class ActionController::Parameters

params.fetch(:none) { 'Francesco' } # => "Francesco"
params.fetch(:none, 'Francesco') # => "Francesco"
params.fetch(:none) # => ActionController::ParameterMissing: param is missing or the value is empty: none
params.fetch(:person) # => {"name"=>"Francesco"}
params = ActionController::Parameters.new(person: { name: 'Francesco' })

is given, then that will be run and its result returned.
if more arguments are given, then that will be returned; if a block
it will raise an ActionController::ParameterMissing error;
can't be found, there are several options: With no other arguments,
Returns a parameter for the given +key+. If the +key+

def fetch(key, *args)
  convert_value_to_parameters(
    @parameters.fetch(key) {
      if block_given?
        yield
      else
        args.fetch(0) { raise ActionController::ParameterMissing.new(key) }
      end
    }
  )
end