ezyang’s blog

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MVC and Purity

Attention conservation notice. Purely functional programming demonstrates the same practices recommended by object-oriented MVC practice.

Model-View-Controller is a widely used object-oriented design pattern for organizing functionality in an application with a user interface. I first ran across it in my early days programming web applications. The Model/View separation made deep intuitive sense to me as a PHP programmer: without it, you’d end up with spaghetti templates with HTML print statements interleaved with MySQL queries. But Controller was always a little wishy-washy. What exactly did it do? It was some sort of “glue” code, the kind of stuff that bound together the Model and View and gave them orders. But this was always a sort of half-hearted answer for me (where should input validation go?), and soon I left the world of web applications, my questions unanswered.

Having been exposed to purely functional programming, I now believe that the controller and model/view separation is precisely the separation between side-effectful code (IO) and pure code.

The controller depends on the model and the view, but the model and view should not (directly) depend on the controller. Pure code and impure code don't mix freely. In particular, you're not allowed to reference impure code from pure code (unless you use unsafePerformIO). However, impure code can call pure code (although there may be some technical details involved), and the resulting code is impure. So, if the Controller is impure code and the Model/View is pure code, separating the two is simply making sure that if we have any code that is impure, we've extracted as much of the pure computation out of it as possible. Stated differently, if I have a function that reads and writes data, and there are lines in it that don't have anything to do with IO, I should move them into their own function. Maybe those lines are the templating system, in which case it’s View; maybe those lines are running some complicated equation, in which case it’s Model. Pure/impure doesn't capture the model/view distinction.

The controller receives input and initiates a response. So, the controller is input-output, i.e. IO.

The controller handles events that affect the model or the view. Pure code sort of lives in a vacuum: it can do computation, but it can't do anything useful, since it can’t have any side effects and thus has no way for us to tell it what to compute, or to view the result of the computation. Impure code is the way we get anything done by handing off this information to our pure code.

There are several possible objections to this division. Here are a few of them:

Most object-oriented models are stateful, and state is not pure! There is a common misconception that state isn't pure, possibly arising from the fact that both IO and State are monads. However, I can turn a state monad into a single, pure value by simply running the state machine: code that is stateful is monadic, but it is also pure, since it doesn't have any external side effects. Shared state is a bit trickier, and usually not pure.

Controller code doesn’t have to be impure and here’s an example. Here I’ll indulge in a bit of prescriptivism: I bet you have a model, but one that is only tangentially related to your core business logic. If you have code that parses binary strings into message objects (but doesn’t actually handle transmitting or receiving those binary strings on the network), you have a mini-model of network messages. You should probably keep it separate from your real model, but for testability you should also keep it separate from your network code. Separation of concerns may be malleable, but the little IO in your type signature is always honest.

Some parting words about the purity hair-shirt: it is fairly widely acknowledged that busting out the MVC pattern makes your application more complex initially, and in a purely functional language, you’re forced to respect the distinction from the very beginning. Thus, writing small programs can be frustrating in a purely functional language because you don’t want to use the bulky but scalable engineering practice yet, and the language is making you do so from the very beginning. Haskell gives you a lot of rope to make it pleasant again, but it takes a while to get used to. On the plus side, when your program grows, this separation will continue to be enforced, and a messy refactoring may be averted.

8 Responses to “MVC and Purity”

  1. Martin Cron says:

    For me, the MVC light came on when I realized that it’s just a specific permutation of the mediator design pattern, where one type’s responsibility is to two or more other types remain ignorant/independent of each other. Along the lines of your pure/impure distinction, sometimes a controller’s responsibility is to encapsulate the messiness into one place.

    After getting into MVC, I’m still working to keep my controllers (as well as the Models and Views) very thin, and have them delegate anything meaningful to (pure) domain objects.

  2. Omar Gómez says:

    Dear Edward,

    Controller’s function is about decoupling user input from view elements. Is not that complicated. Can be implemented in many ways but its goal should be the same.

    Read this:

  3. Yes, Controller plays another important role in making sure the Model and View don’t know anything (to a point) about each other, which the pure/impure distinction doesn’t really capture.

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  5. More than 1 MVC says:

    The original MVC did have communication between View and Model! And quite a lot!

    The controller was a mediator for the user input, and when there were changes to the Model (INSERT, UPDATE, DELETE).

    But for simple data views (SELECTS), the view was able to directly access the model.

    It’s in the MVC pattern sponsored by Sun that the Controller is completely in between the Model and the View.

    Just look for J2EE MVC…

  6. I find this (the fragmentation and proliferation of differing interpretations) one of the annoying aspects of traditional design patterns, which is why I like looking to math for fishing out design principles.

  7. Hakara says:

    Hello Edward,

    I come from a Systems Engineering background (Mechanical, Electronics and Computing, have studied and gained experience in both imperative and functional languages, I thought I’d share my view as I’ve been thinking about this recently.

    Yes I do partially agree with you however I feel that the Controller in MVC should be made from pure functional code.
    The controller is the application logic which should perform make decisions based on View/Model states and make a decision. The decision should then invoke non-pure View or Model code to get the job done. The Controller does not need to maintain state, it should just watch the View and Model states, then make decisions.

    What do you think?

  8. At the end of the day, someone has to talk to the outside world. If that is not the controller, what is it? I don’t think the controller usually needs to maintain state: the model and the real world can maintain all of the necessary state. But sometimes it might need to. If the controller has complex logic not related to IO, this may be something that should be factored into its own model.

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