NAME

ValueType - A role to create Value Type classes

SYNOPSIS

use ValueType;

class Point does ValueType {
    has $.x = 0;
    has $.y = 0;
    has $.distance is hidden-from-ValueType is built(False);

    method TWEAK() {
        $!distance = sqrt $!x² + $!y²;
    }
}

say Point.new.WHICH;                     # Point|Int|0|Int|0

say Point.new(x => 3, y => 4).distance;  # 5

# fill a bag with random Points
my $bag = bag (^1000).map: {
  Point.new: x => (-10..10).roll, y => (-10..10).roll
}
say $bag.elems;  # less than 1000

DESCRIPTION

The ValueType role mixes the logic of creating a proper value type into a class. A class is considered to be a value type if the .WHICH method returns an object of the ValueObjAt class: that then indicates that objects that return the same WHICH value, are in fact identical and can be used interchangeably.

This is specifically important when using set operators (such as (elem), or Sets, Bags or Mixes, or any other functionality that is based on the === operator functionality, such as unique and squish.

The format of the value that is being returned by WHICH is only valid during a run of a process. So it should not be stored in any permanent medium.

EXCLUDING ATTRIBUTES

Sometimes a class has an extra attribute that depends on the other attributes, but which cannot be set, and doesn't need to be included in the calculation of the WHICH value. Such attributes can be marked with the is hidden-from-ValueType attribute.

THEORY OF OPERATION

At role composition time, each attribute will be checked for mutability. If any mutable attributes are found, then a compilation error will occur.

Then, the first time the WHICH method (mixed in by this role) is called, it will check all of its attribute values for being a value type. If they all are, then it will construct a ValueObjAt object for its WHICH value, save it for future reference, and return it. If any of the attribute values are not a value type, then an exception will be thrown.

AUTHOR

Elizabeth Mattijsen liz@raku.rocks

Source can be located at: https://github.com/lizmat/ValueType . Comments and Pull Requests are welcome.

If you like this module, or what I’m doing more generally, committing to a small sponsorship would mean a great deal to me!

COPYRIGHT AND LICENSE

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.