Possibly - Maybe monad for Ruby

Maybe monad implementation for Ruby

puts Maybe(User.find_by_id("123")).username.downcase.or_else { "N/A" }

=> # puts downcased username if user "123" can be found, otherwise puts "N/A"

Installation

gem install possibly

Getting started

require 'possibly'

first_name = Maybe(deep_hash)[:account][:profile][:first_name].or_else { "No first name available" }

Documentation

Maybe monad is a programming pattern that allows to treat nil values that same way as non-nil values. This is done by wrapping the value, which may or may not be nil to, a wrapper class.

The implementation includes three different classes: Maybe, Some and None. Some represents a value, None represents a non-value and Maybe is a constructor, which results either Some, or None.

Maybe("I'm a value")    => #<Some:0x007ff7a85621e0 @value="I'm a value">
Maybe(nil)              => #<None:0x007ff7a852bd20>

Both Some and None implement four trivial methods: is_some?, is_none?, get and or_else

Maybe("I'm a value").is_some?               => true
Maybe("I'm a value").is_none?               => false
Maybe(nil).is_some?                         => false
Maybe(nil).is_none?                         => true
Maybe("I'm a value").get                    => "I'm a value"
Maybe("I'm a value").or_else { "No value" } => "I'm a value"
Maybe(nil).get                              => RuntimeError: No such element
Maybe(nil).or_else { "No value" }           => "No value"

In addition, Some and None implement Enumerable, so all methods available for Enumerable are available for Some and None:

Maybe("Print me!").each { |v| puts v }      => it puts "Print me!"
Maybe(nil).each { |v| puts v }              => puts nothing
Maybe(4).map { |v| Math.sqrt(v) }           => #<Some:0x007ff7ac8697b8 @value=2.0>
Maybe(nil).map { |v| Math.sqrt(v) }         => #<None:0x007ff7ac809b10>
Maybe(2).inject(3) { |a, b| a + b }         => 5
None().inject(3) { |a, b| a + b }           => 3

All the other methods you call on Some are forwarded to the value.

Maybe("I'm a value").upcase                 => #<Some:0x007ffe198e6128 @value="I'M A VALUE">
Maybe(nil).upcase                           => None

Case expression

Maybe implements threequals method #===, so it can be used in case expressions:

value = Maybe([nil, 1, 2, 3, 4, 5, 6].sample)

case value
when Some
  puts "Got Some: #{value.get}"
when None
  puts "Got None"
end

If the type of Maybe is Some, you can also match the value:

value = Maybe([nil, 0, 1, 2, 3, 4, 5, 6].sample)

case value
when Some(0)
  puts "Got zero"
when Some((1..3))
  puts "Got a low number: #{value.get}"
when Some((4..6))
  puts "Got a high number: #{value.get}"
when None
  puts "Got nothing"
end

For more complicated matching you can use Procs and lambdas. Proc class aliases #=== to the #call method. In practice this means that you can use Procs and lambdas in case expressions. It works also nicely with Maybe:

even? = ->(a) { a % 2 == 0 }
odd? = ->(a) { a % 2 != 0 }

value = Maybe([nil, 1, 2, 3, 4, 5, 6].sample)

case value
when Some(even?)
  puts "Got even value: #{value.get}"
when Some(odd?)
  puts "Got odd value: #{value.get}"
when None
  puts "Got None"
end

case_of

Maybe implements the threequals method in a way which allows it to be used in case expressions pretty neatly: You can match both the type (None or Some) and the value of Some. It almost feels like pattern matching in functional language. However, there's one big shortcoming. Ruby doesn't allow binding the value of Some to a variable, so you must use the get method to do that. The following does not work in Ruby:

case apples
when Some(1)
  puts "one apple"
when Some(x)
  puts "#{x} apples"
when None
  puts "Unknown number of apples"
end

Instead, you have to use get, which is cubersome:

case apples
when Some(1)
  puts "one apple"
when Some
  x = apples.get
  puts "#{x} apples"
when None
  puts "Unknown number of apples"
end

To workaround this limitation, Maybe implements a method case_of, which internally uses case expression:

apples.case_of(Some(1)) { |_| puts "1 apple" }
apples.case_of(Some(Integer)) { |x| puts "#{x} apples" }
apples.case_of(None) { |_| puts "Unknown number of apples" }

- apples.case_of(Some(1)) do
  = "1 apple"

- apples.case_of(Some(Integer)) do |x|
  = puts "#{x} apples"

- apples.case_of(None) do
  = "Unknown number of apples"

Examples

Instead of using if-clauses to define whether a value is a nil, you can wrap the value with Maybe() and threat it the same way whether or not it is a nil

Without Maybe():

user = User.find_by_id(user_id)
number_of_friends = if user && user.friends
  user.friends.count
else
  0
end

With Maybe():

number_of_friends = Maybe(User.find_by_id(user_id)).friends.count.or_else { 0 }

Same in HAML view, without Maybe():

- if @user && @user.friends
  = @user.friends.count
- else
  0

= Maybe(@user).friends.count.or_else { 0 }

Tests

rspec spec/spec.rb

License

MIT

Author

Mikko Koski / @rap1ds

ekylibre/possibly