Clojure Koans 11 Lazy Sequences Notebook

{:hello (clojure-version)}

;; "There are many ways to generate a sequence"

(= '(1 2 3 4) (range 1 5))

;; skill 11001: Generate a sequence from number X to number Y by using the `range` function

;; "The range starts at the beginning by default"

(= '(0 1 2 3 4) (range 5))

;; skill 11002: Generate a sequence starting at zero by using the `range` function

;; "Only take what you need when the sequence is large"

(= [0 1 2 3 4 5 6 7 8 9]

   (take 10 (range 100)))

;; skill 11003: Get the first N items of a sequence by using the `take` function

;; "Or limit results by dropping what you don't need"

(= [95 96 97 98 99]

   (drop 95 (range 100)))

;; skill 11004: Get all items of a sequence except for the first N items by using the `drop` function

;; "Iteration provides an infinite lazy sequence"

(= '(1 2 4 8 16 32 64 128) (take 8 (iterate (fn [x] (* x 2)) 1)))

;; skill 11005: Create a lazy sequence by using the `iterate` function

;; question 11001: How can the above skill be made more relevant and/or directly useful seeming?

;; question 11002: What exactly is a lazy sequence?

;;          11003 How does it differ/relate to a non-lazy sequence?

;;          11004 How are lazy sequences made/used?

;;          11005 Where are they best used/avoided?

;; "Repetition is key"

(= [:a :a :a :a :a :a :a :a :a :a]

   (repeat 10 :a))

;; skill 11006: Populate a sequence with a keyword using the `repeat` function

;; question 11006: How can the above skill be generalized further? suggestion: replace keyword with item/thing/element (?) ... TODO: see docs

;; "Iteration can be used for repetition"

(= (repeat 100 "hello")

   (take 100 (iterate #(str %) "hello")))

;; question 11007: What are the differences between `repeat` and the combo of `take` and `iterate` (beyond just sytanx semantics)?