Getting started
Installation
Add this line to your Gemfile
gem 'dry-monads'
Then run
$ bundle
Usage
Every monad has corresponding value constructors. For example, the Maybe monad has two of them: Some(...) and None(). It also has the Maybe(...) method. All three methods start with a capital letter similarly to built-in Ruby methods like Kernel#Array(...) and Kernel#Hash(...). Value constructors are not available globally, you need to add them with a mixin.
To add the Maybe constructors add Dry::Monads[:maybe] to your class:
require 'dry/monads'
class CreateUser
# this line loads the Maybe monad and adds
# Some(...), None(), and Maybe(...) to CreateUser
include Dry::Monads[:maybe]
def call(params)
# ...
if valid?(params)
Some(create_user(params))
else
None()
end
end
end
Example in the docs may use extend Dry::Monads[...] for brevity but you normally want to use include in production code.
Including multiple monads
require 'dry/monads'
class CreateUser
# Adds Maybe and Result. The order doesn't matter
include Dry::Monads[:maybe, :result]
end
Using with do notation
A very common case is using the Result monad with do notation:
require 'dry/monads'
class ResultCalculator
include Dry::Monads[:result, :do]
def calculate(input)
value = Integer(input)
value = yield add_3(value)
value = yield mult_2(value)
Success(value)
end
def add_3(value)
if value > 1
Success(value + 3)
else
Failure("value was less than 1")
end
end
def mult_2(value)
if value % 2 == 0
Success(value * 2)
else
Failure("value was not even")
end
end
end
c = ResultCalculator.new
c.calculate(3) # => Success(12)
c.calculate(0) # => Failure("value was less than 1")
c.calculate(2) # => Failure("value was not even")
Constructing array values
Some constructors have shortcuts for wrapping arrays:
require 'dry/monads'
class CreateUser
include Dry::Monads[:result]
def call(params)
# ...
# Same as Failure([:user_exists, params: params])
Failure[:user_exists, params: params]
end
end
Interaction between monads and constructors availability
Some values can be converted to others or they can have methods that use other monads. By default, dry-monads doesn't load all monads so you may have troubles like this:
extend Dry::Monads[:result]
Success(:foo).to_maybe # RuntimeError: Load Maybe first with require 'dry/monads/maybe'
To work around you may either load dry/monads/maybe add maybe to the mixin:
extend Dry::Monads[:result, :maybe]
Success(:foo).to_maybe # => Some(:foo)
For the same reason Dry::Monads.Some(...), Dry::Monads.Success(...), and some other constructors are not available until you explicitly load the monads with require 'dry/monads/%{monad_name}'.
Loading everything
Just require 'dry/monads/all'