Immutable objects and Dependency Injection

At some point we had:

	while ( my $data = $self->next ) {
	my $featues = Text::FeatureCount->new->analyze($data->{content});

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in our code.

Text::FeatureCount was at that time an accumulator that was counting the document features and saving them in internal structures. Multiple subroutines were using these structures - so it made sense to make them the object attributes. But it also meant that changing Text::FeatureCount into something else was a big deal. We could make the class name variable: $feature_counter_class->new->analyze(... and add an attribute to store it to the main object. But I decided to make Text::FeatureCounter an immutable object instead and inject a ready made Text::FeatureCounte into the object doing the work above. Now it can be used in the loop without re-constructing it to clean the internal structures. When coding with immutable objects you have to pass around the input data from one method to another explicitly instead of keeping it readily available in the object attributes:

	sub frobnicate {
	my( $self, $data ) = @_;
	...
	$self->twiddle( $data );
	...
	}
	sub twiddle {
	my( $self, $data ) = @_;
	...
	}
	# VERSUS
	sub frobnicate {
	my $self = shift;
	...
	$self->twiddle();
	...
	}
	sub twiddle {
	my $self = shift;
	my $data = $self->data;
	...
	}

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This makes it is slightly un-object-oriented, but the benefit is that you don't need to re-construct the object and so you can use Dependency Injection on it and make the code more flexible. It is also easier to reason about the algorithm when some parts are immutable. Often this is a good trade-off.

PS. After much other refactoring Text::FeatureCount mutated into Text::WordCounter (and AI::Classifier::Text::Analyzer) - soon to be released to CPAN.