Functional programming in Elixir

Introduction#

Let’s try to implement the basic higher orders functions like map and reduce using Elixir

Map

Map is a function which will take an array and a function and return an array after applying that function to each element in that list

defmodule MyList do
  def map([], _func) do
    []
  end

  def map([head | tail], func) do
    [func.(head) | map(tail, func)]
  end
end

Copy paste in iex and execute:

MyList.map [1,2,3], fn a -> a * 5 end

Shorthand syntax is MyList.map [1,2,3], &(&1 * 5)

Reduce

Reduce is a function which will take an array, function and accumulator and use accumulator as seed to start the iteration with the first element to give next accumulator and the iteration continues for all the elements in the array (refer below example)

defmodule MyList do
  def reduce([], _func, acc) do
    acc
  end

  def reduce([head | tail], func, acc) do
    reduce(tail, func, func.(acc, head))
  end
end

Copy paste the above snippet in iex:

1. To add all numbers in an array: MyList.reduce [1,2,3,4], fn acc, element -> acc + element end, 0
2. To mutliply all numbers in an array: MyList.reduce [1,2,3,4], fn acc, element -> acc * element end, 1

Explanation for example 1:

Iteration 1 => acc = 0, element = 1 ==> 0 + 1 ===> 1 = next accumulator
Iteration 2 => acc = 1, element = 2 ==> 1 + 2 ===> 3 = next accumulator
Iteration 3 => acc = 3, element = 3 ==> 3 + 3 ===> 6 = next accumulator
Iteration 4 => acc = 6, element = 4 ==> 6 + 4 ===> 10 = next accumulator = result(as all elements are done)

Filter the list using reduce

MyList.reduce [1,2,3,4], fn acc, element -> if rem(element,2) == 0 do acc else acc ++ [element] end end, []