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Scheduling thingy for Raku

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# Static configuration;

use Chronic;

react {
    # Every minute
    whenever Chronic.every() -> $v {
        say "One: $v";
    # Every five minutes
    whenever Chronic.every(minute => '*/5') -> $v {
        say "Five: $v";
    # At 21:31 every day
    whenever Chronic.every(minute => 31, hour => 21) -> $v {
        say "21:31 $v";


# Dynamic configuration

use Chronic;

my @events = (
        schedule => {},
        code     => sub ($v) { say "One: $v" },
        schedule => { minute => '*/2' },
        code     => sub ($v) { say "Two: $v" },
        schedule => { minute => '*/5' },
        code     => sub ($v) { say "Five: $v" },
        schedule => { minute => 31, hour => 21 },
        code     => sub ($v) {  say "21:31 $v"; },

for @events -> $event {

# This has the effect of waiting forever


This module provides a low-level scheduling mechanism, that be used to create cron-like schedules, the specifications can be provided as cron expression strings, lists of integer values or Junctions of values.

There is a class method every that takes a schedule specification and returns a Supply that will emit a value (a DateTime) on the schedule specified. There is also a method at (also a class method) that returns a Promise that will be kept at a specified point in time (as opposed to Promise.in which will return a Promise that will be kept after a specified number of seconds.)

This can be used to build custom scheduling services like cron with additional code to read the specification from a file and arrange the execution of the required thing or it could be used in a larger program that may require to execute some code asynchronously periodically.

There is a single base Supply that emits an event at a 1 second frequency in order to preserve the accuracy of the timings (in testing it may drift by up to 59 seconds on a long run due to system latency if it didn't match the seconds too,) so this may be a problem on a heavily loaded single core computer. The sub-minute granularity isn't provided for in the interface as it is easily achieved anyway with a basic supply, it isn't supported by a standard cron and I think most code that would want to be executed with that frequency would be more highly optimised then this may allow.


If you have a working Rakudo you can install directly with zef:

# From the source directory

zef install .

# Remote installation

zef install Chronic


Suggestions/patches are welcomed via github at https://github.com/jonathanstowe/Chronic


This is free software.

Please see the LICENCE file in the distribution.

© Jonathan Stowe 2015, 2016, 2017, 2019, 2020