pierre.exs

	
# Inspired by:
# http://learnyouahaskell.com/a-fistful-of-monads#walk-the-line
# Put simply, pierre is a tight rope walker. birds land on the left
# or right of his pole. If the difference is > 3 he falls off

ExUnit.start

defmodule WalkTheLine do
  use ExUnit.Case

  def landLeft({l, r}, n) when abs((l + n) - r) > 3 do
    :felloff
  end

  def landLeft(:felloff, _) do
    :felloff
  end

  def landLeft({l, r}, n) do
    {l + n, r}
  end

  def landRight({l, r}, n) when abs((r + n) - l) > 3 do
    :felloff
  end

  def landRight(:felloff, _) do
    :felloff
  end

  def landRight({l, r}, n) do
    {l, r + n}
  end

  test "nasty syntax" do
    assert {5,4} == landRight(landLeft(landRight(landLeft({0,0}, 3), 3), 2), 1)
  end

  test "using the pipe operator" do
    assert {5,4} == {0,0} |> landLeft(3) |> landRight(3) |> landLeft(2) |> landRight(1)
  end

  # ok so far, but we don't deal with failure
  # for failure we're going to return :felloff

  test "testing for failure" do
    assert :felloff == {0,0} |> landLeft(3) |> landRight(3) |> landLeft(8) |> landRight(2)
  end

  test "correct testing for failure" do
    assert :felloff == {0,0} |> landLeft(1) |> landRight(4) |> landLeft(-1) |> landRight(-2)
  end

end