gistfile1.rb

module ParamsValidator
  
  def validator array_of_values = [], default _value, param
    array_of_values.include? param  ? param : default_value
  end
  
end


class ParamsValidator
  
  class << self
    def validate
      # properly initialize and set the hash to return
    end
  
    private
    
    def validator array_of_values = [], default _value, param
      array_of_values.include? param  ? param : default_value
    end
    
  end
  
end

# Inheritance 

class ProductValidator < ParamsValidator
  class << self
    
    def validate params
      @hash ||= {
        limit: validate_limit params[:limit],
        sort:  validate_sort params[:sort],
        page: validate_limit params[:page],
        category_id: params[:category_id]
      }
    end
    
    private
    
    def validate_limit limit_param
      validator %w(15 30 50), 15, limit_param
    end
    
    def validate_sort sort_param
      validator %w(most_recent a-z z-a), 'most_recent', sort_param
    end
    
  end
end


# as a module?

class ProductValidator 
  
  include ParamsValidator
  
  class << self
    
    def validate params
      @hash ||= {
        limit: validate_limit params[:limit],
        sort:  validate_sort params[:sort],
        page: validate_limit params[:page],
        category_id: params[:category_id]
      }
    end
    
    private
    
    def validate_limit limit_param
      validator %w(15 30 50), 15, limit_param
    end
    
    def validate_sort sort_param
      validator %w(most_recent a-z z-a), 'most_recent', sort_param
    end
    
  end
end

# As a DSL?

class Product
  validator :limit, %w(15 30 50), 15
  validator :page, %w(1..50), 15, 1
end

# controller
private
def product_param
  Product.validate(params)
end