Yes, but don’t!

February 15, 2023 Leave a comment

masukomi, likes to play with fire and who am I to stop him? In fact, I shall aid him by answering his question: “#RakuLang is there a way to augment / monkeypatch a class to give it another parent class ?”.

There are a few obstacles. First, a class tends to be composed when we get hold of it and secondly, the list of parents is in fact a List. Both problems vanish when we use nqp.

class A1 { }
class A2 { method cheating { say ‘Yes, but don't!’ } }

class B is A1 { }

use nqp;

my \parents := nqp::getattr(B.HOW, Metamodel::ClassHOW, '@!parents');
nqp::push(parents, A2);

dd parents; # (A1, A2)
say B.^mro; # ((B) (A1) (A2) (Any) (Mu)); # Yes, but don't!

In addition to re-.compose we have to re-compute the MRO, as there is some caching going on. In fact, you should expect to upset the compiler quite a bit when fooling around with things that a meant to be rather static. If you burn yourself with this spell … well, it’s a level 9 fire spell after all.

The proper way to get the same result would be as follows.


class A3 { }

augment class B { also is A3 }

# ===SORRY!=== Error while compiling /home/dex/projects/raku/tmp/2021-03-08.raku
# Parents cannot be added to class 'B'after it has been composed

If I can muster the courage I shall challenge jnthn with a Rakubug.

Categories: Raku

Recursive Cinderella

October 3, 2022 Leave a comment

PWC 184 asked to split an array into numbers and letters.

for ( 'a 1 2 b 0', '3 c 4 d'), ( '1 2', 'p q r', 's 3', '4 5 t') -> @a is copy {
    @a.=map: *.split(' ').cache;
    my @numbers = @a.deepmap({ / <[0..9]> / ?? .Numeric !! Empty }).grep(+*)».Array;
    my @letters = @a.deepmap({ / <[a..z]> / ?? .Str !! Empty }).grep(+*)».Array;
    say @numbers, ‘ and ’, @letters;

I use deepmap with Empty to separate to weed from the chaff and remove the extra structure added by deepmap with a grep. That extra structure raised the question, what would be needed to split a tree in twain. This could be useful to gain parts of a XML Document with a single parser pass, while maintaining the structure of the tree. Relation of child nodes and parent nodes often carries meaning.

for ( 'a 1 2 b 0', '3 c 4 d'), ( '1 2', 'p q r', 's 3', '4 5 t'), [ '1', [ [ 'a' ], '2 b'], '3 c' ] -> @a is copy {

    multi sub cinderella(@data, @a, $needle-a, @b, $needle-b) {
        for @data {
            @a[$++] := my $new-a;
            @b[$++] := my $new-b;
            cinderella($_, $new-a, $needle-a, $new-b, $needle-b)

    multi sub cinderella(@data, Any:U $a is raw, $needle-a, Any:U $b is raw, $needle-b) {
        my @new-a;
        my @new-b;
        for @data {
            cinderella($_, $a, $needle-a, $b, $needle-b);
            @new-a.push: $a;
            @new-b.push: $b;
        $a = @new-a;
        $b = @new-b;

    multi sub cinderella(Str:D $s, $a is raw, $needle-a, $b is raw, $needle-b) {
        cinderella($_, my @new-a, $needle-a, my @new-b, $needle-b) for $s.split(' ');
        $a = @new-a ?? @new-a.join(' ') !! Empty;
        $b = @new-b ?? @new-b.join(' ') !! Empty;

    multi sub cinderella(Str:D $s where *.chars == 1, @a, $needle-a, @b, $needle-b) {
        given $s {
            when $needle-a { @a.push: $s }
            when $needle-b { @b.push: $s }
            default { fail('dunno where to put: ' ~ $s) }

    cinderella @a, my @numbers, / <[0..9]> /, my @letters, / <[a..z]> /;

    dd @numbers, @letters;

# OUTPUT: Array @numbers = ["1 2 0", "3 4"]
          Array @letters = ["a b", "c d"]
          Array @numbers = ["1 2", Empty, "3", "4 5"]
          Array @letters = [Empty, "p q r", "s", "t"]
          Array @numbers = ["1", [[[],], "2"], "3"]
          Array @letters = [[], [["a"], "b"], "c"]

The leaves in the target tree can be either a Str or (empty) Array. However, the first multi candidate doesn’t make the decision what item is added to the target Arrays. Instead I create a fresh scalar container and bind it to a container within the Arrays. I then pass that container on to the next multi-candidate where it might be filled with Positional– or Str-object. Please note the distinction. MMD doesn’t care about the container type we use, it looks for the properties of the value. This allows me to split the strings on a white space and pass it on into the next round of MMD matches. The 2nd candidate handles the case where we descent into a nested Array. It can manipulate the scalar container created with @a[$++] := my $new-a; and turn it into a Positional value (here an Array), because that initial container is passed into the multi with is raw.

This is a very powerful concept. Writing the same with a single recursive function would contain a lot of special casing and be no joy to debug.

Not doing what the instructor asked for seems to produce better results. I shall do so more often.

Categories: Raku


September 29, 2022 1 comment

With PWC 182-2 specifically asking for Linux paths to be handled, we need to resolve issues like /../ and symbolic links. Since I didn’t feel like putting a directory called a into my root folder, I wrote a little helper that deals with some of the tripwires that modern filesystems provide.

my @input = qw <

sub resolve(Str:D() $s){
    my @parent-refs = (my @parts = $s.split(rx{ ‘/./’ | ‘/’+ })).pairs.grep(*.value eq '..')».key;
    @parent-refs = flat @parent-refs, @parent-refs »-» 1;

# OUTPUT: /a/b/c/1/ /a/b/c/d/e/2/ /a/b/c/d/3/ /a/b/c/4/ ///a/b/c/d/5/

The last path starts with a triple root, because join assumes holes are actually there. It won’t skip fields that are Empty either, so is default(Empty) doesn’t help. To actually remove elements from an Array we are better of with splice. Since this method doesn’t return self, we can’t just @parts.splice(@parent-refs.any,1).join(‘/’). Both ways to remove elements are mutators and eager. That doesn’t fit well into the rest of the language and spells doom for concurrency. To find a solution I had to go down the rabbit hole that is iteration in Rakudo. The bottom happens to be located in Rakudo/Iterator.pm6.

method pull-one() is raw {
      nqp::islt_i($!i,nqp::elems($!reified)), # found a hole

So a hole in an Array is just a null-pointer in C-land — given that we didn’t overshoot the end of the Array. With that knowledge, building an Iterator that skips elements becomes rather simple.

multi sub prune(@a, Int:D $i --> Seq:D) {
    prune @a, $i .. $i

multi sub prune(@a, +@l is copy --> Seq:D) {
    @l = @l.sort; # this makes checking the index against the needles simpler class :: does Iterator {
        has int $!i;
        has $!reified;
        submethod !SET-SELF(\arr) {
            $!reified := nqp::getattr(@a,List,'$!reified');
            $!i = -1;
        method new(\arr) { nqp::create(self)!SET-SELF(arr) }
        method pull-one is raw {
            loop {
                if @l {
                    @l.shift while +@l && $!i > @l[0].max;
                    next if +@l && @l[0].min ≤ $!i ≤ @l[0].max;
                return nqp::ifnull(
                    nqp::atpos($reified, $!i),
                        nqp::isge_i($!i, nqp::elems($reified)),
                        next # we actually got a hole

@a = ('a' .. 'z').List;

dd @a.&prune( 25 );
dd @a.&prune( 10..15 );
dd @a.&prune( (2,3,10..15, 21..22, 25).pick(5) ); # randomising for testing
# OUTPUT: ("a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y").Seq
#         ("a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z").Seq
#         ("a", "b", "e", "f", "g", "h", "i", "j", "q", "r", "s", "t", "u", "x", "y").Seq

The idea is to loop by default and bail out of that loop if the current index held in $!i is not found in any needle. Since we don’t got real goto in Raku, loop/next becomes a decent substitute. As I don’t really understand what a unreified Array is, I’m right now not qualified to provide a PR.

Dealing with lazy and infinite lists makes Array a beast. I shall not falter until I have tamed it!

Categories: Raku

Valid temperatures

September 5, 2022 1 comment

PWC 181 Task 2 asks for input validation of dates and numbers. OK, it should ask for input validation so my solution isn’t totally over the top. Better safe then sorry.

my $data = q:to/EOH/;
2022-08-01, 20
2022-08-09, 10
2022-08-03, 19
2022-08-06, 24
2022-08-05, 22
2022-08-10, 28
2022-08-07, 20
2022-08-04, 18
2022-08-08, 21
2022-08-02, 25

==> map(* ~~ / $<date> = [ \d+ ‘-’ \d?\d ‘-’ \d?\d ] \s* ‘,’ \s* $<temperature> = [ '-'?\d+ [ '.' \d+ ]? ] /)
==> map(-> (Date(Str:D(Match)) :$date, Numeric(Str:D(Match)) :$temperature, *%) { [ $date, $temperature ] })
==> sort(*.first)
==> map( -> [$date, $temperature ] {
    state $last-temp;
    LEAVE $last-temp = $temperature;
    once next;
    „$date, Δ{abs $last-temp - $temperature}°C“ if $temperature > $last-temp;
==> -> *@a { @a.join($?NL) }()
==> put();

First I use a regex with named captures to get hold of the date and temperature Strs. Those are not overly helpful, as I need to sort by date. The 2nd map solves that problem with nested coercions in a destructuring sub-signature. As Match contains a few extra methods that I don’t care for, I need to gobble those up with an anonymous hash slurpy. Now I have a LoL with a date and temperature sub-list, where the date is a Date and the temperature is something Numeric. I sort by the first and then destructure again. I use a state container to keep a link between consecutive iteration steps. (To make this hyper-friendly, we would need to .rotor.) I always want $last-temp to contain the $temperature of the previous iteration but I don’t want to do anything else in the first iteration. Hence, the LEAVE-phaser and the once. Anything else means to create the output text. Since I don’t want a combo breaker, I use a pointy-block like we need to in a feed-operator-chain to add newlines where they belong.

If you managed to stuff more Raku-features into your solution, you out-convoluted me and I would love to see your code.

Joking aside, I believe to have another candidate for macro-ideas.txt. If we look at the first and second map, there is a pattern. We use the same named-arguments in both. With RakuAST it might be possible to provide a Regex, a block and a list of coercions, to produce a Sub that does that in one go. That would be a neat shortcut for simple parsers that deal with lines of text.

Categories: Raku

Assuming optionality

September 4, 2022 1 comment

PWC 180 Task 1 asks us to find the first unique character in a string. I wanted to have a nice interface where I would write:

say $str.comb.first: &unique-char;

The idea was to curry postcircumfix:<{ }> so it will be bound to a BagHash and always ask for :!exists. Alas, .assuming doesn’t do the right thing if the proto contains optional positions. I found a workaround utilising once.

for ‘Perl Weekly Challenge’, ‘Long Live Perl’ -> $str {
    my &unique-char = { (once $str.comb.BagHash»--){$_}:!exists }
    say $str.comb.first: &unique-char;

I don’t want to build the BagHash and remove single elements every time unique-char is called. There is a slight overhead when using once but that beats .assuming by a mile.

Given all the special cases Signatures provide, we may want to consider turning .assuming into a RakuAST-macro.

Categories: Raku


August 28, 2022 1 comment

While reading solutions to the PWC, I spotted a pattern. There where plenty of .map-calls that contained only simple maths. It must not be so! Task 179-2 leans itself to use vector operations.

use v6.d;

constant @sparks = "▁" .. "█";
constant \steps = +@sparks - 1;

multi MAIN(
    #| space separated list of numbers
) {
    my $scale := @data.&{ (.max - .min) / steps };
    put @sparks[((@data »-» @data.min) »/» $scale)».round].join;

multi MAIN(
    #| output an example sparkline-graph
    Bool :$test
) {
    .&MAIN for <2 4 6 8 10 12 10 8 6 4 2>, <0 1 19 20>, <0 999 4000 4999 7000 7999>;

First, I define a character range (no need for .ord here) and store the number of bar-graph steps. Then I calculate the scale like it is used in a map (those paper-things, that always work and don’t break when dropped). Now I can use that scale to shrink (or grow) values. I believe rounding is better here then to cut of to integers.

When we feed a list to a postcircumfix:<[ ]> we get a list of values in return. However, we break the method-call chain when we do so. Since operators are just Subs with a funny syntax, we can solve that issue easily.

constant &sparks = {
    constant @sparks = "▁" .. "█";
    constant \steps = +@sparks - 1;

    &postcircumfix:<[ ]>.assuming(@sparks) but role :: { has $.steps = steps }

my $scale := @data.&{ (.max - .min) / &sparks.steps };
((@data »-» @data.min) »/» $scale)».round.&sparks.join.put;

The constants are hidden within a block-scope and only steps is exposed as part of a Subs “namespace”. By declaring &sparks as constant, I move the currying to compile time.

Hypers are not getting much use in PWC-solutions. I believe another entry in raku-did-you-know is in order.

Categories: Raku

Suspicious introspection

August 22, 2022 Leave a comment

One of my routines is acting suspicious. I’m almost certain it wilfully misinterprets arguments just so it can throw an exception. There is a way to spy on it.

sub suspicious($a, :$b, *@rest, *%opts) {

    CATCH {
        put (my $sig = &?ROUTINE.signature.gist) ~ ' -> ' ~ $sig.trans(<*@ *%> => <@ %>).EVAL».raku.join(',');

suspicious(1, :2b, 3,4,5);
# OUTPUT: ($a, :$b, *@rest, *%opts) -> 1,:b(2),3 4 5,
            in sub suspicious at tmp/2021-03-08.raku line 1910
            in block <unit> at tmp/2021-03-08.raku line 1917

Please note, that the CATCH-block doesn’t do anything to the exception. It is just there to trigger the debugging code. The proper place to put that would be a macro. Alas, macro can’t inject CATCH-blocks that are actually used as exception handlers. It will have to wait for RakuAST, what would also allow me to replace the EVAL-shenanigans with proper code. Another entry in macro-ideas.txt until then.

Categories: Raku

Defeating definedness

August 18, 2022 1 comment

In his latest blogpost, p6steve went through a lot of trouble to enforce definedness. I managed to shorten it a bit.

None: Nil;
subset Option of Any where Any:D | None;

my Option $defined = 42;
my Option $nothing = None;
my Option $undefined = Mu;

.&dd for $defined, $nothing, $undefined;

# OUTPUT: Type check failed in assignment to $undefined; expected Option but got Mu (Mu)

I would really like to see the content of $nothing, and while I’m on it show why Steve’s hard labour was in vain.

try {
    my Option $defined = 42;
    my Option $nothing = None;
    my Option $undefined = Mu;

    .&dd for $defined, $nothing, $undefined;

    CATCH { default { note .message; .resume } }

# OUTPUT: Type check failed in assignment to $undefined; expected Option but got Mu (Mu)
#         Int $defined = 42
#         Label $nothing = => "None", file => "tmp/2021-03-08.raku", line => 1880)
#         Any $undefined = Any

Raku is a dynamic, late bound language. There are language constructs, like resumable exceptions and NativeCall, that can defeat any type-check. Trying to use Raku like Rust, Haskell or any other strictly typed language will eventually fail. No amount of :Ds will substitute testing, testing and even more testing.

I hope not to have tested p6steve’s patience and try to calm the waves with a new repo.

Categories: Raku


August 14, 2022 1 comment

On IRC deoac wished to know how to print the name of a variable. This question is ambiguous. To get the name of the container is easy.

my $foo;
say $;
# OUTPUT: $foo

If binding or constants involved, things get odd or fail.

my $bar := $foo;
constant $never-changes = 'war';
# OUTPUT: $foo
          No such method 'name' for invocant of type 'Str'.  Did you mean any of
          these: 'Date', 'Num', 'are', 'none', 'note', 'take'?

The second meaning of “variable” might be “symbol” for deoac’s use case. Getting hold of a symbol name requires early compile time measures.

use experimental :macros;
macro symbol-name($symbol) {
    quasi { $symbol.Str }

say symbol-name($foo);
say symbol-name($bar);
say symbol-name($never-changes);
# OUTPUT: ($foo)

The value of $symbol is an AST object and stringifies (in this case) to a list of Str. I didn’t dig deeper, because with RakuAST things will change.

However, this once again shows that we might do better by explaining what my actually does instead of assuming that one can guess what we mean when using the term “variable”.

Categories: Raku

Swarming Sundays

July 30, 2022 1 comment

To not miss any Sundays, PWC 175 is asking us to find them. Raku helps us a great deal, because we can easily find the last day in a month, calculate it’s distance to the next Sunday and turn the clock back by that many days.

for 2022 -> $year {
    for 1..12 -> $month {
        my $last-day-in-month =$year, $month, *);
        my $Δsunday = $ % 7;
        say $last-day-in-month.earlier(:days($Δsunday));

If you follow this blog, you will have spotted my distaste for simple loops. They get in the way of using interesting language features and thus fun. will only ever return a single date. With an Array of dates I could use vector operations to calculate all last Sundays in a month in one go. So I need a way to create a swarm of Dates, depending on a pattern. Signatures are patterns (and just fancy lists), so let’s use those.

sub swarm(::Type, @sig is copy, Range $range) {
    my $var;
    my $marker-index = @sig.first([], :k);
    @sig[$marker-index] := $var;
    gather for @$range {
        $var = $_;
        take |@sig;

my @months = swarm(Date, (2022, @, *), 1..12);
my @Δsunday = @months».day-of-week »%» 7;
.say for{ .earlier(:days(@Δsunday.shift)) });

The sub swarm takes a type, a parameter-list meant to be used on that types’ new method and a range. It finds the empty list created by @ and replaced it with a container. That container is then filled with a value taken from the range. This is as neat as it is unnecessary. Raku will do the whole shebang for us if we use a Junction. Getting hold of the results requires a module by lizmat.

use eigenstates;

my @months =, (1..12).any, *).&eigenstates;
my @Δsunday = @months».day-of-week »%» 7;
.say for{ .earlier(:days(@Δsunday.shift)) });

# OUTPUT: No such method 'List' for invocant of type 'BOOTArray'.  Found 'List'
on type 'Any'
  in sub eigenstates at /usr/local/src/rakudo/install/share/perl6/site/sources/EDAA4AE0C8813633A2EA392374FB72F6B4D61047 (eigenstates) line 4

Thank you MoarVM, for creating a nice BOOTArray for us that we can’t easily turn into a List. Some bewilderment and a PR later, the result changed. You may wish to zef upgrade if you use eigenstates.


Just displaying those Sundays wont justify getting rid of the loop. If we need that in an API, returning a list of Dates without much hassle seems valuable. Please note, that the BOOTArray indicates eagerness and I don’t think it has to be. Junctions might get lazier and faster in the future.

Categories: Raku