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) { (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
- Wikipedia Monad
- Learn You a Haskell - for a few monads more
- Monad equivalend in Ruby
- Option Type
- NullObject and Falsiness by @avdi
- andand
- ick
- Monads in Ruby
- The Maybe Monad in Ruby
- Monads in Ruby with nice syntax
- Maybe in Ruby
- Monads on the Cheap
- Rumonade
- Monads for functional programming
Installation
Add this line to your application's Gemfile:
gem 'monadic'
And then execute:
$ bundle
Or install it yourself as:
$ gem install monadic
Contributing
- Fork it
- Create your feature branch (
git checkout -b my-new-feature
) - Commit your changes (
git commit -am 'Added some feature'
) - Push to the branch (
git push origin my-new-feature
) - Create new Pull Request