ROP - Railway Oriented Programming in Elixir

---

Some macros to enable railsway-oriented programming in Elixir. Collected from code snippets and wrapped into a simple library for your convenience.

For more examples please check the tests here:

• Examples

Sources for inspiration + copying

• https://github.com/remiq/railway-oriented-programming-elixir
• https://gist.github.com/zabirauf/17ced02bdf9829b6956e (Railway Oriented Programming macros in Elixir)

Installation

If available in Hex, the package can be installed as:

1. Add rop to your list of dependencies in mix.exs:

def deps do
  [{:rop, "~> 0.5"}]
end

Usage

Call

use Rop

in your module. That will give you access to following macros/functions:

>>>

No need to stop pipelining in case of an error somewhere in the middle

used like: 1 |> fn1 >>> fn2 >>> fn3 >>> fn4

defmodule TripleArrowExample do
  use Rop
  def tagged_inc(v) do
    IO.puts "inc for #{v}" # sideeffect for demonstration
    {:ok, v + 1}
  end

  def error_fn(_) do
    {:error, "I'm a bad fn!"}
  end

  def raising_fn(_) do
    raise "I'm raising!"
  end

  def result do
    1 |> tagged_inc >>> tagged_inc >>> tagged_inc
  end

  def error_result do
    1 |> tagged_inc >>> tagged_inc >>> error_fn >>> tagged_inc
  end

  def raising_result do
    1 |> tagged_inc >>> tagged_inc >>> raising_fn >>> tagged_inc
  end
end

iex> TripleArrowExample.result
inc for 1
inc for 2
inc for 3
{:ok, 4}

### increases twice, errors and tries to increase again
### notice that after errored result we don't execute any function anymore in the pipeline,
### e.g. only tagged_inc before error_fn were executed.
iex> TripleArrowExample.error_result
inc for 1
inc for 2
{:error, "I'm a bad fn!"}

### raises... We'll fix it in a later example for try_catch!
iex> TripleArrowExample.raising_result
inc for 1
inc for 2
** (RuntimeError) I'm raising!

bind

Wraps a simple function to return a tagged tuple with :ok to comply to the protocol {:ok, result}: e.g.

defmodule BindExample do
  use Rop
  def inc(v) do
    v + 1
  end

  def only_last_pipe_tagged_result(v) do
    v |> inc |> bind(inc)
  end

  def result_fully_tagged(v) do
    v |> bind(inc) >>> bind(inc) >>> bind(inc)
  end
end
iex> BindExample.only_last_pipe_tagged_result(2)
{:ok, 4}

iex> BindExample.result_fully_tagged(2)
{:ok, 5}

try_catch

Wraps raising functions to return a tagged tuple {:error, ErrorMessage} to comply with the protocol

# modified example from TripleArrowExample to handle raising functions
defmodule TryCatchExample do
  use Rop
  def tagged_inc(v) do
    IO.puts "inc for #{v}" # sideeffect for demonstration
    {:ok, v + 1}
  end

  def raising_fn(_) do
    raise "I'm raising!"
  end

  def result do
    1 |> tagged_inc >>> tagged_inc >>> tagged_inc
  end

  def raising_result_wrapped(v) do
    v |> tagged_inc >>> tagged_inc >>> try_catch(raising_fn) >>> tagged_inc
  end
end

iex> TryCatchExample.raising_result_wrapped(1)
inc for 1
inc for 2
{:error, %RuntimeError{message: "I'm raising!"}}

tee

Like a similar Unix utility it does some work and returns the input. See tee (command), Unix.

defmodule TeeExample do
  use Rop
  def tagged_inc(v) do
    IO.puts "inc for #{v}" # sideeffect for demonstration
    {:ok, v + 1}
  end

  def calc(v) do
    v |> tee(tagged_inc) >>> tee(tagged_inc) >>> tee(tagged_inc)
  end
end

# notice how the incremented value is not passed through the pipeline,
# but just the original argument `1`
iex> TeeExample.calc(1)
inc for 1
inc for 1
inc for 1
_{:ok, 1}

ok

A simple utility function to extract the value from {:ok, result} tuple and to raise the error in {:error, ErrorStruct}.

defmodule OkExample do
  use Rop
  def ok_result do
    {:ok, 1} |> ok
  end

  def error_result do
    {:error, %ArithmeticError{}} |> ok
  end

  def any_value_result do
    "bad value" |> ok
  end
end

iex> OkExample.ok_result
1

iex> OkExample.error_result
** (ArithmeticError) bad argument in arithmetic expression
    (rop) lib/rop.ex:70: Rop.ok/1


iex> OkExample.any_value_result
** (RuntimeError) bad value
    (rop) lib/rop.ex:71: Rop.ok/1

Background information

Some discussions about Railsway programming:

• http://insights.workshop14.io/2015/10/18/handling-errors-in-elixir-no-one-say-monad.html
• http://blog.danielberkompas.com/2015/09/03/better-pipelines-with-monadex.html
• http://onor.io/2015/08/27/railway-oriented-programming-in-elixir/
• http://www.zohaib.me/railway-programming-pattern-in-elixir/
• http://www.zohaib.me/monads-in-elixir-2/
• http://fsharpforfunandprofit.com/posts/recipe-part2/
• https://www.reddit.com/r/programming/comments/30coly/railway_oriented_programming_in_elixir/

Code (Railsway Programming)

• https://github.com/remiq/railway-oriented-programming-elixir/blob/master/lib/rop.ex
• https://gist.github.com/zabirauf/17ced02bdf9829b6956e (Railway Oriented Programming macros in Elixir) -> Rop
• https://github.com/CrowdHailer/OK/blob/master/lib/ok.ex
• https://gist.github.com/danielberkompas/52216db76d764a68dfa3 -> pipeline.ex

Code (Monads)

• https://github.com/rmies/monad.git
• https://github.com/rob-brown/MonadEx.git

Altenatives

• https://github.com/batate/elixir-pipes - Macros for more flexible composition with the Elixir Pipe operator