Do not stop not thinking negatively

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Have you ever run across a variable called something like undisable? It takes some mental lifting to turn it into enable. The conversion from something into its negation is almost like removing a layer of abstraction.

With this in mind, it's usually clearer to write if something than it is to write if not something. Of course, there are times you don't have a choice - you have to use not. But the other day I ran across a case where I didn't immediately realize I had a choice. I caught myself staring at something like:

if ( ! previouslyCached || ! linked ) {
    methodA();
} else {
    methodB();
}

I was looking at this piece of code because a path was dropping into the wrong fork. The two nots do not help, and in this case, the or gets in the way too.

Is this clearer?

if ( previouslyCached && linked ) {
    methodB();
} else {
    methodA();
}

The two code snippets above are logically equivalent. They accomplish the same thing, but I think the second one is much easier to read.

Removing the nots makes it clearer, and in this case, switching to and makes it clearer. There are four cases we are concerned with: one of the variables is true while the other is false (2 possibilities for this one), both are true, or both are false. With the first if, the or is capturing three of these possibilities, with the second snippet - the and - we are only capturing one. In this case specificity means clarity. It's easier to imagine both previouslyCached and linked being true then it is to imagine the either one or both of the opposites of previouslyCached or linked being true. The else acts as a natural "catch all" in the second snippet, while it acts as a strangely specific and narrow filter in the second one.