composition operator fun (using ruby 2.7 preview 1)

compositional_fun.rb

class SubtractTwoInteraction
  def self.call(number)
    number - 2
  end
end

class MultiplyByInteraction
  def self.call(number:, by:)
    number * by
  end
end

class DivideByInteraction
  def self.call(number:, by:)
    number / by
  end
end

module Workflow # aka Organizer in collectiveidea/interactor gem
  def call(*args)
    function.call(*args)
  end

  def function
    raise NotImplementedError
  end
end

class SimpleWorkflow
  extend Workflow

  def self.function
    SubtractTwoInteraction.:call >>
      ->(n) { { number: n, by: 2 } } >> # "glue" between two interactions
      MultiplyByInteraction.:call
  end
end

class ConditionalWorkflow
  extend Workflow

  def self.function
    SubtractTwoInteraction.:call >>
      ->(n) { { number: n, by: 2 } } >>
      ->(params) {
        klass = params[:number] > 10 ? DivideByInteraction : MultiplyByInteraction
        params.then(&klass.:call)
      }
  end
end

compositional_fun_tests.rb

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'minitest', require: false
end

require 'minitest/autorun'
require_relative './compositional_fun'

class WorkflowTest < Minitest::Test
  def test_simple_workflow
    assert_equal SimpleWorkflow.call(7), 10
  end

  def test_conditional_workflow
    assert_equal ConditionalWorkflow.call(7), 10
    assert_equal ConditionalWorkflow.call(13), 5
  end
end