Math With Protocols

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.8.0", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

Navigation

Return Home Report An Issue

Math With Protocols

In this exercise, you’re going to create a Math protocol that handles adding and subtracting different data types.

Create a Math protocol implementation for integers, strings, and lists. You do not need to handling adding or subtracting different data types.

Math.add(1, 2)
3

Math.add("Hello, ", "World!")
"Hello, World!"

Math.add([1, 2], [3, 4])
[1, 2, 3, 4]

Math.subtract(10, 2)
8

Math.subtract("hello there", "hello ")
"there"

Math.subtract([1, 2, 3, 4], [1, 3])
[2, 4]

Hint: Subtracting Strings

Consider converting your strings to a list, then subtract the two lists together then join your result back into a string.

Example Solution

defprotocol Math do
  def add(value1, value2)
  def subtract(value1, value2)
end

defimpl Math, for: Integer do
  def add(integer1, integer2) do
    integer1 + integer2
  end

  def subtract(integer1, integer2) do
    integer1 - integer2
  end
end

defimpl Math, for: BitString do
  def add(string1, string2) do
    string1 <> string2
  end

  def subtract(string1, string2) do
    (String.split(string1, "") -- String.split(string2, ""))
    |> Enum.join()
  end
end

defimpl Math, for: List do
  def add(list1, list2) do
    list1 ++ list2
  end

  def subtract(list1, list2) do
    list1 -- list2
  end
end

Define implementations for the Math protocol as documented below.

defmodule MyStruct do
  defstruct [:name]
end

Map.replace!(%MyStruct{}, :name, "hello")

defprotocol Math do
  @doc """
  Add two integers, lists, or strings.

  ## Examples

    iex> Math.add(2, 2)
    4
    iex> Math.add(4, 4)
    8

    Math.add([1, 2], [3, 4])
    [1, 2, 3, 4]
    Math.add([1, 2, 3], [4, 5, 6])
    [1, 2, 3, 4, 5, 6]

    iex> Math.add("abc", "def")
    "abcdef"
    iex> Math.add("123", "456")
    "123456"

    iex> Math.add(%{}, %{})
    ** (Protocol.UndefinedError) protocol Math not implemented for %{} of type Map

    iex> Math.add({}, {})
    ** (Protocol.UndefinedError) protocol Math not implemented for {} of type Tuple
  """
  def add(value1, value2)

  @doc """
  Subtract two integers, lists, or strings.

  ## Examples
    iex> Math.subtract(10, 2)
    8
    iex> Math.subtract(20, 2)
    18

    iex> Math.subtract([1, 2], [1])
    [2]
    iex> Math.subtract([1, 2, 3, 4], [1, 3])
    [2, 4]

    iex> Math.subtract("abcd", "abc")
    "d"
    iex> Math.subtract("cbad", "abc")
    "d"
    iex> Math.subtract("abc", "xyz")
    "abc"
    iex> Math.subtract("abc", "xyza")
    "bc"

    iex> Math.subtract(1.0, 2.0)
    ** (Protocol.UndefinedError) protocol Math not implemented for 1.0 of type Float
  """
  def subtract(value1, value2)
end

Mark As Completed

file_name = Path.basename(Regex.replace(~r/#.+/, __ENV__.file, ""), ".livemd")

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "math_with_protocols_reading"
    "exercises" -> "math_with_protocols_exercise"
  end

progress_path = __DIR__ <> "/../progress.json"
existing_progress = File.read!(progress_path) |> Jason.decode!()

default = Map.get(existing_progress, save_name, false)

form =
  Kino.Control.form(
    [
      completed: input = Kino.Input.checkbox("Mark As Completed", default: default)
    ],
    report_changes: true
  )

Task.async(fn ->
  for %{data: %{completed: completed}} <- Kino.Control.stream(form) do
    File.write!(
      progress_path,
      Jason.encode!(Map.put(existing_progress, save_name, completed), pretty: true)
    )
  end
end)

form

Commit Your Progress

Run the following in your command line from the curriculum folder to track and save your progress in a Git commit. Ensure that you do not already have undesired or unrelated changes by running git status or by checking the source control tab in Visual Studio Code.

$ git checkout -b math-with-protocols-exercise
$ git add .
$ git commit -m "finish math with protocols exercise"
$ git push origin math-with-protocols-exercise

Create a pull request from your math-with-protocols-exercise branch to your solutions branch. Please do not create a pull request to the DockYard Academy repository as this will spam our PR tracker.

DockYard Academy Students Only:

Notify your teacher by including @BrooklinJazz in your PR description to get feedback. You (or your teacher) may merge your PR into your solutions branch after review.

If you are interested in joining the next academy cohort, sign up here to receive more news when it is available.

Up Next