Functional Programming explained to my grandma

Anne-Honyme

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Higher Order Function

In functional programming, a function is defined as a "first order citizen". This means it includes all the properties generally available to any other element, such as the possibility of being affected to a name, returned as a result or passed as a parameter.

A higher-order function is a function that accepts other functions as parameters and/or use a function as the return value.
Let's try it out! Implement the greeting function to return "Hello, [name]" if the name is defined. Otherwise, return "who are you?".

More detailed informations

[Check the scaladoc!] (http://docs.scala-lang.org/tutorials/tour/higher-order-functions.html)

How to pass a function as a parameter?

def apply(f: Int => String, v: Int) = f(v)

Greetings

package example
object Hof{
  /**
    * return hello to someone
    */
  def hello (string: String): String = {
    "Hello, "+string
  }
  /**
    * if string is not empty say something, else ask "Who are you"
    */
  def greetings = ???
  /**
    * should say hello if string is empty, else ask "Who are you"
    */
  def helloOrAskName(string: String): String = ???/*{
      greetings(string, hello)
  }*/
}

What make high order functions awesome is this ability to pass a function in parameter.

Let's try to implement the following example: we want to use different sorts. Let's try to use the already-implemented sort function to define the other ones.

_ Language tips _ For this example you can use the wildcard character _ which can match any parameter. It's useful to compare elements to each other.

Implement the differents sort

package example
/**
  * Created by charlotte on 13/05/17.
  */
object HofSort {
  def sort(cmp: (String, String)=>Boolean, list: List[String]):List[String] ={
    list.sortWith(cmp)
  }
  def comparator(a: String, b: String): Boolean ={
    if(a < b) true
    else false
  }
  def sortAlphabetically(list: List[String]):List[String]= {
    ??? //TODO use the sort method with a function to comparate elements
  }
  def sortReverseAlphabetically(list: List[String]):List[String]={
    ??? //TODO use the sort method with a function to comparate elements
  }
  def sortIncreasingSize(list: List[String]):List[String]={
    ??? //TODO use the sort method with a function to comparate elements
  }
}

You can use functions of the collection API the same way you use map and reduce.

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3/9 What is Higher Order Function?

Higher Order Function

More detailed informations

How to pass a function as a parameter?

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