gistfile1.ex

defmodule P5 do
  def work(num, list, target, result \\ "")

  def work(num, [], target, result) do
    if num == target, do: IO.puts "#{result}#{num |> abs |> to_string}"
  end

  def work(num, [head | tail], target, result) do
    work(head, tail, target - num, "#{result}#{abs(num)} + ")
    work(-head, tail, target - num, "#{result}#{abs(num)} - ")
    work(concat(num, head), tail, target, result)
  end

  def concat(a, b) do
    zeroes = trunc(:math.log10(b)) + 1
    result = abs(a) * trunc(:math.pow(10, zeroes)) + b
    if abs(a) == a, do: result, else: -result
  end
end

target = 100
[head | tail] = Enum.to_list(1..9)

P5.work(head, tail, target)