# Modular Programming

We've been building very small programs. When a program is small enough, we can keep all of the details of the program in our heads at once. Real application programs are 100 to 10000 times larger than any program you have likely written (or maybe even worked on); they are simply too large and complex to hold all their details in our heads. They are also written by multiple authors. To build large software systems requires techniques we haven't talked about so far.

One key solution to managing complexity of large software is modular programming: the code is composed of many different code modules that are developed separately. This allows different developers to take on discrete pieces of the system and design and implement them without having to understand all the rest. But to build large programs out of modules effectively, we need to be able to write modules that we can convince ourselves are correct in isolation from the rest of the program. Rather than have to think about every other part of the program when developing a code module, we need to be able to use local reasoning: that is, reasoning about just the module and the contract it needs to satisfy with respect to the rest of the program. If everyone has done their job, separately developed code modules can be plugged together to form a working program without every developer needing to understand everything done by every other developer in the team. This is the key idea of modular programming.

Therefore, to build large programs that work, we must use abstraction to make it manageable to think about the program. Abstraction is simply the removal of detail. A well-written program has the property that we can think about its components (such as functions) abstractly, without concerning ourselves with all the details of how those components are implemented.

Modules are abstracted by giving specifications of what they are supposed to do. A good module specification is clear, understandable, and gives just enough information about what the module does for clients to successfully use it. This abstraction makes the programmer's job much easier; it is helpful even when there is only one programmer working on a moderately large program, and it is crucial when there is more than one programmer.

Languages often contain mechanisms that support modules directly. OCaml is one, as we will see shortly. In general (i.e. across programming languags), a module specification is known as an interface, which provides information to clients about the module's functionality while hiding the implementation. Object-oriented languages support modular programming with classes. The Java interface construct is one example of a mechanism for specifying the interface to a class (but by no means the only one). A Java interface informs clients of the available functionality in any class that implements it without revealing the details of the implementation. But even just the public methods of a class constitute an interface in the more general sense—an abstract description of what the module can do.

Once we have defined a module and its interface, developers working with the module take on distinct roles. It is likely that most developers are clients of the module who understand the interface but do not need to understand the implementation of the module. A developer who works on the module implementation is an implementer. The module interface is a contract between the client and the implementer, defining the responsibilities of both. Contracts are very important because they help us isolate the source of the problem when something goes wrong—and know who to blame!

It is good practice to involve both clients and implementers in the design of a module's interface. Interfaces designed solely by one or the other can be seriously deficient, because each side may have its own view of what the final product should look like, and these may not align. So mutual agreement on the contract is essential. It is also important to think hard about global module structure and interfaces early, even before any coding is done, because changing an interface becomes more and more difficult as the development proceeds and more of the code comes to depend on it. Finally, it is important to be completely unambiguous in the specification. In OCaml, the signature is part of writing an unambiguous specification, but is by no means the whole story. While beyond the scope of this course, Interface Description (or Definition) Languages (IDL's) are used to specify interfaces in a language-independent way so that different modules do not even necessarily need to be implemented in the same language.

In modular programming, modules are used only through their declared interfaces, which the language may help enforce. This is true even when the client and the implementer are the same person. Modules decouple the system design and implementation problem into separate tasks that can be carried out largely independently. When a module is used only through its interface, the implementer has the flexibility to change the module as long as the module still satisfies its interface.