Higher-order Functions

included in dev

2016-12-16 423 words 2 minutes

Contents

Introduction

Martin Odersky¹ in his Functional Programming in Scala course illustrates higher-order functions² with a simple example in Scala. Scala is a modern functional programming language where functions are first-class citizen so they can be used, like any other value, as a parameter and returned as a result.

// Simple sum function using a recursion to perform an operation on integers between a and b
def sum(f: Int => Int, a: Int, b: Int): Int =
  if (a > b) 0
  else f(a) + sum(f, a + 1, b)

// Using anonymous function -- addict to syntactic sugar -- to define
// The sum of integers between a and b
def sumInts(a: Int, b: Int) = sum(x => x, a, b)
// The sum of the cubes of integers between, a and b
def sumCubes(a: Int, b: Int) = sum(x => x * x * x, a, b)

> sumInts(1, 10)
Int = 55
> sumCubes(1,10)
Int = 3025

I love the Scala lean syntax–and its syntactic sugars–, but Python and R–my old friends–are also, among other paradigms, functional languages. So how is it possible to perform the same things?

In Python

The syntax is very close–and the results are the same :-). Anonymous functions are called lambda.

# The way to define this recursive function is almost the same -- just sad to have to write return statements
def sum(f, a, b):
    if (a > b): return 0
    else: return f(a) + sum(f, a + 1, b)

# In Python anonymous functions are called lambda and are defined as follow
def sum_ints(a, b): 
    return sum(lambda x: x, a, b)

def sum_cubes(a, b): 
    return sum(lambda x: x * x * x, a, b)

>>> sum_ints(1, 10)
55
>>> sum_cubes(1, 10)
3025

In R

Mainly the same thing. I’m not an expert in R and maybe there is a way to use the formula notation (~) instead of the anonymous function notation, but I don’t know.

# Brackets have been ommited
sum <- function(f, a, b) {
  if (a > b) return(0)
  else return(f(a) + sum(f, a + 1, b))
}

# Anonymous function are also possible
sum_ints <- function(a, b) {
return(sum((function(x) x), a, b))
}

sum_cubes <- function(a, b) {
return(sum((function(x) x * x * x), a, b))
}

> sum_ints(1, 10)
55 
> sum_cubes(1, 10)
3025

In the next course, he introduces more compact syntax, but this is another story.

Martin Odersky has designed the Scala programming language. ↩︎
Lecture 2.1 - Higher-Order Functions - École Polytechnique Fédérale de Lausanne | Coursera ↩︎