Ruby Example of Railway Oriented Programming

railway_oriented_programming_in_ruby.rb

require 'json'

##
# Combinator module
#   contains .map, .bind, .apply, .compose
#   a toolkit for building functional programs
##
module Combinator
  Result = Data.define(:success, :error, :value) do
    def call(&block)
      if self.success
        self.value
      else
        self.error
      end
    end

    def tee(&block)
      if self.success
        p block.call(self.value)
      end
      self
    end

    def map(&block)
      if self.success
        Result.new(true, nil, block.call(self.value))
      else
        self
      end
    end

    def bind(&block)
      if self.success
        lambda { Result.new(true, nil, block.call(self.value)) }
      else
        Result.new(false, "#{self.error}, line no: #{__FILE__}", nil)
      end
    end

    def apply(&block)
      if self.success
        bind { block.call(self.value) }.call
      else
        self
      end
    end
  end

  def self.map(&block)
    begin
      Result.new(true, nil, block.call)
    rescue => e
      Result.new(false, e, nil)
    end
  end

  def self.bind(&block)
    lambda { |x| Result.new(true, nil, block.call(x)) }
  end

  def self.apply(&block)
    bind { |x| block.call(x) }.call
  end

  def self.compose(method1, method2)
    lambda { |x| Result.new(true, nil, method1.call(x)).apply { |y| method2.call(y) } }
  end
end


##
# validate_input, canonicalize_email!, encrypt_email, update_db!, log
#   these are the methods defined to demonstrate the use of the module
#   in a web application context
## 

def validate_input(user)
  if user.email.nil? || user.email.strip.empty?
    raise "ValidationError: Email is Blank, line no: #{__FILE__}/#{__LINE__}"
  end

  if user.name.length > 50
    raise "ValidationError: Name is too long, line no: #{__FILE__}/#{__LINE__}"
  end

  if user.email !~ /@.*\./
    raise "ValidationError: Email is invalid, line no: #{__FILE__}/#{__LINE__}"
  end

  if user.name.nil? || user.name.strip.empty?
    raise "ValidationError: Name is Blank, line no: #{__FILE__}/#{__LINE__}"
  end

  user
end

def canonicalize_email!(user)
  User.new(user.name, user.email.downcase)
end

def encrypt_email(user)
  User.new(user.name, user.email.reverse)
end

def update_db!(user)
  # let's pretend there's mutation here
  "Updating DB with #{user.name} and #{user.email}"
end

def log(user)
  "Logging user #{user.name}"
end

##
# Example usage
# imagine a web app where the user is created
## 

User = Data.define(:name, :email)
user = User.new("John", "john@example.com")

result = Combinator
  .map { validate_input(user) }
  .map { |user| canonicalize_email!(user) }
  .tee { |user| update_db!(user) }
  .tee { |user| log(user) }

if result.success
  json = { "status" => 200, "data" => { "message": "User of name: #{result.value.name} and email: #{result.value.email} created" } }.to_json
  puts json
else
  json = { "status" => 400, "data" => { "error": result.error } }.to_json
  puts json
end

Based on this: https://www.youtube.com/watch?v=fYo3LN9Vf_M