The Law of Demeter Is Not A Dot Counting Exercise

0 comments suggest edit

Recently I read a discussion on an internal mailing list on whether or not it would be worthwhile to add a null dereferencing operator to C#.

For example, one proposed idea would allow the following expression.

object a = foo.?bar.?baz.?qux;

This would assign the variable a the value null if any one of foo, bar, or baz is null instead ofthrowing a NullReferenceException. It’s a small, but potentially helpful, mitigation for the billion dollar mistake.

Sure enough, it did not take long for someone to claim that this syntax would be unnecessary if the code here was not violating the sacred Law of Demeter (or LoD for short). I think this phenomena is an analog to Godwin’s Law and deserves its own name. Let’s call it the “LoD Dot Counting Law”:

As a discussion of a code expression with more than one dot grows longer, the probability that someone claims a Law of Demeter violation approaches 1.

dippindots Count the dots and win a prize!

What is wrong with the claim that the above expression violates LoD? To answer that let’s briefly cover the Law of Demeter. I’m not going to cover it in detail but rather point to posts that describe it in much better detail than I would.

The Many Forms of Demeter

The formal object form of the law can be summarized as:

A method of an object may only call methods of:

  1. The object itself.
  2. An argument of the method.
  3. Any object created within the method.
  4. Any direct properties/fields of the object.

A general formulation of the law is:

Each unit should have only limited knowledge about other units: only units “closely” related to the current unit. Or: Each unit should only talk to its friends; Don’t talk to strangers.

This of course leads to the succinct form of the law:

Don’t talk to strangers

In other words, try to avoid calling methods of an object that was returned by calling another method. Often, people shorten the law to simply state “use only one dot”.

One of the key benefits of applying LoD is low coupling via encapsulation. In his paper, The Paperboy, The Wallet, and The Law of Demeter (PDF) (it’s a relatively quick read so go ahead, I’ll be here), David Bock provides a great illustration of this law with an analogy of a paperboy and a customer. Rather than having a customer hand over his wallet to pay the paperboy, he instead has the paperboy request payment from the customer.

In answer to “Why is this better?” David Bock gives these three reasons.

The first reason that this is better is because it better models the real world scenario…  The Paperboy code is now ‘asking’ the customer for a payment.  The paperboy does not have direct access to the wallet.  

The second reason that this is better is because the Wallet class can now change, and the paperboy is completely isolated from that change…

The third, and probably most ‘object-oriented’ answer is that we are now free to change the implementation of ‘getPayment()’.

Note that the first benefit is an improvement not only in encapsulation but the abstraction is also improved.

Dot Counting Is Not The Point

You’ll note that David doesn’t list “50% less dots in your code!” as a benefit of applying LoD. The focus is on reduced coupling and improved encapsulation.

So going back to the initial expression, does foo.bar.baz.qux violate LoD? Like most things, it depends.

For example, suppose that foo is of type Something and it contains properties named Bar, Baz, and Qux which each simply return this.

In this semi-contrived example, the expression is not an LoD violation because each property returns the object itself and according to the first rule of LoD, “you do not talk about LoD” … wait … sorry… “a method is free to call any properties of the object itself” (in a future post, I will cover the class form of LoD which seems to indicate that if Bar, Baz, and Qux return the same type, whether it’s the same object or not, LoD is preserved).

This pattern is actually quite common with fluent interfaces where each method in a calling chain might return the object itself, but transformed in some way.

So we see that counting dots is not enough to indicate an LoD violation. But lets dig deeper. Are there other cases where counting dots leads do not indicate an LoD violation? More importantly, is it always a bad thing to violate LoD? Are there cases where an LoD violation might even be the right thing to do?

Go Directly to Jail, Do Not Pass Go

Despite its name, violating the Law of Demeter will not get you on an episode of Cops nor is it some inviolable law of nature.

As the original paper points out, it was developed during design and implementation of the Demeter system (hence the name) and was held to be a law for the developers of that system.

The designers of the system felt that this practice ensured good Object-Oriented design:

The motivation behind the Law of Demeter is to ensure that the software is as modular as possible. The law effectively reduces the occurrences of nested message sendings (function calls) and simplifies the methods.

However, while it was a law in the context of the Demeter system, whether it should hold the weight that calling it a Law implies is open to debate.

David Bock refers to it as an idiom:

This paper is going to talk about a particluar (sic) ‘trick’ I like, one that is probably better classified as an ‘idiom’ than a design pattern (although it is a component in many different design patterns).

 Martin Fowler suggests (emphasis mine)

I’d prefer it to be called the Occasionally Useful Suggestion of Demeter.

Personally, I think most developers are guilty of bad encapsulation and tight coupling. I’m a bit more worried about that than applying this law inappropriately (though I worry about that too). Those who have deep understanding of this guideline are the ones who are likely to know when it shouldn’t be applied.

For the rest of us mere mortals, I think it’s important to at least think about this guideline and be intentional about applying or not applying it.

I Fought The Law and The Law Won

So what are the occasions when the Law of Demeter doesn’t necessarily apply? There’s some debate out there on the issue.

In his post, Misunderstanding the Law of Demeter, Daniel Manges argues that web page views aren’t domain objects and thus shouldn’t be subject to the Law of Demeter. His argument hinges on a Rails example where you send an Order object to the view, but the view needs to display the customer’s name.

<%= @order.customer.name %>

Counting two dots, he considers the change that would make it fit LoD:

<%= @order.customer_name %>

He then asks:

Why should an order have a customer_name? We’re working with objects, an order should have a customer who has a name.

…when rendering a view, it’s natural and expected that the view needs to branch out into the domain model.

Alex Blabs of Pivotal Labs takes issue with Daniel’s post and argues that views aredomain objects and an order ought to have a customer_name property.

It’s an interesting argument, but the following snippet of a comment by Zak Tamsen summarizes where I currently am on this subject (though my mind is open).

because they don’t. the primary job of the views (under discussion) is to expose the internal state of objects for display purposes. that is, they are expressly for data showing, not data hiding. and there’s the rub: these kind of views flagrantly violate encapsulation, LoD is all about encapsulation, and no amount of attribute delegation can reconcile this.

The problem as I see it with Alex’s approach is where do you stop? Does the Order object encapsulate every property of Customer? What about sub-properties of the Customer’s properties? It seems the decision to encapsulate the Customer’s name is driven by the view’s need to display it. I wouldn’t want my domain object’s interface to be driven by the needs of the view as that would violate separation of concerns.

There’s another option which might be more common in the ASP.NET MVC world than in the Rails world, I’m not sure. Why not have a view specific model object. This would effectively be the bridge between the domain objects and the view and could encapsulate many of the these properties that the view needs to display.

Another case where an LoD violation might not be such a bad idea is in cases where the object structure is public and unlikely to change. While in Norway, I had the opportunity to briefly chat with Michael Feathers about LoD and he pointed out the example of Excel’s object model for tables and cells. If LoD is about encapsulation (aka information hiding) then why would you hide the structure of an object where the structure is exactly what people are interested in and unlikely to change?

Use It or Lose It

When I learn a new guideline or principle, I really like to dig into where the guideline breaks down. Knowing where a guideline works, and what its advantages are is only half the story in really understanding it. When I can explain where it doesn’t work and what its disadvantages are, only then do I feel I’m starting to gain understanding.

However, in writing about my attempts at understanding, it may come across that I’m being critical of the guideline. I want to be clear that I think the Law of Demeter is a very useful guideline and it applies in more cases than not. It’s one of the few principles that can point to an empirical study that may point to its efficacy.

In a Validation of Object-Oriented Design Metrics as Quality Indicators, the authors of the study provide evidence that suggests The Law of Demeter reduces the probability of software faults.

the-count Still, I would hope that those who apply it don’t do it blindly by counting dots. Dot counting can help you find where to look for violations, but always keep in mind that the end goal is reducing coupling, not dots.

Found a typo or error? Suggest an edit! If accepted, your contribution is listed automatically here.

Comments

avatar

22 responses

  1. Avatar for Daniel Lindh
    Daniel Lindh March 11th, 2010

    I have the last couple of days been thinking a bit about The Law of Demeter and Composition, and it feels like this article is the sum of all my thoughts. Thanks

  2. Avatar for Zohirul Alam Tiemoon
    Zohirul Alam Tiemoon March 13th, 2010

    Good explanation on LoD.

  3. Avatar for Rus Pornoları
    Rus Pornoları August 20th, 2010

    Yes Really Good explanation

  4. Avatar for David V. Corbin
    David V. Corbin September 29th, 2010

    Although you mention ASP.NET MVC...you completely miss the MVVM pattern used by WPF/Silverlight...
    In this case a View Model exposes exactly what the view should interact with.
    Using the specific example, exposting the Customer object (to get the name) also exposes the remainder of the customer object. This is dangerous. What if the view creator (often a graphic designer rather than a programmer) accidently binds a button to Order.Customer.DeleteAllInformation() !!!!
    Why should a Order FORM (screen/report/etc) be dependent on a customer object at all? Sure it needs a name (probably address, phone, etc), but could that not be some entity that is not a "Customer" in the underlying system?

  5. Avatar for Tom
    Tom October 22nd, 2010

    hahah that purple guy

  6. Avatar for Rob Edwards
    Rob Edwards November 4th, 2010

    It seems to me that in order to make things separate in the Order/Customer scenario, the following would be considered. The Order doesn't need access to all of the properties of a customer, just a subset. That subset could be encapsulated into a class, like Billing Info or Shipping Info that contains a customer's name, address and the like. Just like the wallet example has the Officer class consuming the Driver's License class, the Order class should consume a BillingInfo or ShippingInfo class that Customer provides.

  7. Avatar for ADK
    ADK November 14th, 2010

    Good coverage of the LoD argument. I would go on to point out that the suggested "fixes" to LoD violations that are usually covered in such discussions don't really cover the bases. IMHO, the tenet "tell, don't ask" leads us toward a solution more in keeping with the spirit of the law.
    e.g. man.getWallet().getMoney(double amount) would become man.pay(Payee payee) where Payee is an interface implemented by the Paperboy. In fact, it may even be implemented by the Paperboy's Wallet, which he passes to the Man. The Man neither knows nor cares - which is kind of the point.
    To apply this to Order.getCustomer().getName() we have to think about what we plan to do with the Customer's Name. Are we just displaying it somewhere? If so, perhaps order.display(OrderDisplay display) would be an alternative solution. You would implement the OrderDisplay to do what you wanted to do with the order details.
    IMHO, this solution is really what the LoD should lead us towards. I should also point out that this will not always be "better" than the LoD violation, as it can tend to make the class take on too many responsibilities. e.g. is it really the job of an Order to display itself? Perhaps it is in your model - or perhaps not. Some judgement is required.

  8. Avatar for Gary
    Gary December 5th, 2010

    I thought Dementers were something to do with Harry Potter.

  9. Avatar for Headlight Converters
    Headlight Converters May 23rd, 2011

    well done, nice informations about LoD

  10. Avatar for Alan
    Alan June 21st, 2011

    It does seem pretty silly to say that data must be normalized in the DB and it must be fully denormalized in the application.

  11. Avatar for James
    James February 21st, 2012

    Good summary.
    As with all software design principles, LoD is mostly a good suggestion. For most internal method calls, if the object is purposefully tightly coupled to another object (by design, where they are separate objects conceptually, but not really decoupled functionally), violating LoD can be OK, because it wouldn't really be able to decouple anything without creating a sea of objects that "don't make sense" to most people. For external calls, LoD really ought to be followed, because it is far too easy to provide unseen security holes by sending an entire object, as opposed to a specific set of data that can be verified as secure.
    The page view case is more in the "internal" category, since it is still local code, and some items like reports are intrinsically bound to the underlying data, no matter how many layers you put between the view and the data. But if there are many reports that look at similar data, it can make sense to centralize that aspect of the data model, and create a specific viewmodel that flattens it out for each slightly different report. That way, if the underlying data structure changes, you only need change the properties in that single viewmodel, instead of in each of the dozens of reports that reference it.

  12. Avatar for Theodore R. Smith
    Theodore R. Smith August 14th, 2012

    It looks like this article is a **blatant** plagiarism of http://www.dan-manges.com/blog/37, written exactly two years earlier... Care to explain?

  13. Avatar for haacked
    haacked August 14th, 2012

    Well we both discuss the same topic, but it's clearly not plagiarism. The approach we take and the language used is very different. The only similarity is our use of metasynctatic variables foo.bar.baz.

  14. Avatar for haacked
    haacked August 14th, 2012

    Oh, the other similarity is that I reference and properly attribute the paperboy example by David Bock.
    The post you mentioned also uses that example but fails to attribute it. Perhaps that led to your false charge of plagiarism.

  15. Avatar for Theodore R. Smith
    Theodore R. Smith September 12th, 2012

    haacked, that could very well be it.
    I looked again at the article you referenced. Its example looks to be in C# and both of your guys is in Python.
    The thing that looked the weirdest was how you both have
    <%= @order.customer.name %>
    and then

    <%= @order.customer_name %>
    But that could very well be chance, and you're right, the other guy doesn't attribute at all, further muddying the waters.
    I apologize.

  16. Avatar for Sergey Akulinin
    Sergey Akulinin July 25th, 2013

    Very good article!
    As for me, next example shows real reason of LoD usability:

    public function order($application) {
    $application->getProduct()->addToCart();
    $application->getCustomer()->checkout();
    }


    Why we don't pass real dependency for the function (customer and product), but pass application object? LoD is violated here and shows us bad architecture.
    In next example I will never want to follow LoD:

    public function testStructure($root) {
    $directoryElements = $root->getDirectory()->getSubDirectory()->getChildren();
    $this->assertGreaterThan(0, count($directoryElements));
    $anotherDirectoryElements = $root->getAnotherDirectory()->getChildren();
    $this->assertGreaterThan(0, count($anotherDirectoryElements));
    }


    IMHO LoD should be re-defined in the way to handle the difference of 2 above examples.

  17. Avatar for Christian Tietze
    Christian Tietze April 30th, 2014

    I wonder: what is your opinion on (1) "presenter" objects, and (2) "data-transfer object"-like representations of the `Order` model for rendering purposes?

    In Rails-land, both approaches seem like over-engineering at first.

    I think it's reasonable to give `Order` the ability to compile an `OrderRepresentation` or something similar which is just a value object containing all the data the view likes to have. This way, the presentation layer can't access the model objects. Although we wouldn't do this intentionally of course, this way we can't even modify model objects by accident when we're writing view templates.

    I think this separation has its benefits: security and a buffering layer between model and presentation. But it adds complexity to the overall application because it introduces a presentation object for every aggregate (or every entity).

  18. Avatar for Programaths
    Programaths February 13th, 2015

    I am glad that PHP uses arrows "->" and that XQuery uses slahses "/" !

  19. Avatar for JeanFrederic Nault
    JeanFrederic Nault August 16th, 2015

    For the examble of the view with the order. Order should not have a property username, it should have a method getUserName that will use the user object.

    So the view know the order
    The order now the user

    The order expose through its public interface a method to communicate with its user object

  20. Avatar for Jeffrey L. Whitledge
    Jeffrey L. Whitledge October 7th, 2015

    A piece of code that is designed to display an order does not need to know—and should not know—anything about customer names. To do so is not only a violation of the LoD, but it is also (more importantly) a bad design.

    Is the customer name stored in one field or two or several? It is common to have first_name and last_name. It could also be prefix and sort_name, to better handle international names. Perhaps titles are stored separately. Perhaps there is a formal name and a familiar name. Is there a maximum length for the name?

    Can any of these things change? And if they change, how much code is expected to change to support that?

    Why should the programmer who is responsible for displaying orders need to get into the nuts-and-bolts of customer names?

    The correct design clear and simple: The code that displays the order information does not display customer's names, but rather it displays a Customer. An entirely different piece of code will be responsible for figuring out how the customer is to be displayed.

  21. Avatar for Alex Perez
    Alex Perez December 21st, 2016

    Regarding the example with the view, wouldn't using a Data Transfer Object (https://en.wikipedia.org/wi... be the better choice versus the existing two? In that case, there's no violation of LOD in the view itself since all the data used in the View are fields in the DTO, which becomes the only place where where the coupling should happen, AKA the only object that needs to talk to the View's dependencies is the DTO.

  22. Avatar for Roy Stegers
    Roy Stegers May 19th, 2017

    Very good acticle!