Monadic

helps dealing with exceptional situations, it comes from the sphere of functional programming and bringing the goodies I have come to love in Scala to my ruby projects (hence I will be using more Scala like constructs than Haskell).

My motivation to create this gem was that I often work with nested Hashes and need to reach deeply inside of them so my code is sprinkled with things like some_hash.fetch(:one, {}).fetch(:two, {}).fetch(:three, "unknown").

We have the following monadics:

  • Option (Maybe in Haskell) - Scala like with a rubyesque flavour
  • Either - more Haskell like

What's the point of using monads in ruby? To me it started with having a safe way to deal with nil objects and other exceptions. Thus you contain the erroneous behaviour within a monad - an indivisible, impenetrable unit.

Usage

Option

Is an optional type, which helps to handle error conditions gracefully. The one thing to remember about option is: 'What goes into the Option, stays in the Option'.

Option(User.find(123)).name._         # ._ is a shortcut for .fetch 

# if you prefer the alias Maybe instead of option
Maybe(User.find(123)).name._

# confidently diving into nested hashes
Maybe({})[:a][:b][:c]                   == None
Maybe({})[:a][:b][:c].fetch('unknown')  == None
Maybe(a: 1)[:a]._                       == 1

Basic usage examples:

# handling nil (None serves as NullObject)
obj = nil
Option(obj).a.b.c            == None

# None stays None
Option(nil)._                == "None"
"#{Option(nil)}"             == "None"
Option(nil)._("unknown")     == "unknown"
Option(nil).empty?           == true
Option(nil).truly?           == false

# Some stays Some, unless you unbox it
Option('FOO').downcase       == Some('foo') 
Option('FOO').downcase.fetch == "foo"
Option('FOO').downcase._     == "foo"
Option('foo').empty?         == false
Option('foo').truly?         == true

Map, select:

Option(123).map   { |value| User.find(value) } == Option(someUser)    # if user found
Option(0).map     { |value| User.find(value) } == None                # if user not found
Option([1,2]).map { |value| value.to_s }       == Option(["1", "2"])  # for all Enumerables

Option('foo').select { |value| value.start_with?('f') } == Some('foo')
Option('bar').select { |value| value.start_with?('f') } == None

Treat it like an array:

Option(123).to_a         == [123]
Option([123, 456]).to_a  == [123, 456]
Option(nil)              == []

Falsey values (kind-of) examples:

user = Option(User.find(123))
user.name._

user.fetch('You are not logged in') { |user| "You are logged in as #{user.name}" }.should == 'You are logged in as foo'

if user != nil
  "You are logged in as foo"
else
  "You are not logged in"

user.subscribed?              # always true
user.subscribed?.truly?       # true if subscribed is true
user.subscribed?.fetch(false) # same as above
user.subscribed?.or(false)    # same as above

Remember! an Option is never false (in Ruby terms), if you want to know if it is false, call #empty? of #truly?

#truly? will return true or false, always.

Slug example

# instead of 
def slug(title)
  if title
    title.strip.downcase.tr_s('^[a-z0-9]', '-')
  end
end

# or 

def slug(title)
  title && title.strip.downcase.tr_s('^[a-z0-9]', '-')
end

# do it with a default
def slug(title)
  Option(title).strip.downcase.tr_s('^[a-z0-9]', '-')._('unknown-title')
end

Either

Its main purpose here to handle errors gracefully, by chaining multiple calls in a functional way and stop evaluating them as soon as the first fails. Assume you need several calls to construct some object in order to be useful, after each you need to check for success. Also you want to catch exceptions and not let them bubble upwards.

Success represents a successfull execution of an operation (Right in Scala, Haskell).
Failure represents a failure to execute an operation (Left in Scala, Haskell).

The Either() wrapper will treat nil, false or empty? as a Failure and all others as Success.

result = parse_and_validate_params(params).
            bind ->(user_id) { User.find(user_id) }.        # if #find returns null it will become a Failure
            bind ->(user)    { authorized?(user); user }.   # if authorized? raises an Exception, it will be a Failure 
            bind ->(user)    { UserDecorator(user) }

if result.success?
  @user = result.fetch                                      # result.fetch or result._ contains the 
  render 'page'
else
  @error = result.fetch
  render 'error_page'
end

You can use alternate syntaxes to achieve the same goal:

# block and Haskell like >= operator
Either(operation).
  >= { successful_method }.
  >= { failful_operation }

# start with a Success, for instance a parameter
Success('pzol').
  bind ->(previous) { good }.
  bind ->           { bad  }

Either.chain do
  bind ->                   { good   }                     # >= is not supported for Either.chain, only bind
  bind ->                   { better }                     # better returns Success(some_int)
  bind ->(previous_result)  { previous_result + 1 }
end

either = Either(something)
either += truth? Success('truth, only the truth') : Failure('lies, damn lies')

Exceptions are wrapped into a Failure:

Either(true).
  bind -> { fail 'get me out of here' }                    # return a Failure(RuntimeError)

Another example:

Success(params).
  bind ->(params)   { Either(params.fetch(:path)) }
  bind ->(path)     { load_stuff(params) }

References

Installation

Add this line to your application's Gemfile:

gem 'monadic'

And then execute:

$ bundle

Or install it yourself as:

$ gem install monadic

Contributing

  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Added some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request