Rail

Rail is a helper macros for "Railway oriented programming".

It helps you handle error cases at almost no cost with rail, >>>, and def macro.

If you are not comfortable with "Railway oriented programming", see Railway oriented programming

This library is mostly copied from SeokminHong/reather-lite and removed reader monad related stuffs to lower learning curve.

Installation

def deps do
  [
    {:rail, "~> 0.6.0"}
  ]
end

Usage

Basic usage

use Rail introduces new syntax left <- right,

• which bind value to left when right is {:ok, value} or value
• or skips entire code block when right is {:error, err} or :error.

defmodule Target do
  use Rail

  def div(num, denom) do
    denom <- check_denom(denom)
    num / denom
  end

  def check_denom(0) do
    {:error, :div_by_zero}
  end

  def check_denom(n) do
    # same with {:ok, n}
    n
  end
end

iex> Calc.div(10, 2)
5.0
iex> Calc.div(10, 0)
{:error, :div_by_zero}

rail/1 is available inside other code blocks.

iex> rail do
...    x <- {:ok, 1}
...    y <- {:ok, 2}
...
...    x + y
...  end
3

left >>> right is similar to |>, but

• applies right function only when left is value or {:ok, value}.
• support both function and call expression, which makes it compatible with |>.

iex> 1 >>> fn v -> Integer.to_string(v) end
"1"
iex> {:ok, 1} >>> fn v -> Integer.to_string(v) end
"1"
iex> {:ok, 1} >>> Integer.to_string()
"1"
iex> {:error, :div_by_zero} >>> Integer.to_string()
{:error, :div_by_zero}

LICENSE

MIT