What Is Functional Programming

Functional programming should not be confused with imperative (or procedural) programming. Neither is it like object oriented programming. It is something different. Not radically so, since the concepts that we will be exploring are familiar programming concepts, just expressed in a different way. The philosophy behind how these concepts are applied to solving problems is also a little different.

Functional programming is all about expressions. In fact another way to describe FP might be to term it expression oriented programming since in FP everything reduces to an expression. You should recall that an expression is a collection of operations and variables that results in a single value. Thus x == 5 is a boolean expression. 5 + (7-Y) is an arithmetic expression. And "Hello world".uppercase() is a string expression. The latter is also a function call (Or more strictly a method call) on the string object "Hello world" and, as we shall see, functions are very important in FP (You might already have guessed that from the name!).

Functions are used as objects in FP. That is they are often passed around within a program in much the same way as other variables. We have seen examples of this in our GUI programs where we assigned the name of a function to the command attribute of a Button control. We treated the event handler function as an object and assigned a reference to the function to the Button. This idea of passing functions around our program is key to FP.

Functional Programs also tend to be heavily List oriented.

Finally FP tries to focus on the what rather than the how of problem solving. That is, a functional program should describe the problem to be solved rather than focus on the mechanism of solution. There are several programming languages which aim to work in this way, one of the most widely used is Haskell and the Haskell web site ( www.haskell.org) has numerous papers describing the philosophy of FP as well as the Haskell language. (My personal opinion is that this goal, however laudable, is rather overstated by FP's advocates.)

A pure functional program is structured by defining an expression which captures the intent of the program. Each term of the expression is in turn a statement of a characteristic of the problem (maybe encapsulated as another expression) and the evaluation of each of these terms eventually yields a solution.

Well, that's the theory. Does it work? Yes, sometimes it works very well. For some types of problem it is a natural and powerful technique. Unfortunately for many other problems it requires a fairly abstract thinking style, heavily influenced by mathematical principles. The resultant code is often far from readable to the layman programmer. The resultant code is also very often much shorter than the equivalent imperative code and more reliable.

It is these latter qualities of conciseness and reliability that have drawn many conventional imperative or object oriented programmers to investigate FP. Even if not embraced whole heartedly there are several powerful tools that can be used by all.