Functional Design: Principles, Patterns, and Practices

4

Laziness

Consider the following boldfaced change to our program that calculates a list of Fibonacci numbers:

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(declare fib)

(defn fib-w [n]
  (cond
    (< n 1) nil
    (<= n 2) 1
    :else (+ (fib (dec n)) (fib (- n 2)))))

(def fib (memoize fib-w))


(defn lazy-fibs []
  (map fib (rest (range)))
  )

The lazy-fibs function may look a little strange to you. Let’s walk through it. You already understand the map function. The rest function takes a list and returns that list without the first element. And that brings us to the range function.

The range function, as called here, returns a list of integers starting at zero. How many integers, ...