Russian Peasant Multiplication — with Elixir — Part 4

In this article we will be working on Higher order function with statements. We want to explore how to encapsulate all the mess that a function deals with inside a box and pass it around.

If you have followed part 1, part 2, part 3 by now you would know what we are doing and how are we tackling this problem.

We have decrement, increment, combine and now we need filter. Last time we ended up with with this:

[{13, 238}, {6, 476}, {3, 952}, {1, 1904}]

We need to loop through this list, if the first number is even then we need to remove it and return a list of second numbers. We need to end up with something like this:

[238, 952, 1904]

As you can see we can do this with Enum.filter easily. With Enum.filter we push a higher order function and then we pipe the result into an Enum.map where we can patten match and return our list (not fun :) and too easy.

Test

defmodule RussianPeasantMultiplication.FilterTest do
  use ExUnit.Case
  doctest RussianPeasantMultiplication.Filter
end

Code

As usual we are using Monad.Result to keep things the same across our applications.

line 34 We are using we are creating a function to pattern match and remove the even number if the first number in our tuple is even.

Then we start the recursion. Again in line 50 we are creating our own concat/2 version. It will unwrap!/1 our Monad.Result and update the state.

line 54 you can make your own concat where add new item to your list, however to keep things short I used Enum.concat

Run the test and it should pass.

I think this should do it for now. Tune in to next for the last piece of the puzzle where we create a simple sum with recursion.

Technical Debt

We still got our 2 technical debt remains. We should be able to look after them in later date.

Cheers.