Either monad chain.

The typespec of either is {:ok, term} | {:error, term}.

iex> witha Witha.Error, x1 <- {:ok, 1}, do: x1 + 1
{:ok, 2}

iex> witha Witha.Error, x1 <- {:error, "駄目"}, do: x1
{:error, "駄目"}

:ok can chain.

iex> witha Witha.Error, [
iex>   x1 <- {:ok, 1},
iex>   x2 <- {:ok, x1 + 1},
iex> ], do: x1 + x2
{:ok, 3}

:error & raise stops the chain.

iex> witha Witha.Error, [
iex>   x1 <- {:error, "駄目"},
iex>   x2 <- {:ok, x1 + 1},
iex> ], do: x1 + x2
{:error, "駄目"}

iex> witha Witha.Error, [
iex>   x1 <- raise("駄目"),
iex>   x2 <- {:ok, x1 + 1},
iex> ], do: x1 + x2
{:error, %RuntimeError{message: "駄目"}}