eval_attempt.exs

defmodule Eval do
  def calculate(str), do: _calculate(str)

  defp _calculate(str) do
    {op, i} = find(str)
    list1 = Enum.slice(str, 0..i-1)
    list2 = Enum.slice(str, i+2..length(str))
    _calculate(op, list1, list2)
  end
  defp _calculate(?+, list1, list2), do: number(list1) + number(list2)
  defp _calculate(?-, list1, list2), do: number(list1) - number(list2)
  defp _calculate(?*, list1, list2), do: number(list1) * number(list2)
  defp _calculate(?/, list1, list2), do: number(list1) / number(list2)

  defp find(str), do: _find(str, 0)

  defp _find([], _), do: raise "No operator"
  defp _find([ h | t ], i) do
    if h in '+-*/' do
      {h, i}
    else
      _find(t, i+1)
    end
  end

  def number(str), do: _number(str, 0)

  defp _number([], value),      do: value
  defp _number([ ?\s ], value), do: value
  defp _number([ digit | tail ], value)
  when digit in '0123456789' do
    _number(tail, value*10 + digit - ?0)
  end
  defp _number([ non_digit | _ ], _), do: raise "Invalid digit #{[non_digit]}"
end