Archive for July, 2022

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

Coercive bits

July 13, 2022 1 comment

Altreus was looking for a way to convert a list of bitmask names, provided as a string by the user, to a bitmask. He wished for BUILDARGS, as provided by Perl’s Moose but was given good advise how to solve the problem without making object instantiation even more complex. Nobody can stop me from doing just that.

With Moose, BUILDARGS allows to modify values before they are bound to attributes of a class. We can do the same by using the COERCE-protocol, not just for Routine-arguments.

role BitMask[@names] {
    has int $.mask;
    sub names-to-bits(@a) {
        my int $mask;
        for @a -> $s {
            $mask = $mask +^ (1 +< @names.first($s, :k) // fail('bad‼'))

    sub bits-to-names($mask) {
        ($mask.base(2).comb.reverse Z @names).map: -> [Int() $is-it, $name] { $is-it ?? $name !! Empty }

    multi method COERCE(Str $s) { self.COERCE: $s.split(' ').list }
    multi method COERCE(List $l) { :mask($l.&names-to-bits) }

    method raku { "BitMask[<@names[]>](<{$!mask.&bits-to-names}>)" }
    method bits(--> Str) { $.mask.fmt('%b') }

class C {
    has BitMask[<ENODOC ENOSPEC LTA SEGV>]() $.attr is required;

my $c = :attr<ENODOC ENOSPEC SEGV>;
say $c;
say $c.attr.bits;
#         1011 

Here I build a role that carries the names of bits in a bit-field and will create the bit-field when give a Str or List containing those names. Since I use a parametrised role, my bitfield is type-safe and the type is tied to the actual names of the bits. As a consequence a user of the module that exports C can extend the accepted types by using that specific role-candidate.

enum IssueTypes ( ENODOC => 0b0001, ENOSPEC => 0b0010, LTA => 0b0100, SEGV => 0b1001 );

class IssueTypesBits does BitMask[<ENODOC ENOSPEC LTA SEGV>] {
    method new(*@a where .all ~~ IssueTypes) {
        self.bless: mask => [+|] @a

sub bitmask(*@a where .all ~~ IssueTypes) { |@a

my $c3 = attr => BEGIN bitmask(ENODOC, LTA, SEGV);
say $c3;
say $c3.attr.bits;

Here I define an enum and provide the same names in the same order as in does BitMask. With the shim bitmask I create the type-safety bridge between IssueTypes and BitMask[<ENODOC ENOSPEC LTA SEGV>].

It is very pleasing how well the new COERCE-protocol ties into the rest of the language, because we can use a coercing-type at any place in the source code which takes a normal type as well. What is not pleasing are the LTA-messages X::Coerce::Impossible is producing.

my $c4 = attr => 42;
# OUTPUT: Impossible coercion from 'Int' into 'BitMask[List]': method new returned a type object NQPMu
            in block <unit> at parameterised-attribute.raku line 60

This is surpring because we can get hold of the candidates for COERCE at runtime.

    when X::Coerce::Impossible && .target-type.HOW === Metamodel::CurriedRoleHOW {
        put "Sadly, COERCE has failed for {.from-type.^name}. Available candidates are: ",^lookup('COERCE').candidates».signature».params[*;1]»*.^name)

# OUTPUT: Sadly, COERCE has failed for Int. Available candidates are: Str List

What we can’t see are the candidates of IssueTypesBits, because that is hidden behind the constructor new. I tried to use the MOP to add another multi candidate but failed. When parametrising the underlying type-object gets cloned. Any change to the multi-chain wont propagate to any specialised types.

The COERCE-protocol is quite useful indeed. Maybe it’s time to document it.

Categories: Raku

The truth is a hard problem

July 11, 2022 2 comments

In a recent article, stevied promised a detailed walk through of code. I asked him if he would be interested in a critique of his writings. He foolishly agreed.

He claims that Arrays are eager and fill all memory if given the chance. Let’s check this claim.

my @a = 1, 1, * + * … ∞;
say @a.WHAT;

# OUTPUT: (Array)

Array.STORE goes out of it’s way to maintain iterables (not all things that are iterable do Iterable) and only reify container slots that we actively ask for. It will cache lazy lists by default, assuming that when we use .AT-POS (usually indirectly with a positional subscript) we want to use the values again. Dealing with lazy lists is a hard problem and many get it wrong (as my next blog post will discuss in more detail).

In the next paragraph I found another inaccurate statement. Int:D does not check for definedness.

multi sub bar(Any:D $) { say 'defined'; }
multi sub bar(Any:U $) { say 'undefined'; }

bar(Int); # undefined

class C { method defined { False } }
my $c =; # undefined
bar(C); # undefined
bar($c); # defined
say defined $c; # False
say $c.DEFINITE; # True
say $c // 'undefined'; # undefined

In Raku objects are either type objects or definite objects. The latter are the result of nqp::create. Raku and most user code expects objects to provide a basic set of methods that can be found in Mu or Any. The default type-check is against Mu. We can check if an object is definite with the macro .DEFINITE (you can overload a method of the same name, but Rakudo will ignore it) or, if we also want to know if we got an ordinary object, with $foo ~~ Mu:D. There is a clear distinction of definedness and being definite. Raku needs to make that differentiation to support “unusual” values and types like Nil and Failure. Please note, that type-objects are not definite but can be defined. Types are a hard problem.

In the same paragraph stevied writes: “defined integer object”, even though he makes a type-check against Int.

multi sub foo(Int:D $i) { }
multi sub foo($i where { $i - $i.truncate == 0 }) { say 'lolwut‽' }
multi sub foo($) { say 'not integer'; }


# OUTPUT: lolwut‽
          not integer

Int:D will fail for any value that doesn’t got (Int) in its .^mro or is a subset of Int or any of Int‘s sub-classes. Raku sports an excellent coercion protocol and there is no reason not use it.

subset PositiveInteger of Numeric:D() where { $^i > 0 && ($i - $i.truncate == 0) || fail("Expected a positive and an integer value but got $i.")}
sub get-prime(PositiveInteger $nth where * > 0) {
    say ($x.grep: *.is-prime)[$nth - 1];
# OUTPUT: 11
          Expected a positive and an integer value but got 0.5.
            in block  at 2021-03-08.raku line 1809
            in block <unit> at 2021-03-08.raku line 1288

A value check is needed in this instance because .AT-POS will coerce to Int (by truncating) what may give a reasonable answer for an unreasonable question. No matter how sharp the knife, we wont get the ½th prime number. Being positive and integer is a hard problem.

Please don’t use say unless you actually want to run .gist.

say Nil;
put Nil;
          Use of Nil in string context
            in block  at 2021-03-08.raku line 1818

When you say you may miss subtile bugs, especially when running CI-tests.

Further down we find Sequence but grep returns a Seq – in this example. I never had to make the distinction between Seq and HyperSeq but the latter is not a child of Cool so a lot of interfaces are missing. The same is true for the role Sequence. The build-in types are a hard problem.

If we must ask what exactly * is (I wouldn’t, because that’s a really hard question.), it is best to provide the correct answer.

my &b = * + *;
say .WHAT, .signature with &b;
my &b2 = { $^a + $^b };
say .WHAT, .signature with &b2;
# OUTPUT: (WhateverCode)(;; $whatevercode_arg_11 is raw, $whatevercode_arg_12 is raw)
          (Block)($a, $b)

A WhateverCode-object will have all its arguments as is raw, what means it will always have access to the containers used by the callee (There are many things we can do, but probably shouldn’t, with .VAR). It will never have control exceptions (no return or phasers, etc.), doesn’t have a proper scope (no use-statement) and a few more things. The idea is to have a subclass of Code that is so simple that the compiler can always inline it. Code-objects are a hard problem.

I would have written that code a little different.

sub get-primes(*@nth) {
    (^∞).hyper.grep(-> Int:D() $_ is raw { .is-prime })[@nth »-» 1]

.put for get-primes(5, 50, 500, 5000, 50000);

I spend a pretty penny on that Threadripper, so I better .hyper as often as I can. If sensible I try to use a slurpy to move the burden of dealing with plurals from the user to the implementer. We can ask .AT-POS for a list of values, so there is no reason not to.

There are quite a few more inaccuracies in Steve’s article. Bless him, Raku is a bitch and we are not good at explaining how the language actually works. For most programs that doesn’t matter, they will run just fine even if the programmer is sloppy. I learned most of the gritty details when tracking down bugs. Granted, I stick my arm deep into the machinery, so it’s my own bloody fault when I get pinched. Since bugs happen we need to get better at explaining how to hunt them down. Raku is a hard problem.

Categories: Raku