The Evolution of Programming Paradigms

My career as a programmer has spanned more than forty years. In that time I have seen a number of programming paradigms promise to make programming easier, more robust, and less error prone. Let’s review these paradigms in roughly chronological order.

Early in my career the latest advance in programming paradigms was Structured Programming.  The essential insight of Structured Programming was that programs should be written using a small set of control structures and that the primitive goto instruction should be shunned. The benefit was more readable code that could be more easily analyzed. The goto ridden code was often referred to as spaghetti code.

Next to come into vogue was the paradigm called Object Oriented Programming. Objects were intended to provide several benefits. First they were aimed at grouping functions with the data that it operated on. It also introduced the concept of encapsulation or data hiding. Data, and functions for that matter, that were not specified as public could not be accessed or modified by code outside the object.

Most popular Object Oriented languages were based on the idea of Classes that specified the data and functional template from which individual object instances were created. These Classes were often organized in a hierarchy such that the more general Classes specified the common data and functional characteristics of a group of objects. The more specific members of the class hierarchy would then add or override data or functional characteristics of the subclass to refine the representation and behavior of the instances of the subclass.

As it turns out, in most cases class hierarchies just added unnecessary complexity to programs. They also introduced problems such as what if you have a class that inherits from two different parent classes. For example, suppose that you had a BankAccount class that represented the ledger of a bank account, the deposits, the withdrawals, and the current balance. Suppose there was another class that represented a PrintableReport. Suppose that you wanted to create a class BankAccountReport that inherited attributes from both the BankAccount class and the PrintableReport class. Now here’s the quandary. Both superclasses have an operation called addItem. Which operation should the child class inherit, the one from BankAccount or the one from PrintableReport? This created so many problems that many Object Oriented languages only allowed a class to have a single super class.

Next to the scene was Aspect Oriented Programming. It’s claim to fame was a solution to the problem of multiple inheritance or, as it referred to it, cross cutting concerns. Aspects were a mechanism that allowed the programmer to conditionally alter the behavior of an object by modifying the behavior of a class without modifying its implementation. It did this by capturing calls to it’s methods and allowing the programmer to intervene before or after the call to the aspected operation of the underlying class.

The latest paradigm is not really new. Functional Programming goes back to the early days of Lisp. It says that functions, in the mathematical sense, should map inputs to outputs without causing any side effects. Functions should further be first class entities in the language. This means that they should be allowed to be stored in variables and passed as arguments to other functions.

Strict functional programming is difficult if not impossible to achieve practical results with. Most programs take input from somewhere and output results to somewhere. Taking input and outputting results are both violations of the constraint that the output of a function should depend solely on it’s input. Most functional languages have well defined accommodations for operations that aren’t strictly functional.

This was a whirlwind tour. I hope it gave an overview of the evolution of programming paradigms over the last forty years. Look these programming paradigms up on Wikipedia if you want to know more about them. Or if I have managed to confuse you totally.

Sweet dreams, don’t forget to tell the ones you love that you love them, and most important of all, be kind.