What are some interesting uses of higher-order functions?

Question

I\'m currently doing a Functional Programming course and I\'m quite amused by the concept of higher-order functions and functions as first class citizens. However, I can\'t yet

Answer 1

One interesting and slightly crazy thing you can do is simulate an object-oriented system using a function and storing data in the function's scope (i.e. in a closure). It's higher-order in the sense that the object generator function is a function which returns the object (another function).

My Haskell is rather rusty so I can't easily give you a Haskell example, but here's a simplified Clojure example which hopefully conveys the concept:

(defn make-object [initial-value]
  (let [data (atom {:value initial-value})]
      (fn [op & args]
        (case op 
          :set (swap! data assoc :value (first args))
          :get (:value @data)))))

Usage:

(def a (make-object 10))

(a :get)
=> 10

(a :set 40)

(a :get)
=> 40

Same principle would work in Haskell (except that you'd probably need to change the set operation to return a new function since Haskell is purely functional)