Things I Learnt The Hard Way - The Magical Number Seven, Plus Or Minus Two

3 minute read Published: 2019-06-26

"The magical number" is a psychology article about the number of things one can keep in their mind at the same time.

I've seen twice this weird construction on where a function would do some processing, but its return value was the return of this processing, plus the result of a second function and some bit of processing. Nothing major. But the second function would also do some processing and call a third function. And the third function would call a fourth. And the fourth a fifth. And the fifth, a sixth function.

And the "processing" was not something small, like "add two" or "multiply by itself or a configurable value".

Something like this

func_1
 +-- func_2
     +-- func_3
         +-- func_4
             +-- func_5
                 +-- func6

(If you need the real processing I saw, it was a class that had a function with some processing and then it would call a function in an injected dependency -- which is pretty nice and dandy. But the injected dependency had an injected dependency, and the third injected dependency also had an injected dependency, and so forth).

Now, when you're trying to understand this kind of code to find a problem, you'll have to keep in mind what the first, second, third, fourth, fifth and sixth functions do, 'cause they are all calling each other (inside them).

This causes some serious mental overflow that shouldn't be necessary.

Not only that, but imagine that you put a log before and after func_1: The log before points the data that's being send to func_1, and the log after its result.

So you'd end up with the impression that func_1 does a lot of stuff, when it actually is passing the transformation along.

(I got a weird experience with a function called expand, which logging before the call would show some raw, compressed data, but the after was not the expanded data, but actually a list of already processed data from the compressed data.)

What would be a better solution, you may ask?

Well, if instead of making func_1 call func_2, you can make it return the result (which may not be the final result, anyway) and then call func_2 with that result.

Something like:

result1 = func_1
result2 = func_2(result1)
result3 = func_3(result2)
result4 = func_4(result3)
result5 = func_5(result4)
result6 = func_6(result5)
result7 = func_7(result6)

(If we go back to the dependency injection chain, you may think that instead of making DI7 receive DI6 as dependency [which would receive DI5 as dependency, which would receive DI4 as dependency, which would receive DI3 as dependency and so forth] you would actually create all the DI (dependency injections) in one single pass and then the starting function would call the dependencies in a single shot, not chaining them.)

Now you can see exactly how the data is being transformed -- and, obviously, the functions would have better names, like expand, break_lines, name_fields and so on, so you can see that that compressed data I mentioned before is actually being decompressed, the content is being broke line by line, the lines are getting names in its fields and so on (and one could even claim that it would make things clear if there was a function after break_lines which would just break_fields, which would make name_fields more obvious -- and in a construction like this it would be almost trivial to add this additional step).

"But that isn't performant!" someone may cry. Well, maybe it's just a bit less performant than the original chained-calls ('cause it wouldn't create and destroy frames in the stack, it would just pile them up and then "unstack" them all in the end), but heck, optimization is for compilers, not people. Your job is to make the code readable and understandable. If you need performance, you can think of a better sequence of steps, not some "let's make this a mess to read" solution.

Just a quick note: Although the famous paper mentions that the number is around 7, new research is actually pointing that the number is way lower than that, at 4. So simply making func_1 call func_2, which would call func_3, which would call func_4 may be enough to overload someone and make them lose the track of what the code does.

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