Does Scala's pattern matching violate the Open/Closed Principle?

Question

If I add a new case class, does that mean I need to search through all of the pattern matching code and find out where the new class needs to be handled? I've been learning the language recently, and as I read about some of the arguments for and against pattern matching, I've been confused about where it should be used. See the following:

Pro: Odersky1 and Odersky2

Con: Beust

The comments are pretty good in each case, too. So is pattern matching something to be excited about or something I should avoid using? Actually, I imagine the answer is "it depends on when you use it," but what are some positive use cases for it and what are some negative ones?

Answer 1

Jeff, I think you have the right intuition: it depends.

Object-oriented class hierarchies with virtual method dispatch are good when you have a relatively fixed set of methods that need to be implemented, but many potential subclasses that might inherit from the root of the hierarchy and implement those methods. In such a setup, it's relatively easy to add new subclasses (just implement all the methods), but relatively difficult to add new methods (you have to modify all the subclasses to make sure they properly implement the new method).

Data types with functionality based on pattern matching are good when you have a relatively fixed set of classes that belong to a data type, but many potential functions that operate on that data type. In such a setup, it's relatively easy to add new functionality for a data type (just pattern match on all its classes), but relatively difficult to add new classes that are part of the data type (you have to modify all the functions that match on the data type to make sure they properly support the new class).

The canonical example for the OO approach is GUI programming. GUI elements need to support very little functionality (drawing themselves on the screen is the bare minimum), but new GUI elements are added all the time (buttons, tables, charts, sliders, etc). The canonical example for the pattern matching approach is a compiler. Programming languages usually have a relatively fixed syntax, so the elements of the syntax tree will change rarely (if ever), but new operations on syntax trees are constantly being added (faster optimizations, more thorough type analysis, etc).

Fortunately, Scala lets you combine both approaches. Case classes can both be pattern matched and support virtual method dispatch. Regular classes support virtual method dispatch and can be pattern matched by defining an extractor in the corresponding companion object. It's up to the programmer to decide when each approach is appropriate, but I think both are useful.

Answer 2

While I respect Cedric, he's completely wrong on this issue. Scala's pattern matching can be fully-encapsulated from class changes when desired. While it is true that a change to a case class would require changing any corresponding pattern matching instances, this is only when using such classes in a naive fashion.

Scala's pattern matching always delegates to the deconstructor of a class's companion object. With a case class, this deconstructor is automatically generated (along with a factory method in the companion object), though it is still possible to override this auto-generated version. At all times, you can assert complete control over the pattern matching process, insulating any patterns from potential changes in the class itself. Thus, pattern matching is simply another way of accessing class data through the safe filter of encapsulation, just like any other method.

So, Dr. Odersky's opinion would be the one to trust here, particularly given the sheer volume of research he has performed in the area of object-oriented programming and design.

As for where it should be used, that is entirely according to taste. If it makes your code more concise and maintainable, use it! Otherwise, don't. For most object-oriented programs, pattern matching is unnecessary. However, once you begin to integrate more functional idioms (Option, List, etc) I think you'll find that pattern matching will significantly reduce syntactic overhead as well as improving the safety offered by the type system. In general, any time you want to extract data while simultaneously testing some condition (e.g. extracting a value from Some), pattern matching will likely be of use.

Answer 3

Pattern matching is definitely good if you are doing functional programming. In case of OO, there are some cases where it is good. In Cedric's example itself, it depends on how you view the print() method conceptually. Is it a behavior of each Term object? Or is it something outside it? I would say it is outside, and makes sense to do pattern matching. On the other hand if you have an Employee class with various subclasses, it is a poor design choice to do pattern matching on an attribute of it (say name) in the base class.

Also pattern matching offers an elegant way of unpacking members of a class.