ex1.exs

defmodule Num do
  defstruct [:val]

  defimpl String.Chars do
    def to_string(element), do: "#{element.val}"
  end
end

defmodule Add do
  defstruct [:left, :right]

  defimpl String.Chars do
    def to_string(element), do: "#{element.left} + #{element.right}"
  end
end

defmodule Mul do
  defstruct [:left, :right]

  defimpl String.Chars do
    def to_string(element), do: "#{element.left} * #{element.right}"
  end
end

defmodule VM do
  def eval(formula) do
    IO.puts "humanized: #{formula}"
    IO.puts reduce(formula)
  end

  def reduce(%Num{val: val}), do: %Num{val: val}
  def reduce(%Add{left: left, right: right}), do: %Num{val: reduce(left).val + reduce(right).val}
  def reduce(%Mul{left: left, right: right}), do: %Num{val: reduce(left).val * reduce(right).val}
end

defmodule App do
  def run do
    num = &(%Num{val: &1})
    add = &(%Add{left: &1, right: &2})
    mul = &(%Mul{left: &1, right: &2})

    formula = add.(
      mul.(num.(1), num.(2)),
      mul.(num.(3), num.(4))
    )

    VM.eval(formula)
  end
end

App.run

ex2.exs

defprotocol Reduce do
  def reduce(element)
end

defmodule Num do
  defstruct [:val]

  defimpl String.Chars do
    def to_string(element), do: "#{element.val}"
  end

  defimpl Reduce do
    def reduce(element), do: element
  end
end

defmodule Add do
  defstruct [:left, :right]

  defimpl String.Chars do
    def to_string(element), do: "#{element.left} + #{element.right}"
  end

  defimpl Reduce do
    def reduce(element), do: %Num{val: reduce(element.left).val + reduce(element.right).val}
  end
end

defmodule Mul do
  defstruct [:left, :right]

  defimpl String.Chars do
    def to_string(element), do: "#{element.left} * #{element.right}"
  end

  defimpl Reduce do
    def reduce(element), do: %Num{val: reduce(element.left).val * reduce(element.right).val}
  end
end

defmodule VM do
  import Reduce

  def eval(formula) do
    IO.puts "humanized: #{formula}"
    IO.puts reduce(formula)
  end
end

defmodule App do
  def run do
    num = &(%Num{val: &1})
    add = &(%Add{left: &1, right: &2})
    mul = &(%Mul{left: &1, right: &2})

    formula = add.(
      mul.(num.(1), num.(2)),
      mul.(num.(3), num.(4))
    )

    VM.eval(formula)
  end
end

App.run