My, my, look at all those notations. You can infix: 2 + 2. You can prefix: + 2 2. You can postfix: 2 2 +. All three notations are perfectly arbitrary for the purposes of mathematics. (2 + 2) * 3 = 12, so what’s really different about these codes?
x = 2 + 2 y = x * 3 print(y)
x := 2 plus: 2 y := x times: 3 y print
2 2 + 3 * print
(let* ( (x (+ 2 2)) (y (* x 3))) (print y))
Code 1 is typical of procedural programming (Python1, C, Basic, Pascal). You declare variables, store data in variables, and display variables.
Code 3 is typical of stack programming (Factor, Joy, Forth). You push values onto the stack, push functions onto the stack, and display the stack.
Code 4 is typical of functional programming (Haskell, ML, Erlang, Lisp). You pass expressions, evaluate expressions, and display expressions.
But what’s really different?
Codes 1 and 2 are fundamentally the same: they’re stateful computations. Code 1 stores state in variables, Code 2 stores state in objects, but it’s all the same. Code 2 is syntactical sugar for Code 1. And Code 1 is syntactical sugar for:
add x,2,2 mul y,x,3 put y
In other words, you’re still explicitly performing register transactions. Do this calculation then STORE THE RESULT IN HERE. Do that calculation then MOVE THE BYTES TO THERE.
By contrast, Codes 3 and 4 are an island all their own: they’re functional computations. Instead of manipulating machine registers, they manipulate mathematical expressions3. Code becomes data becomes code again; you can pass functions as arguments to other functions as if they were pure mathematical constructs. Code 4 named the expressions “x” and “y”, while Code 3 didn’t, but as Code 3 shows, naming isn’t really necessary, it’s just a convenience.
So what’s really different? Functional programming provides a whole new level of abstraction, one that allows you to write more complex code than you could in low-level languages like Java. Really. Do you like moving bytes around, or would you prefer to leave that as an exercise for the compiler?
2 Ditto for Ruby and Smalltalk. They have blocks, and therefore can do functional programming, but it’s not emphasized. With all the OOP hullabaloo, the incentive is to write stateful methods instead of pure methods, especially in Ruby where the line between f and f! blurs because the result of the last evaluated expression is always returned.
3 Most functional languages have stateful capabilities, but they’re strictly only necessary for specialized calculations such as pseudo-random number generation, concurrent programming, and I/O. Also, functional language compilers must be stateful in order to compile functional programs that provide the abstraction of pure, stateless code. (see Haskell).