Fibonacci sequence in Swift

fibs.swift

/// Generator for Fibonacci sequence
func fibonacciGenerator() -> AnyGenerator<Int> {
    var a = -1
    var b = 1
    return anyGenerator {
        let next = a + b
        a = b
        b = next
        return next
    }
}

/// Infinite lazy sequence of Fibonacci numbers
let fibs = AnySequence(fibonacciGenerator)

// Print first 20 elements
for n in fibs.prefix(20) { print(n) }