use v6.d;

role ValueType:ver<0.0.4>:auth<zef:lizmat> {
    has $!WHICH;

    my @attributes = ::?CLASS.^attributes.grep: *.name ne 'WHICH';

    multi method WHICH(::?CLASS:D: --> ValueObjAt:D) {
        $!WHICH.defined
          ?? $!WHICH
          !! self!WHICH
    }

    method !WHICH() {
        my str @rest = self.^name;
        for @attributes -> $attribute {
            my $WHICH := $attribute.get_value(self).WHICH;
            $WHICH ~~ ValueObjAt
              ?? @rest.push($WHICH.Str)
              !! die "Attribute '$attribute.name()' is not a Value Type";
        }
        $!WHICH := ValueObjAt.new(@rest.join('|'))
    }
}

=begin pod

=head1 NAME

ValueType - A role to create Value Type classes

=head1 SYNOPSIS

  use ValueType;

  class Point does ValueType {
      has $.x = 0;
      has $.y = 0;
  }

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

  # 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

=head1 DESCRIPTION

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

This is specifically important when using set operators (such as C<(elem)>,
or C<Set>s, C<Bag>s or C<Mix>es, or any other functionality that is based
on the C<===> operator functionality, such as C<unique> and C<squish>.

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

=head1 THEORY OF OPERATION

The first time the C<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 C<ValueObjAt> object for its C<WHICH>
value, save it for future reference, and return it.  If any of the attribute
values are B<not> a value type, then an exception will be thrown.

=head1 AUTHOR

Elizabeth Mattijsen <[email protected]>

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

=head1 COPYRIGHT AND LICENSE

Copyright 2020, 2021 Elizabeth Mattijsen

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

=end pod

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