Chapter 24

Exercise: Protocols-1

defprotocol Caesar do
  def encrypt(string, shift)
  def rot13(string)
end

defimpl Caesar, for: List do
  def encrypt(string, shift), do: Enum.map(string, &(?a + rem(&1 + shift - ?a, 26)))
  def rot13(string), do: encrypt(string, 13)
end

defimpl Caesar, for: BitString do
  def encrypt(string, shift), do: string |> to_charlist() |> Caesar.encrypt(shift) |> to_string()
  def rot13(string), do: encrypt(string, 13)
end

'elixir' = Caesar.encrypt('ryvkve', 13)
'elixir' = Caesar.rot13('ryvkve')
"elixir" = Caesar.encrypt("ryvkve", 13)
"elixir" = Caesar.rot13("ryvkve")

Exercise: Protocols-2

Exercise: Protocols-3

defmodule MyEnum do
  def each(enumerable, fun) do
    Enum.reduce(enumerable, :ok, fn elem, _acc -> fun.(elem) end)
  end

  def filter(enumerable, fun) do
    Enum.reduce(enumerable, [], fn elem, acc ->
      if fun.(elem) do
        acc ++ [elem]
      else
        acc
      end
    end)
  end

  def map(enumerable, fun) do
    Enum.reduce(enumerable, [], fn elem, acc -> acc ++ [fun.(elem)] end)
  end
end

ExUnit.start(auto_run: false)

defmodule MyEnumTest do
  use ExUnit.Case
  import ExUnit.CaptureIO

  test "each/2 works" do
    result = capture_io(fn -> MyEnum.each(["some", "example"], fn x -> IO.puts(x) end) end)
    assert result == "some\nexample\n"
    assert MyEnum.each([], &IO.puts/1) == :ok
  end

  test "filter/2 works" do
    assert Enum.filter([1, 2, 3], fn x -> rem(x, 2) == 0 end) == [2]
  end

  test "map/2 works" do
    assert MyEnum.map([1, 2, 3], fn x -> x * 2 end) == [2, 4, 6]
    assert MyEnum.map([a: 1, b: 2], fn {k, v} -> {k, -v} end) == [a: -1, b: -2]
  end
end

ExUnit.run()

Exercise: Protocols-4