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Functions use func. Parameters have external labels and internal names. The external label is what callers write at the call site:

func greet(name: String, greeting: String = "Hello") -> String {
    return "\(greeting), \(name)!"
}

print(greet(name: "Alice"))                           // Hello, Alice!
print(greet(name: "Bob", greeting: "Howdy"))          // Howdy, Bob!

To omit the label at the call site, prefix with _:

func add(_ a: Int, _ b: Int) -> Int { a + b }
add(3, 4)   // no labels

For single-expression functions, drop the return. The return type is -> Type after the params. Void (or nothing) means no return.