Constants
I already said that variables have that name because their values can change over time, and that is often useful. However, very often you want to set a value once and never change it, and so we have an alternative to the var keyword called let.
The let keyword creates constants, which are values that can be set once and never again. For example:
let taylor = "swift"If you try to change that, Xcode will refuse to run your code. It’s a form of safety: when you use constants you can no longer change something by accident.
When you write your own Swift code, you should always use let unless you specifically want to change a value. In fact, Xcode will warn you if you use var then don’t change the variable.
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