046 - Fliping out - higher order functions

src/four_clojure/046_flipping_out.clj

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+(ns four-clojure.046-flipping-out)
+
+;; #46 - Flipping out
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; http://www.4clojure.com/problem/46
+
+;; Difficulty:	Medium
+;; Topics:	higher-order-functions
+
+;; Write a higher-order function which flips the order of the arguments of an input function.
+
+;; (= 3 ((__ nth) 2 [1 2 3 4 5]))
+;; (= true ((__ >) 7 8))
+;; (= 4 ((__ quot) 2 8))
+;; (= [1 2 3] ((__ take) [1 2 3 4 5] 3))
+
+;; Hint
+;; A lesson in calling funcitons and defining functions
+
+;; Clojure syntax - functions that return functions
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; How to write a function that returns a function
+
+;; (fn [arg]
+;;   (fn [arg2 arg3]
+;;     (str "The outer function returns the inner function definition")))
+
+;; This can go very deep, although if you do then there may be an abstraction you missed
+
+;; (fn [arg]
+;;   (fn [arg1 arg2]
+;;     (fn [arg3 arg4 arg5]
+;;       (str "Matryoshka dolls, also known as Russian nesting dolls, stacking dolls"))))
+
+;; Now we know how to write a function that returns a function
+
+;; deconstructing the problem
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; We need to define a function that takes one argument,
+;; and returns a function definition
+
+(fn [function]
+  (fn []))
+
+;; our return function should do something with the argument it received
+;; as the argument is a function we want to call, then we should put it in a list
+
+(fn [function]
+  (fn []
+    (function)))
+
+;; we are not finished yet, but if we use data from the first test,
+;; and called out function, then we would get:
+
+;; (nth 2 [1 2 3 4 5])
+
+;; This wouldnt work because the arguments are in the wrong order.
+;; Also the function we return is not taking arguments, so would cause an error when called
+
+;; So lets get our function definition to return a function that takes two arguments
+
+(fn [function]
+  (fn [arg1 arg2]
+    (function arg2 arg1)))
+
+;; We are almost there.
+
+;; The function we return needs to swap around the arguments
+;; otherwise we get the wrong results
+
+;; We could use the `reverse` function to swap the arguments around
+;; if we get the arguments as a collection,
+;; changing our function signature to (fn [& args])
+
+(reverse [:arg1 :arg2])
+;; => (:arg2 :arg1)
+
+(reverse [2 [1 2 3 4 5]])
+;; => ([1 2 3 4 5] 2)
+
+;; This adds more complexity to our code and we would have to apply our function
+;; over our reversed arguments as they are now in a collection.
+
+;; However, we can make the swap by simply changing the order of the arguments in the return function body
+
+(fn [function]
+  (fn [arg1 arg2]
+    (function arg2 arg1)))
+
+
+;; Answers summary
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(fn [function]
+  (fn [arg1 arg2]
+    (function arg2 arg1)))
+
+;; Lowest golf score
+(fn [f] #(f %2 %1))