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Why does Swift have constants as well as variables?

Paul Hudson    @twostraws   

Updated for Xcode 12.5

Variables are a great way to store temporary data in your programs, but Swift gives us a second option that’s even better: constants. They are identical to variables in every way, with one important difference: we can’t change their values once they are set.

Swift really loves constants, and in fact will recommend you use one if you created a variable then never changed its value. The reason for this is about avoiding problems: any variable you create can be changed by you whenever you want and as often as you want, so you lose some control – that important piece of user data you stashed away might be removed or replaced at any point in the future.

Constants don’t let us change values once they are set, so it’s a bit like a contract with Swift: you’re saying “this value matters, don’t let me change it no matter what I do.” Sure, you could try to make the same contract with a variable, but one slip of your keyboard could screw things up and Swift wouldn’t be able to help. By using a constant instead – just by changing var to let – you’re asking Swift to make sure the value never changes, which is another thing you no longer need to worry about.

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