Lisp’s Functional Programming advantage

Functional Programming is the art of writing programs that work by returning values, instead of modifying things. Functional Programming also enables you to create fabulously powerful and very efficient abstract programs – it is a mathematical approach to programming. In math, for at least the last 400 years, functions have played a very central role. Functions express the connection between parameters (the ‘input’) and the result (the ‘output’) of certain processes. In each computation, the result depends, in a certain way, on the parameters. Therefore, a function is a good way of specifying a computation. This is the basis of the functional programming style. A ‘program’ consists of the definition of one or more functions. With the ‘execution’ of a program, the function is provided with parameters, and the result must be calculated. Writing code in a functional style guarantees that a function does only one thing (returns a value), and is dependent on one thing (the parameters passed to it). This equips you to control side effects.

However, some side effects are almost always necessary for a program to actually do something. This means that you can’t write a useful program that has the entirety of its code written in the functional style. In Article 8, where we talked about CLOS, I pointed out James Hague’s assessment of functional programming, where he argues that, “100 per cent pure functional programming doesn’t work. Even 98 per cent pure functional programming doesn’t work. But if the slider between functional purity and 1980s BASICstyle imperative messiness is kicked down a few notches—say, to 85 per cent—then it really does work. You get all the advantages of functional programming, but without the extreme mental effort and unmaintainability that increases as you get closer and closer to perfectly pure.”