Functional programming has been getting a lot of attention lately and seems to be the new hotness among software developers. As new as it seems, it's actually been around from the beginning of programming. The second high level programming language was LISP in 1958 and is purely functional. Only FORTRAN is older, by one year.

Functional programming is based on mathematical functions rather than imperative statements. Imperative languages mimic the way a computer works, by making statements one at a time in sequence. Assembly languages are probably the best examples.

Most modern languages have functional capabilities along with imperative statements, like JavaScript, Go and Tcl. Others, like LISP, ML, OCaml and Haskell have built-in functional capabilities like currying, which composes functions out of simple ones that take just one argument. In other languages with partial functional support, add-on libraries provide this. In JavaScript Underscore is the most well known.

The most commonly used and well known functional language is SQL. In an imperative language, the programmer tells the computer what to do, step by step. In SQL, the programmer makes declarations about what the results should be, not how to do them. That's the hallmark of a functional language.

It's been pointed out many times that functional programming is much more than just learning another language. It requires looking at programming in a new way.

In my humble opinion, the best way to understand functional programming is by learning Haskell. There are many books and web sites about functional programming in JavaScript, and most of them refer to Haskell in illustrations and pseudo code. Fine, if you already know Haskell, but frustrating if you don't, like happened to me at first.

There a lots of advantages of functional programming. For one, it's based on mathematical proofs, which means they must be correct if written properly. If a program is not written properly, the compiler will catch it first so it's very unlikely that bugs will show up at runtime.

Being math based, Haskell programs look a lot like math functions. In fact, they are math functions, written is computer-friendly UTF-8 rather than Greek letters. Without knowing the language of mathematics, programs don't make a lot of sense. Be prepared to learn or catch up on your math, but I consider that more of an asset than a liability.

Functional programming isn't for everyone, and isn't always the best approach for solving particular problems. A survey of programmers recently showed that Haskell programmers use it for their own home projects at a much higher percentage than for business. I suspect that it's not used much for projects in larger organizations. Not that it's a toy language, by any means. The Pandoc document converter is written entirely in Haskell, and is quite popular. I use it regularly. It is part of the automated process I used to create this document

I agree with the philosophy behind function programming, and apply it to code written in other languages when we can, for all the reasons shown above. If you think this will help when working on one of your projects, keep me in mind.