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Scala-inspired Monads for Ruby
branch: master

NOTICE: Re-thinking is happening...

Scala has evolved quite a bit since this project began in 2011, and mainstream Ruby usage has evolved as well.

In the branch experimental-0.5, new things are going to be tried.

If you're interested in sharing your ideas, join the mailing list.


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Project: github


A Ruby Monad Library, Inspired by Scala

Are you working in both the Scala and Ruby worlds, and finding that you miss some of the practical benefits of Scala's monads in Ruby? If so, then Rumonade is for you.

The goal of this library is to make the most common and useful Scala monadic idioms available in Ruby via the following classes:

Syntactic support for scala-like for-comprehensions will be implemented as a sequence of calls to flat_map, select, etc, modeling Scala's approach.

Support for an all_catch idiom will be implemented to turn blocks which might throw exceptions into Option or Either results. If this proves useful (and a good fit for Ruby), then more narrow functional catchers can be implemented as well.

Usage Examples

Option: handle possibly nil values in a functional fashion:

def format_date_in_march(time_or_date_or_nil)
  Option(time_or_date_or_nil).    # wraps possibly-nil value in an Option monad (Some or None)
    map(&:to_date).               # transforms a contained Time value into a Date value
    select {|d| d.month == 3}.    # filters out non-matching Date values (Some becomes None)
    map(&:to_s).                  # transforms a contained Date value into a String value
    map {|s| s.gsub('-', '')}.    # transforms a contained String value by removing '-'
    get_or_else("not in march!")  # returns the contained value, or the alternative if None

format_date_in_march(nil)                            # => "not in march!"
format_date_in_march(Time.parse('2009-01-01 01:02')) # => "not in march!"
format_date_in_march(Time.parse('2011-03-21 12:34')) # => "20110321"


  • each step of the chained computations above are functionally isolated
  • the value can notionally start as nil, or become nil during a computation, without effecting any other chained computations

Either: handle failures (Left) and successes (Right) in a functional fashion:

def find_person(name)
  case name
    when /Jack/i, /John/i
      Left("No such person: #{name.capitalize}")

# success looks like this:
# => Right("Jack")

# failure looks like this:
# => Left("No such person: Jill")

# lift the contained values into Array, in order to combine them:
# => Left(["No such person: Joan"])

# on the 'happy path', combine and transform successes into a single success result:
(find_person("Jack").lift_to_a + 
 find_person("John").lift_to_a) { |*names| names.join(" and ") }
# => Right("Jack and John")

# but if there were errors, we still have a Left with all the errors inside:
(find_person("Jack").lift_to_a +
 find_person("John").lift_to_a +
 find_person("Jill").lift_to_a +
 find_person("Joan").lift_to_a) { |*names| names.join(" and ") }
# => Left(["No such person: Jill", "No such person: Joan"])

# equivalent to the previous example, but shorter:
%w(Jack John Jill Joan).
  map { |nm| find_person(nm).lift_to_a }.inject(:+). { |*names| names.join(" and ") }
# => Left(["No such person: Jill", "No such person: Joan"])

Also, see the Either class in action in the Ruby version of A Tale of Three Nightclubs validation example in F#, and compare it to the Scala version using scalaz.

Hash: flat_map returns a Hash for each key/value pair; get returns an Option

h = { "Foo" => 1, "Bar" => 2, "Baz" => 3 }

h = h.flat_map { |k, v| { k => v, k.upcase => v * 10 } }
# => {"Foo"=>1, "FOO"=>10, "Bar"=>2, "BAR"=>20, "Baz"=>3, "BAZ"=>30}

h = { |k, v| k =~ /^b/i }
# => {"Bar"=>2, "BAR"=>20, "Baz"=>3, "BAZ"=>30}

# => Some(2)

# => None


There have been many posts and discussions about monads in Ruby, which have sparked a number of approaches.

Rumonade wants to be a practical drop-in Monad solution that will fit well into the Ruby world.

The priorities for Rumonade are:

  1. Practical usability in day-to-day Ruby
    • don't mess up normal idioms of the language (e.g., Hash#map)
    • don't slow down normal idioms of the language (e.g., Array#map)
  2. Rubyish-ness of usage
    • Monad is a mix-in, requiring methods self.unit and #bind be implemented by target class
    • Prefer blocks to lambda/Procs where possible, but allow both
  3. Equivalent idioms to Scala where possible


Option, Either, Array, and Hash are already usable.

Supported Ruby versions: MRI 2.0.0, 1.9.3, 1.9.2, JRuby in 1.9 mode, and Rubinius in 1.9 mode.

Please try it out, and let me know what you think! Suggestions are always welcome.

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