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Keyword: ‘coerce’

Coercing the unspeakable

April 17, 2021 3 comments

My wish for typed Supply would be rather limited if we could not coerce to roles.

role R[::T] {}

class A {
    method m(R[Int]() $_) { say $_ ~~ R[Int] }

class B {
    method R[Int]() {}

# OUTPUT: Missing block
          at /home/dex/projects/raku/tmp/typed-supply.raku:35
          ------>     method R⏏[Int]() {}

So a Signature can ask for a coercion to a parametrised role but a class can’t provide such a method because the compiler doesn’t like the name. From the standpoint of the compiler method names are just strings. The class keyword is just veneer for the MOP.

B.^add_method('R[Int]', method {
    class :: does R[Int] {

# OUTPUT: True

Having a dynamic compiler for a dynamic language does come with perks. However, using silly method names is not specced. So a problem solving issue is still in order.

Categories: Raku

Raku is a match for *

March 11, 2021 2 comments

PimDaniel asked an interesting question.

How do i test match is True while matching : this does NOT work :
if my ($type,$a,$b,$c) = ($v ~~ /^ ('horiz'|'vertic') '_' (\d+) '_' (\d+) '_' (\d+) $/)>>.Str { ... }
Well i made it in 2 times 1/ capture and test the match, 2/ convert the match to Str.

There was no prompt answer and no improvement at all. I couldn’t find a nice way to do this quickly either. In fact it took me the better part of an hour to crack this nut. The main issue here is that a failed match will produce Nil that .Str will complain about. So lets separate boolean check of if and the conversion to Str.

my $a = '1 B';

if $a ~~ /(<digit>) \s (<alpha>)/ -> $_ {
    my ($one, $B) = .deepmap: *.Str;
    say "$one $B";

By forcing the result of the condition expression into the topic, we can run any method on the result of the match, but only if Match.bool returns true. I don’t got a degree in CS* but would be very surprised if Raku-signatures would not turn out to turing complete.

if $a ~~ /(<digit>) \s (<alpha>)/ -> Match (Str() $one, Str() $B) {
    dd $one;
    dd $B;
# OUTPUT: "1"

The signature of the if block coerces the Match to a list. We pick two elements of it and coerce those to Str. Of cause we could coerce to anything we like based on the position of the captures.

Regexes in Raku are compiled to the same byte code then the rest of the program. In fact grammars are just classes with a funky syntax. That’s why we can run Raku code inside a regex with ease. That means we can turn the whole thing inside out.

my @a = <1 B 3 D 4>;
my @b;

my $hit;

for @a -> $e {
    @b.push: ($e ~~ /(<alpha>) || { next } /).Str;

say @b;

Here we skip the .push if the match does not succeed by skipping the rest of the loop body with next. We could fire any control exception inside the regex. That means we could stick the whole thing into a sub and return the value we are looking for from within the regex.

sub cherry-pick-numeric(Str $matchee) {
    $matchee ~~ m/(<digit>) && { return .Numeric }/;

@b = do .&cherry-pick-numeric for @a;

dd @b;
# OUTPUT: Array @b = [1, 3, 4]

Raku has been in the making for 10 years. This was an gargantuan task. Now comes the hard bit. We have to take that large language and find all the nice idioms. Good things come to those who wait (on IRC).

*) read: Don’t believe anything I write. You have been warned.


In truly lazy fashion I came up with a way to turn a match into a lazy list after the work should have been done.

$a = '1B3D4';

my \ll := gather $a ~~ m:g/
      [ <alpha> && { take $/<alpha>.Str } ]
    | [ <digit> && { take $/.<digit>.Numeric } ]
    | [ { say 'step' } ]
say ll[0];
say ll[3];

The trick is force the match to run all the way to the end of the string with the :g adverb. This run will be interrupted by a take (by throwing CX::Take) and resumed when the next value is asked from the Seq returned by gather. I don’t know if this is memory efficient thought. There may be a Match instance kept around for each take.

Categories: Raku

Assumed predictability

December 27, 2020 2 comments

Vadim does not agree with me. Nor should he. I hardly ever agree with myself. Also, I’m happy for his disagreement because it allows me to write about a topic that I got in the queue for quite some time.

The basic statement is that enforcing types allows reasoning about interfaces at compile time and maybe even earlier — at brain time. A reasonable thing to do in a statically typed language. When objects are involved, Raku does plenty of its thinking at runtime. Let’s have a look at two examples.

class Foo {
    submethod shift-right($i) { }

sub foo(Foo() $handle) {


class Bar is Foo {


# OUTPUT: No such method 'shift-right' for invocant of type 'Bar'

We ask the compiler for a type check against Foo that Bar satisfied. Then we proceed to call a method on an instance of Bar that is only supplied by Foo. The compiler tried to help us but couldn’t. In this example our interface is not Foo and its methods but only Foo — excluding its child classes — and its methods. This is a subtle difference that will hurt us at runtime. With another example I can illustrate quite clearly why.

class Catchall {
    has $.the-object;
    method FALLBACK($name, |c) {

sub foo(Catchall $c) {

say foo( the-object =>
    class :: {
        method any-name($i) { 42 ~~ $i ?? 'Universe' !! 'wut‽'  }
# OUTPUT: Universe

Can you name the interface that is being used by sub foo? That’s a mean question of course, as the class that supplies the interface got no name. The type constraint provides the interface of an accessor method to $.the-object and anything inherited from Any. The latter might change with new language versions btw. Consequently, the real interface is more like *.any-name() with a type constraint of Any. Those two are simple examples. Any module you use might use MONKEY-TYPING or fiddle with the MOP. Runtime interfaces are utterly unpredictable in Raku and we all do well to use Test.

That being said, Vadim is right to uphold the principle of least surprise. We start the names of roles with a capital to indicate it to be a type object and thus the wish to honour some sort of interface. I would be happy for a more general solution to the problem of “slurpy” coercion. Technically, Raku got the syntax for it already.

sub Filish(Any:D $handle where * ~~ Str|IO::Handle|IO::Path --> IO::Handle) {
    # ... coerce away here

sub foo(&Filish() $handle) {
    $handle.put: "This would make me happy!";

This form would basically say: “I want the coercion handled by a sub called Filish“. It would allow the code in Filish to be reused and as such provide flexibility, without giving the impression to promise interface. At least in this example the signature of the coercion sub contains its own documentation. There may even be room for some compile time checks, so long as we don’t use a multi. The parameter $handle must satisfy the signature of Filish. Having a sub would allow a module user to .wrap it.

Being in general disagreement with myself can be challenging but does allow me to change my mind easily. Since this is likely my last blog post this year I wish you all exclusively nice reasons to change your minds in 2021.

Categories: Raku

Coercive files

December 25, 2020 2 comments

Many APIs have a routine or two that take a file as an argument. In Raku we can easily turn Str into IO::Path and subsequently into IO::Handle. As a module author it’s a polite move to provide MMD variants, so the user can supply what they like.

sub s($file is copy where { $_ ~~ Str|IO::Path|IO::Handle or fail(&? ~ ' likes Str, IO::Path and IO::Handle.') } ) {
    given $file {
        when Str { $file =; }
        when IO::Path { $file = .open; }
        when IO::Handle { }

    say ?$file;

This is boilerplate. Which in the kingdom of Raku is almost banned. Using the new coercion protocol, we can implement a role to happily save time ever after.

role Filish[*%mode] is IO::Handle {
    proto method COERCE($) {*}
    multi method COERCE(Str:D $s) {
        my $handle = :path($s.IO);
        $ |%mode
    multi method COERCE(IO::Path:D $p) {
        my $handle = :path($p);
        $ |%mode
    multi method COERCE(IO::Handle:D $h) {

sub f(Filish[:w, :a, :!bin]() $handle) {
    $handle.put: "foo" xx 42;


With the coercing type constraint Filish[:w, :a, :!bin]() we basically say: “Give me something that represents a file, that I will open for writing in the appending fashion.”. I was not aware of the possibility to use a slurpy in the parameter list of a role (*@a works too). This seems to be an ENODOC. Since it makes my file easier I wont complain.

The new coercion protocol is very useful but got one flaw. It forces me to return the type that contains the COERCE-method. In a role that doesn’t make much sense and it forces me to juggle with IO::Handle. It took me 30 minutes to figure out how to successfully subclass it. There may be classes in modules that are even worse. Some programmers really like their private attributes. It would be nice to drop that restrictions on roles and/or if a return type is explicitly provided. With the current model, something easy is made hard.

Anyway, I got it working and might stick a module into the ecosystem, once I came up with a good name.

Categories: Raku

Guarding dynamics

August 14, 2020 1 comment

Dynamic variables are a nice way to get the benefits of global variables without taking the drawbacks. They pass information up the call tree without forcing a declaration upon the caller. However, dynvars share a burden with exceptions. The callee knows how to do what the caller might not expect.

use Module::By::Author::A;
use Module::By::Author::B;

my $*dynvar = 42;

sub-from-A(); # expects $*dynvar to be Int
sub-from-B(); # expects $*dynvar to be IO(Str)
sub-from-C(); # expects $*dynvar to be IO::Handle

In this example sub-from-B() will silently fail until it tries to open the file named “42”. While sub-from-C() will try to coerce 42 to become a file handle and throw. So there lies a danger in dynvars expected to be set by independent modules. Worse, the behaviour might suddenly pop up after any zef --install. Raku is a dynamic language that will try its best to convert values automatically and fail at runtime. It is good advice to support the compiler by failing early.

I want to use dynvars in a few modules to provide semi-global flags to remove boilerplate. The following seems to provide protection against unintentional dynvar-spill over.

class Switch is Mu {
    has $.name;
    method gist { $.name }
    method Str { die('invalid coersion') }

constant on = :name<on>;
constant off = :name<off>;

sub s() {
    # put $*d; # this will die
    say $*d;
    dd $*d;
my $*d = off;
# OUTPUT: off
#         Switch $*d = => "off")

I derive from Mu to get rid of all the coercers from Cool and overload Str to take that one out of the loop as well. Using say for anything but debugging is a bug anyway, so I might support it properly with .gist. Since I create a type with the class I can then go and protect my subs and methods with a whereception.

sub dyn-var-typecheck(Mu:U \T, $name) {
    sub {
        DYNAMIC::($name) ~~ T || die(„$name is set to an unexpected value“)

constant &dyn-s-typecheck = dyn-var-typecheck(Switch, ‚$*d‘);

sub s($? where dyn-s-typecheck) { }
# OUTPUT: $*d is set to an unexpected value
#         in sub  at /home/dex/projects/blog/shielded-dynvars.raku line 6
#         in sub s at /home/dex/projects/blog/shielded-dynvars.raku line 22
#         in block <unit> at /home/dex/projects/blog/shielded-dynvars.raku line 29

Using the DYNAMIC::($name) in a where clause will slow down any call on MMD. So pulling the check into the sub might be reasonable.

With this measure I feel better to add $*always-capture-stderr to Shell::Piping to get rid of :stderr(Capture) on pretty much any pipe I start. And I feel a lot better when adding $*debug in everywhere.

Raku was not designed with type checks on dynvars in mind. It was designed to be a Perl. That gives us the flexibility to fix problems as they occur.

Categories: Raku

Handling Failure

July 27, 2020 1 comment

After some back and forth I have found a practical way to handle error conditions in Shell::Piping. The practical way is to have more then one way. A process does have an exitinteger (called a code, because it can be quite cryptic) and text output to STDERR to indicate something went wrong. Sometimes we need sloppy error handling, sometimes we need to look into the textual output and react to it.

I found a really nice way to use a Junction and Rakus type system to remove some boilerplate from error handling. Combining both allows us to create a flexible type.

class Exitcode {
    has $.value;
    has $.command;
    method Numeric { $.value }
    method Str { $.command }

So this class produces objects that are both a number and a text. What is actually looked at depends on who is looking. We can use infix:<~~> to make the decision which comparison operator to use.

say $ex ~~ 42 && $ex ~~ ‚find‘; # OUTPUT: True

That’s still quite wordy. We can use a Junction because it binds tighter then ~~.

say $ex ~~ 42 & ‚find‘; # OUTPUT: True

Now we can CATCH an Exception and narrow done the command in a pipe that failed easily.

    when X::Shell::NonZeroExitcode {
        given .exitcode {
            when 42 & ‚find‘ {
                warn ‚Oh now! We found the answer!‘;

Not all users of a module might like to use Exceptions. So we use a construct in a Shell::Pipe object to create a Failure to return from .sink. If the method Shell::Pipe.exitcode is called, we assume the user is dealing with them by hand. We can then call .handled to “abort” the Exception. This has to be easy or it might get skipped. Hence the unusual usage of the coercer methods in the class Exitcode.

Categories: Raku

Nil statement considered harmful

July 24, 2016 1 comment

Nil is the little parent of Failure and represents the absence of a value. It’s the John Doe of the Perl 6 world. Its undefined nature means we can talk about it but we don’t know its value. If we try to coerce it to a presentable value we will be warned.

my $a := Nil; dd $a.Str, $a.Int;
# OUTPUT«Use of Nil in string context  in block <unit> at <tmp> line 1␤Use of Nil
 in numeric context  in block <unit> at <tmp> line 1␤""0␤»

As the output shows it still coerces to the closest thing we have for a undefined string or number. Some times the empty string is outright dangerous.

sub somesub { Nil };
my $basename = somesub;
spurt("$basename.html", "<body>oi!</body>");

If we do that in a loop we would drop plenty of “.html” into the filesystem. Since this can depend on input data, some cunning individual might take advantage of our neglect. We can’t test for Nil in $basename, because assignment of Nil reverts the container to it’s default value. The default default value for the default type is Any. We can protect ourselves against undefined values with a :D-typesmile.

my Str:D $basename = somesub;

That would produce a runtime error for anything but Nil, because the default value for a container of type Str:D is Str:D. A type object that happens to be undefined and wont turn into anything then the empty string. Not healthy when use with filenames.

We still get the warning though, what means that warn is called. As it happens warn will raise a control exception, in this instance of type CX::Warn. We can catch that with a CONTROL block and forward it to die.

sub niler {Nil};
my Str $a = niler();
say("$a.html", "sometext");
say "alive"; # this line is dead code
CONTROL { .die };

That’s quite a good solution to handle interpolation problems stemming from undefined values. Given that any module or native function could produce undefined values makes it hard to reason about our programs. Having the control exception allows us to catch such problems anywhere in the current routine and allows us to deal with more then one place where we interpolate in one go.

Sometimes we want to stop those values early because between an assignment of a undefined value and the output of results minutes or even hours can pass by. Fail loudly and fail early they rightfully say. Type smileys can help us there but for Nil it left me with a nagging feeling. So I nagged and requested judgment, skids kindly provide a patch and judgement was spoken.

Perl 6 will be safe and sound again.

Categories: Perl6