singleton_or_not.rb

# You could have an object for the whole app to use (e.g. in rails initializer)
ChanceGenerator = Chance.new

# But also easily testable (this would be in a test setup):
test_chance = Chance.new(seed: 1)

# The constructor can have a little documentation:
class Chance
  # If seed is omitted, a new seed will be generated for each random call
  def initialize(seed: nil)
    # …
  end
  # …
end

# This takes care of making class testable, and has good hygiene of working with randomness:
# a random class that accepts a seed is only natural.