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Typed filters

The Discord Raku bot is now also an IRC -> Discord bridge. To handle the streams of messages I use a react-block with a few whenevers. I would like to handle filtering of debug output from API::Discord in there as well, without disrupting something simple like printing to the terminal.

In my last post I showed how I can control the behaviour of a module with the use statement. The next step is to divert writes to $*ERR to a Supply.

my $active = False;

sub debug-print(|c) {
    $*ERR.print: |c if $active;

sub debug-print-supply(Supply $in? --> Supply:D) {
    my $result = $in // Supplier::Preserving.new;

    &debug-print.wrap(-> |c {
        $result.emit: |c


multi sub EXPORT('FROM-MODULE') {
        '&debug-print' => &debug-print,

multi sub EXPORT() {
    $active = True;

        '&debug-print' => &debug-print-supply,

As long as the user of the module doesn’t call debug-print, we have the simple case of writing to $*ERR. When debug-print is called, we divert the stream into a Supply. If no Supply is supplied, we create one. To be able to filter output a role is created in API::Discord::Debug.

role LogEventType is export {}

This role is indeed empty and so are the roles that are created to provide filters.

unit module Testmodule;

use API::Discord::Debug <FROM-MODULE>;

role HEARTBEAT does LogEventType is export {}
role PING does HEARTBEAT does LogEventType is export {}

start loop {
    sleep ¼;
    debug-print("ping" but PING);
    debug-print("pong" but PONG) unless 10.rand < 2;

So we create a Str and mixin a role, what is a type object we can check against.

use API::Discord::Debug;
use Testmodule;

react {
    my  $pings = 0;

    whenever debug-say().merge(debug-print()) {
        when PING { $pings++; }
        when PONG { $pings--; }
        default { .note }

    whenever Supply.interval(5) {
        say „$pings heartbeats are lost“ if $pings > 1;

Mixing in an empty role wont upset any unsuspecting bystander and type checks compose well. Here I use them with when/default, a multi dispatch would work as well. Introspection can be done with .WHAT.say.

On CPAN we can find many “complete” modules that kill a specific problem once and for all. For many of those, there tends to be another module suffixed with ::Simple. Simple stuff should be simple because we can’t buy any more days in this life. We also don’t take anything with us. I hope I can leave something simple behind instead.

Categories: Raku, Uncategorized
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  1. June 29, 2021 at 13:06

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