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
end

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"

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

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

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

# lift the contained values into Array, in order to combine them:
find_person("Joan").lift_to_a
# => 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).right.map { |*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).right.map { |*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(:+).
  right.map { |*names| names.join(" and ") }
# => Left(["No such person: Jill", "No such person: Joan"])

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

h.get("Bar")
# => Some(2)

h.get("Foo")
# => None