Notes from Joy of Clojure - Chapter 7 - Functional Programming

joy-of-clojure-ch7-notes.clj

;; Example of using comp
(map (comp keyword #(.toLowerCase %) name) '(a B C))
;=> (:a :b :c)

;; equpivalent code using -> (but which doesn't use functions as data.
(map #(-> % (name) (.toLowerCase) (keyword)) '[a B C])
;=> (:a :b :c)


;; Using partial
((partial + 5) 100 200)
;=> 305

;; equivalent
(#(apply + 5 %&) 100 200)
;=> 305

;;;;;; IMPORTANT POINT 
;; Most collection processing can be performed with some combination of the following functions:
;; map, reduce, filter, for, some, repeatedly, sort-by, keep, take-while, drop-while

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; My own example of functional constraints

(defn ++ [n] (inc n))
;=> #'user/++

(defn ensure-number [f x]
 {:pre [(number? x)]}
 (f x))
;=>#'user/ensure-number

(++ 1)
;=> 2

(ensure-number ++ 1)
;=> 2

user> (ensure-number ++ {})
;=> Assert failed: (number? x)
;    [Thrown class java.lang.AssertionError]

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Closures in Clojure

(defn bot [x y bearing-num]
  {:coords     [x y]
   :bearing    ([:north :east :south :west] bearing-num)
   :forward    (fn [] (bot (+ x (:x (bearings bearing-num)))
                           (+ y (:y (bearings bearing-num)))
                           bearing-num))
   :turn-left  (fn [] (bot x y (mod (+ 1 bearing-num) 4)))
   :turn-right (fn [] (bot x y (mod (- 1 bearing-num) 4)))})
;=> #'user/bot

(:bearing ((:forward ((:forward ((:turn-right (bot 5 5 0))))))))
;=> :east

(:coords ((:forward ((:forward ((:turn-right (bot 5 5 0))))))))
;=> [7 5]