refactoring posted code by creating Enumerable#find_mapped

implementation_in_class.rb

LOOKUP_CHAIN = [:params, :user, :session, :http, :default]

def self.set_by(inputs)
  LOOKUP_CHAIN.find_mapped do |lookup|
    locale = send("by_#{lookup}", inputs[lookup])
  end
end

monkey_patches.rb

module Enumerable
  def find_mapped
    result = nil
    self.each do |lookup|
      result = yield(lookup)
      break if result
    end
    return result
  end
end

find_mapped could have an ifnone parameter added, like find has. See find’s documentation.