Result/Either monad in Elixir

result.ex

defmodule Result do
  @type result :: any
  @type reason :: any
  @type ok :: {:ok, result}
  @type error :: {:error, reason}
  @type t :: ok | error

  @spec ok(result) :: ok
  def ok(result) do
    {:ok, result}
  end
  
  @spec fail(reason) :: error
  def fail(reason) do
    {:error, reason}
  end
  
  @spec and_then(t, (result -> t)) :: t
  def and_then({:error, _reason} = m, _callback), do: m
  def and_then({:ok, result}, callback) do
    callback.(result)
  end

  @spec and_then(t, (result -> result)) :: t
  def map({:error, _reason} = m, _updater), do: m
  def map({:ok, result}, updater) do
    ok updater.(result)
  end

  @spec and_then(t, (reason -> reason)) :: t
  def map_error({:ok, _result} = m, _updater), do: m
  def map_error({:error, reason} = m, updater) do
    fail updater.(reason)
  end
end

defmodule TestModule do
  def fetch do
    Result.ok 9.0
  end

  def div_ten_by_x(x) do
    case x do
      0.0 ->
        Result.fail "zero"
      _ ->
        {:ok, 10 / x} # tuples are ok too
    end
  end
end

TestModule.fetch
|> IO.inspect # {:ok, 9.0}
|> Result.map(&(&1 + 1))
|> IO.inspect # {:ok, 10.0}
|> Result.and_then(&TestModule.div_ten_by_x/1)
|> IO.inspect # {:ok, 1.0}
|> Result.map(&(&1 - 1))
|> IO.inspect # {:ok, 0.0}
|> Result.and_then(&TestModule.div_ten_by_x/1)
|> IO.inspect # {:error, "zero"}
|> Result.map(&(&1 + 1))
|> IO.inspect # {:error, "zero"}
|> Result.and_then(&TestModule.div_ten_by_x/1)
|> IO.inspect # {:error, "zero"}
|> Result.map_error(&String.capitalize/1)
|> IO.inspect # {:error, "Zero"}