Someone asked me this weekend how to get started with functional programming. My answer: Start by writing pure functions in the programming language you’re currently using.
The only input to a pure function is its argument list and the only output is its return value. If you haven’t seen this before, you might think all functions are pure. After all, any function takes in values and returns a value. But in conventional programming there are typically out-of-band ways for information to flow in or out of a function. For example, an impure function may depend on a global variable or class member data. In that case, it’s behavior is not entirely determined by its arguments. Similarly, an impure function might set a global variable or write to a database. In that case the function has a side effect in addition to its return value.
You can write pure functions in any language, though it’s easier in some languages than others. For example, no one would call Fortran a functional language, but there are people (M. J. D. Powell comes to mind) who discipline themselves to write pure functions in Fortran.
Why write pure functions? A pure function has referential transparency, meaning it will always return the same value when given the same inputs. The output does not depend on the system time, the state of a database, which functions were called previously, or anything else that is not explicitly passed as an argument to the function. This means pure functions are easier to understand (and hence easier to debug and test).
You can’t always write pure functions. If you need to stick a value in a database, this cannot be accomplished with a pure function. Or if you’re calling a random number generator, you don’t want it to have referential transparency, always returning the same output! But the goal is to use pure functions when practical. You want to eliminate out-of-band communication when it’s convenient to do so. Total purity is not practical; some argue that the sweet spot is about 85% purity.
So why don’t programmers use pure functions more often? One reason is that pure functions require longer argument lists. In an object oriented language, object methods can have shorter argument lists by implicitly depending on object state. The price to pay for shorter method signatures is that you can’t understand a method by itself. You have to know the state of the object when the method is called. Is it worthwhile to give up referential transparency in order to have shorter method signatures? It depends on your context and your taste, though in my opinion its often worthwhile to use longer function signatures in exchange for more pure functions.
Another reason people give for not writing pure functions is that its too expensive to copy large data structures to pass them into a function. But that’s what pointers are for. You can conceptually pass an object into a function without having to actually make a copy of the object’s bits.
You can also fake purity for the sake of efficiency. For example, Mike Swaim left a comment recently giving an example of how memoization sped up a program by several orders of magnitude. (Memoization is a technique of caching computations. When a function is asked to compute something, it first looks to see whether it has already done the calculation. If so, it returns the cached value. If not, it does the calculation and adds its output to the cache.) A function that uses memoization is not strictly pure — its calculations have a persistent impact on the state of its cache — but such a function can still have referential transparency, always returning the same output given the same input. You could say it’s cheating to call such functions pure, and it is, but if you’re really a stickler about it, all pure functions have side effects.