The Swift Programming Language (Swift 4.1): Patterns
original source : https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Patterns.html
Value-Binding Pattern
A value-binding pattern binds matched values to variable or constant names. Value-binding patterns that bind a matched value to the name of a constant begin with the let keyword; those that bind to the name of variable begin with the var keyword.
Identifiers patterns within a value-binding pattern bind new named variables or constants to their matching values. For example, you can decompose the elements of a tuple and bind the value of each element to a corresponding identifier pattern.
let point = (3, 2)switch point {// Bind x and y to the elements of point.case let (x, y):print("The point is at ((x), (y)).")}// Prints "The point is at (3, 2)."
In the example above, let distributes to each identifier pattern in the tuple pattern (x, y). Because of this behavior, the switch cases case let (x, y): and case (let x, let y): match the same values.