Playing around with promises in Ruby, inspired by Sandi Metz' talk "Nothing Is Something"

promise.rb

class Promise
  attr_reader :object

  def initialize(object)
    @object = object
  end

  def try(method)
    @tries << method
    self
  end

  def otherwise(method)
    @catches << method
    self
  end

  def call
    @tries.each do |function|
      function.call(object, self)
    end
  rescue StandardError => exception
    @catches.each do |function|
      function.call(exception, self)
    end
  end

  def method_missing(_)
    call
  end

  def respond_to?(_)
    true
  end
end

class Object
  def try(&block)
    Promise.new(self).try(block)
  end

  def otherwise(&block)
    Promise.new(self).otherwise(block)
  end
end

class MyObject
  def initialize(name)
    @name = name
  end

  def to_s
    @name
  end
end

obj = MyObj.new('name')

obj.try(&:to_s).try(&:what).otherwise do |error|
  puts error.message
end