SICP Goodness - Why Assignment Is Still Useful in Functional Programming

Guowei Lv

3 minute read

Do you think Computer Science equals building websites and mobile apps?

Are you feeling that you are doing repetitive and not so intelligent work?

Are you feeling a bit sick about reading manuals and copy-pasting code and keep poking around until it works all day long?

Do you want to understand the soul of Computer Science?

If yes, read SICP!!!

In FP world people bash assignment. They say it is the root of all evils. But is it really true?

First we have to ask why assignment? The answer is simply to achieve better modularity in program design. Let’s see an example.

Suppose we are designing a counter, which simply starts from 0, and increase by 1 every time it is called.

Let’s first implement it using assignment.

(define (make-counter)
  (let ((count 0))
    (lambda ()
      (set! count (+ count 1))
      count)))

The make-counter is a constructor, which returns a lambda that has access to a local variable count.

Now let’s try to use it inside exponential function.

(define (exp x n counter)
  (if (> (counter) n)
      1
      (* x (exp x n counter))))

I know this is a bit contrived. But it is still a valid implementation and kind of straightforward.

Now, let’s implement the counter without using any assignment.

(define (counter current-count)
  (+ current-count 1))

A counter is just a function that increase the current count by one, fair enough.

Now let’s use it in the same exponential function.

(define (exp x n counter current-count)
  (let ((new-count (counter current-count)))
    (if (> new-count n)
        1
        (* x (exp x n counter new-count)))))

Notice that the function takes one more parameter current-count. And it is always 0 when calling the function. (exp 4 2 0). Imagine if a procedure A calls exp, then A needs to know to put 0 as the current-count. In other words, the internals of counter has leaked into exp.

If you think this is not too bad, let’s tweak the contrived example to be even more contrived.

Suppose that the we want to control the speed of the counter. Say we want to increase 3 by each count.

In the assignment version of the counter, we just change to (set! count (+ count 3)). No other places needs to change.

While in the non-assignment version. We have to modify the expo to be:

(define (expo x n counter current-count)
  (let ((c1 (counter current-count)))
    (let ((c2 (counter c1)))
      (let ((c3 (counter c2)))
        (if (> c3 n)
            1
            (* x (expo x n counter c3)))))))

Now the internals of the counter logic made a big mess in expo.

If you think my example is too contrived, you can still read the original example given in the book on section 3.1.2.

Hope that after reading these examples, you will understand why assignments are still useful sometimes.

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