Explaining functional programming to object-oriented programmers and less technical people

Question

What are some good examples that I can use to explain functional programming?

The audience would be people with little programming experience, or people who only have ob

Answer 1

The ones with little programming experience are probably easier, since they won't have as many programming concepts to get in the way: functional programming is very similar to mathematical functions as expressions, so show them some math.

For the OO crowd, you could show how closures are like encapsulation, and thus how functional programming encompasses OOP. You could also show how useful higher-order functions are, in particular how they cause some OOP patterns (such as the Strategy and Template patterns, and the Visitor pattern in languages with multimethods) fade away into the language (Peter Norvig asserts that 16 of the 23 GOF patterns are simpler in Dylan and Lisp). As a counterpart, message-passing based OOP (rather than the CLOS approach) is a pattern in FP that is invisible in OOP.

Demonstrate how easy it is to write general arithmetic expressions, along with function composition and differentiation, as programming constructs and you've got both of them.

You may or may not want to show the OO people CLOS's multimethods; they'll either riot or have a geekgasm. Either way, it's likely to be messy.

Some other possibilities to show how flexible FP is: lazy evaluation can be implemented using nullary anonymous functions. FP can support both class-based and prototype-based OOP. There are plenty of other examples (e.g. continuation-passing style), but they generally require familiarity with FP before learning.

SICP has plenty of interesting examples and problems; it should prove inspirational.