Examining the enum type in Perl 6

In Perl 6, you can create enumerations like this:

enum colour <red orange yellow green blue violet>;

Having this said, you can use the new name as a type name and create variables of that type:

my colour $c;

$c = green;
say $c;     # green
say $c.Int; # 3

As you would rightly expect, the type of the variable is very predictable:

say $c.^name; # colour

Now, try to find the class implementation in Rakudo sources. Surprisingly, there is no file src/core/Enum.pm, but instead, there is src/core/Enumeration.pm. Looking at that file, you cannot say how our program works. Let us dig a bit.

In Grammar (src/Perl6/Grammar.nqp), you can find the following piece:

proto token type_declarator { <...> }

token type_declarator:sym<enum> {
    . . .

So, the enum is not a name of the data type but a predefined keyword, one of a few that exist for type declarations (together with subset and constant).

The token starts with consuming the keyword and making some preparations, which are not very interesting for us at the moment:

:my $*IN_DECL := 'enum';
:my $*POD_BLOCK;
    my $line_no := HLL::Compiler.lineof(self.orig(), self.from(), :cache(1));
    if $*PRECEDING_DECL_LINE < $line_no {
        $*PRECEDING_DECL_LINE := $line_no;
        $*PRECEDING_DECL := Mu; # actual declarand comes later, in Actions::type_declarator:sym<enum>

Then, we expect either a name of the new type or a variable or nothing(?):

| <longname>
     . . .
| <variable>
| <?>

The variable part is not yet implemented:

> enum $x <a b c>
===SORRY!=== Error while compiling:
Variable case of enums not yet implemented. Sorry.
at line 2

Our test program falls to the first branch:

      my $longname := $*W.dissect_longname($<longname>);
      my @name := $longname.type_name_parts('enum name', :decl(1));
      if $*W.already_declared($*SCOPE, self.package, $*W.cur_lexpad(), @name) {
          $*W.throw($/, ['X', 'Redeclaration'],
                    symbol => $longname.name(),

For example, if you declare enum colour, then the $longname.name() returns colour colour. Thus, we extracted it. (Also notice how redeclaration is handled.)

Finally, here is the rest of the token body:

{ $*IN_DECL := ''; }
:my %*MYSTERY;
[ <?[<(«]> <term> <.ws> || <.panic: 'An enum must supply an expression using <>, «», or ()'> ]
<.explain_mystery> <.cry_sorrows>

Indeed, we need to explain the mystery here. So, there’s room for optional traits, fine:


There’s another construct that should match to avoid panic messages:

<?[<(«]> <term> <.ws>

Don’t be confused by the different number of opening and closing angle brackets here. The first part is a forward assertion with a character class:

<?  [<(«]  >

It looks if there is one of the <, (, or « opening bracket at this position. The panic message is displayed if it is not found there.

Our next expected guest is a term. Obviously, the whole part <red orange . . . violet> matches with it. Not that bad; what we need to do now is to understand what happens next.

🦋 69. Setting timeouts in Perl 6

In Perl 5, I used to set timeouts using signals (or, at least, that was an easy and predictable way). In Perl 6, you can use promises. Let us see how to do that.

To imitate a long-running task, create an infinite loop that prints its state now and then. Here it is:

for 1 .. * {
    .say if $_ %% 100_000;

As soon as the loop gets control, it will never quit. Our task is to stop the program in a couple of seconds, so the timer should be set before the loop:


for 1 .. * {
    .say if $_ %% 100_000;

Here, the Promise.in method creates a promise that is automatically kept after the given number of seconds. On top of that promise, using then, we add another promise, whose code will be run after the timeout. The only statement in the body here is exit that stops the main program.

Run the program to see how it works:

$ time perl6 timeout.pl
. . .

real 0m2.196s
user 0m2.120s
sys 0m0.068s

The program counts up to about four millions on my computer and quits in two seconds. That is exactly the behaviour we needed.

For comparison, here is the program in Perl 5:

use v5.10;

alarm 2;
$SIG{ALRM} = sub {

for (my $c = 1; ; $c++) {
    say $c unless $c % 1_000_000;

(It manages to count up to 40 million, but that’s another story.)

🔬68. The smartness of the sequence operator in Perl 6, part 1

In Perl 6, you can ask the sequence operator to build a desired sequence for you. It can be arithmetic or geometric progression. All you need is to show the beginning of the sequence to Perl, for example:

.say for 3, 5 ... 11;

This prints numbers 3, 5, 7, 9, and 11. Or:

.say for 2, 4, 8 ... 64;

This code prints powers of 2 from 2 to 64: 2, 4, 8, 16, 32, and 64.

I am going to try understanding how that works in Rakudo. First of all, look into the src/core/operators.pm file, which keeps a lot of different operators, including a few versions of the ... operator. The one we need looks really simple:

multi sub infix:<...>(\a, Mu \b) {
    Seq.new(SEQUENCE(a, b).iterator)

Now, the main work is done inside the SEQUENCE sub. Before we dive there, it is important to understand what its arguments a and b receive.

In the case of, say, 3, 5 ... 11, the first argument is a list 3, 5, and the second argument is a single value 11.

These values land in the parameters of the routine:

sub SEQUENCE(\left, Mu \right, :$exclude_end) {
    . . .

What happens next is not that easy to grasp. Here is a screenshot of the complete function:


It contains about 350 lines of code and includes a couple of functions. Nevertheless, let’s try.

What you see first, is creating iterators for both left and right operands:

my \righti := (nqp::iscont(right) ?? right !! [right]).iterator;
my \lefti := left.iterator;

Then, the code loops over the left operand and builds an array @tail out of its data:

while !((my \value := lefti.pull-one) =:= IterationEnd) {
    $looped = True;
    if nqp::istype(value,Code) { $code = value; last }
    if $end_code_arity != 0 {
        if +@end_tail >= $end_code_arity {
            @end_tail.shift xx (@end_tail.elems - $end_code_arity)
                unless $end_code_arity ~~ -Inf;

            if $endpoint(|@end_tail) {
                $stop = 1;
                @tail.push(value) unless $exclude_end;
    elsif value ~~ $endpoint {
        $stop = 1;
        @tail.push(value) unless $exclude_end;

I leave you reading and understand this piece of code as an exercise, but for the given example, the @tail array will just contain two values: 3 and 5.

> .say for 3,5...11;
multi sub infix:<...>(\a, Mu \b)
List    # nqp::say(a.^name);
~~3     # nqp::say('~~' ~ value);
~~5     # nqp::say('~~' ~ value);
elems=2 # nqp::say('elems='~@tail.elems);
0=3     # nqp::say('0='~@tail[0]);
1=5     # nqp::say('1='~@tail[1]);

This output shows some debug data print outs that I added to the source code to see how it works. The green comments show the corresponding print instructions.

That’s it for today. See you tomorrow with more stuff from the sequence operator. Tomorrow, we have to understand how the list 3, 5 tells Perl 6 to generate increasing values with step 1.

🔬67. Redeclaration of a symbol in Perl 6

Today, we will see how Perl 6 helps to keep our programs better.

Redeclaration of a variable

Examine the following program:

my $x = 1;
my $x = 2;
say $x;

You can immediately see that this program is not entirely correct. Either we meant to assign a new value to $x or to create a new variable with a different name. In either case, compiler has no idea and complains:

$ perl6 redecl.pl 
Potential difficulties:
    Redeclaration of symbol '$x'
    at /Users/ash/redecl.pl:2
    ------> my $x⏏ = 2;

You see a runtime warning, while the program does not stop. Let us find out where it happens in the source code.

When you declare a variable, the grammar matches the corresponding text and calls the variable_declarator action method. It is quite compact but nevertheless I will not quote it completely.

class Perl6::Actions is HLL::Actions does STDActions {
    . . .

    method variable_declarator($/) {
        . . .

    . . .

By the way, you can see here how Perl 6 treats a variable name:

 my $past := $<variable>.ast;
 my $sigil := $<variable><sigil>;
 my $twigil := $<variable><twigil>;
 my $desigilname := ~$<variable><desigilname>;
 my $name := $sigil ~ $twigil ~ $desigilname;

The name of a variable is a concatenation of a sigil, a twigil and an identifier (which is called desigiled name in the code).

Then, if we’ve got a proper variable name, check it against an existing lexpad:

if $<variable><desigilname> {
    my $lex := $*W.cur_lexpad();
    if $lex.symbol($name) {
        $/.typed_worry('X::Redeclaration', symbol => $name);

If the name is known, generate a warning. If everything is fine, create a variable declaration:

make declare_variable($/, $past, ~$sigil, ~$twigil, $desigilname,
                      $<trait>, $<semilist>, :@post);

Redeclaration of a routine

Now, let us try to re-create a subroutine:

sub f() {}
sub f() {}

This may only be OK if the subs are declared as multi-subs. With the given code, the program will not even compile:

===SORRY!=== Error while compiling /Users/ash/redecl.pl
Redeclaration of routine 'f' (did you mean to declare a multi-sub?)
at /Users/ash/redecl.pl:6
------> sub f() {}⏏<EOL>

This time, it happens in a much more complicated method, routine_def:

method routine_def($/) {
     . . .

     my $predeclared := $outer.symbol($name);
     if $predeclared {
         my $Routine := $*W.find_symbol(['Routine'], :setting-only);
         unless nqp::istype( $predeclared<value>, $Routine)
                && nqp::getattr_i($predeclared<value>, $Routine, '$!yada') {
              $*W.throw($/, ['X', 'Redeclaration'],
                        symbol => ~$<deflongname>.ast,
                        what => 'routine',

The exception

The code of the exception is rather simple. Here it is:

my class X::Redeclaration does X::Comp {
    has $.symbol;
    has $.postfix = '';
    has $.what = 'symbol';
    method message() {
        "Redeclaration of $.what '$.symbol'"
        ~ (" $.postfix" if $.postfix)
        ~ (" (did you mean to declare a multi-sub?)" if $.what eq 'routine');

As you see, depending on the value of $.what, it prints either a short message or adds a suggestion to use the multi keyword.

🦋 66. Atomic operations in Perl 6

N. B. The examples below require a fresh Rakudo compiler, at least of the version 2017.09.

Discussing parallel computing earlier or later leads to solving race conditions. Let us look at a simple counter that is incremented by two parallel threads:

my $c = 0;

await do for 1..10 {
    start {
        $c++ for 1 .. 1_000_000

say $c;

If you run the program a few times, you will immediately see that the results are very different:

$ perl6 atomic-1.pl 
$ perl6 atomic-1.pl 
$ perl6 atomic-1.pl 
$ perl6 atomic-1.pl 

Of course, the idea was to increase the counter by 1 million in all of the ten threads, but about ⅓ of the steps were lost. It is quite easy to understand why that happens: the parallel threads read the variable and write to it ignoring the presence of other threads and not thinking that the value can be changed in-between. Thus, some of the threads work with an outdated value of the counter.

Perl 6 offers a solution: atomic operations. The syntax of the language is equipped with the Atom Symbol (U+0x269B) ⚛ character (no idea of why it is displayed in that purple colour). Instead of $c++, you should type $c⚛++.

my atomicint $c = 0;

await do for 1..10 {
    start {
        $c⚛++ for 1 .. 1_000_000

say $c;

And before thinking of the necessity to use a Unicode character, let us look at the result of the updated program:

$ perl6 atomic-2.pl 

This is exactly the result we wanted!

Notice also, that the variable is declared as a variable of the atomicint type. That is a synonym for int, which is a native integer (unlike Int, which is a data type represented by a Perl 6 class).

It is not possible to ask a regular value to be atomic. That attempt will be rejected by the compiler:

$ perl6 -e'my $c; $c⚛++'
Expected a modifiable native int argument for '$target'
  in block  at -e line 1

A few other operators can be atomic, for example, prefix and postfix increments and decrements ++ and --, or += and -=. There are also atomic versions of the assignment operator = and the one for reading: ⚛ (sic!).

If you need atomic operations in your code, you are not forced to use the ⚛ character. There exists a bunch of alternative functions that you can use instead of the operators:

my atomicint $c = 1;

my $x = ⚛$c;  $x = atomic-fetch($c);
$c ⚛= $x;     atomic-assign($c, $x);
$c⚛++;        atomic-fetch-inc($c);
$c⚛--;        atomic-fetch-dec($c);
++⚛$c;        atomic-inc-fetch($c);
--⚛$c;        atomic-dec-fetch($c);
$c ⚛+= $x;    atomic-fetch-add($c,$x);

say $x; # 1
say $c; # 3

🔬65. The EVAL routine in Perl 6, part 2

Welcome back! As you might notice, there was a small gap in the daily post flow.

Before we are back to the Rakudo internals, a couple of words about some changes here. First of all, every post is now marked with either 🦋 or 🔬 (or with indistinguishable rectangles □ if your browser cannot display an emoji :-). These characters mean two categories of posts here: a butterfly stands for Perl 6 syntax, while a microscope is for Perl 6 internals. In the first category, only user-level aspects or Perl 6 are discussed. In the second, we dig into the source codes of Rakudo. All the past post are updated accordingly.

The second change is that I will occasionally post more articles in the Perl 6 syntax category because I found out that non-Russian speakers often like my Russian blog posts. Those posts are mostly short texts explaining interesting features of Perl 6, such as the =~= operator or promises.

* * *

OK, now we have to talk about the EVAL routine. It is defined in the src/core/ForeignCode.pm file as a multi-function. Let us see at their signatures:

proto sub EVAL($code is copy where Blob|Cool, 
               Str() :$lang = 'perl6',
               PseudoStash :$context, *%n)

multi sub EVAL($code, 
               Str :$lang where { ($lang // '') eq 'Perl5' },
               PseudoStash :$context)

Notice that one of the function is a proto, while another is the only multi-candidate. Unlike many other cases that you can see in the sources of Rakudo, this proto routine contains code. Refer to one of the recent blog posts to see how it works.

We start with an example from the first part of the article.

EVAL('say 123');

Here, the passed value is Str, and it is caught by the proto sub, as its first argument can be Cool.

The sub creates a compiler for the given language (which is Perl 6 by default).

my $compiler := nqp::getcomp($lang);

The next step in the sub is to make a string out of $code. In this first example, this task is trivial.

$code = nqp::istype($code,Blob) ?? $code.decode(
    $compiler.cli-options<encoding> // 'utf8'
) !! $code.Str;

Finally, the string is compiled:

my $compiled := $compiler.compile:
    |(:optimize($_) with nqp::getcomp('perl6').cli-options<optimize>),
    |(%(:grammar($LANG<MAIN>), :actions($LANG<MAIN-actions>)) if $LANG);

After compilation, you get an object of the ForeignCode type. This class is a child class of Callable, so the object can be called and returned (actually, it’s not quite clear how it happens):


Now you can understand that single quotes in the second example with curly braces still create an executable code:

EVAL('say {456}');

Here, the whole string is compiled as it was a Perl 6 code, and the code block there is a code block, which Perl should execute, and thus say gets a code block, so it calls its gist method to prepare the output:

> {456}.gist
-> ;; $_? is raw { #`(Block|140388575216888) ... }


🦋64. What does gist do in Perl 6?

When you print an object, say, as say $x, Perl 6 calls the gist method. This method is defined for all built-in types: for some of them, it calls the Str method, for some the perl method, for some types it makes the string representation somehow differently.

Let us see how you can use the method to create your own variant:

class X {
    has $.value;

    method gist {
        '[' ~ $!value ~ ']'

my $x = X.new(value => 42);

say $x; # [42]
$x.say; # [42]

When you call say, the program prints a number in square brackets: [42].

Please notice that the interpolation inside double-quoted strings is using Str, not gist. You can see it here:

say $x.Str; # X<140586830040512>
say "$x";   # X<140586830040512>

If you need a custom interpolation, redefine the Str method:

class X {
    has $.value;

    method gist {
        '[' ~ $!value ~ ']'
    method Str {
        '"' ~ $!value ~ '"'

my $x = X.new(value => 42);

say $x;     # [42]
$x.say;     # [42]

say $x.Str; # "42"
say "$x";   # "42"

Now, we got the desired behaviour.