Sketch of nested strong Rails params

gistfile1.rb

#  Like this:
#
#  params.allow_nested(
#    user: must(
#      name: must,
#      age: maybe,
#      article: must(
#        title: must,
#        publish_at: maybe
#      ),
#      profile: maybe(
#        intro: must,
#        gender: maybe
#      )
#    )
#  )

module NestedStrongParams
  RequiredParamsMissing = Class.new(RuntimeError)
  WhitelistedHashError  = Class.new(RuntimeError)

  module Constants
    MAYBE = Struct.new(:nest)
    MUST  = Struct.new(:nest)
  end

  def maybe(nest=nil)
    Constants::MAYBE.new(nest)
  end

  def must(nest=nil)
    Constants::MUST.new(nest)
  end
end

# Don't actually monkey-patch Hash. Please.
class Hash
  def allow_nested(allowances)
    allowances.each do |key,value|
      case value
      when NestedStrongParams::Constants::MAYBE
        raise NestedStrongParams::WhitelistedHashError.new(key) if self[key].is_a?(Hash) && value.nest.nil?

        if value.nest
          self[key].allow_nested(value.nest)
        end
      when NestedStrongParams::Constants::MUST
        raise NestedStrongParams::RequiredParamsMissing unless has_key?(key)
        raise NestedStrongParams::WhitelistedHashError.new(key) if self[key].is_a?(Hash) && value.nest.nil?

        if value.nest
          self[key].allow_nested(value.nest)
        end
      else
        raise "Encountered unexpected value in nested allowances: #{value.inspect}"
      end
    end

    self
  end
end