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# Cognitive Cost Is The Readability Killer
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"[Cognitive dissonance](https://en.wikipedia.org/wiki/Cognitive_dissonance)"
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is a fancy way of saying "I need to remember two (or more) different and
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contradicting things at the same time to understand this." Keeping those
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different things in your head creates a cost and it keeps accumulating the
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more indirect the things are ('cause you'll have to keep all those in your
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head).
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(Disclaimer: I like to use the expression "cognitive dissonance" to make me
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sound smarter. I usually explain what it means, though.)
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To give you an example of a (very mild) cognitive cost, I'll show you this:
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* You have a function called `sum()`. It does the sum of the numbers of a
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list.
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* You have another function, called `is_pred()`. It gets a value and, if it
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fits the predicate -- a test, basically -- returns True; otherwise,
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returns False.
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So, pretty simple, right? One function sums numbers and another returns a
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boolean.
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Now, what would you say if I shown you this, in Python:
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```python
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sum(is_pred(x) for x in my_list)
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```
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Wait, didn't I say that `sum()` sums numbers? And that `is_pred()` returns a
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boolean. How can I sum booleans? What's the expected result of True + True +
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False?
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Sadly, this works. Because someone, long time ago, didn't think booleans were
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worth a thing and used an integer instead. And everyone else since then did
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the same stupid mistake.
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But, for you, you'll now read a line that says "summing a boolean list returns
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a number". And that's two different, disparate things that you suddenly have
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to keep in mind when reading that line.
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That's why [types are important](../coding/data-types.md). Also, this may
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sound a bit like [the magical number seven](./magical-number-seven.md), 'cause
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you have to keep two things at your mind at the same thing but, although
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that's not near seven, they are not the same, with opposite (for weird
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meanings of "opposite", in this case) meanings.
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