declarative_parameters.rb

# USING THIS IN SINATRA
#
# require 'sinatra'
# require 'declarative_parameters'
# helpers DeclarativeParameters
#
# get '/some/uri' do
#   required :name
#   required :age, :type => :int, :min => 18
#   
#   # I now know that @name is set and not blank, and @age is an int
#   # over 18.
# end


module DeclarativeParameters
  def required(*required_fields)
    options = required_fields.pop if required_fields.last.kind_of? Hash; options ||= {}
    raise RequiredFieldsMissing.new(*required_fields) if required_fields.any?{|f| params[f].blank? }
    optional(*required_fields + [options])
  end
  
  def optional(*optional_fields)
    options = optional_fields.pop if optional_fields.last.kind_of? Hash; options ||= {}
    optional_fields.each do |field|
      instance_variable_set("@#{field}", options[:default]) and next if params[field].nil?
      
      params[field] = params[field].send("to_#{options[:type].to_s[0,1]}")
      raise TooSmall.new(field, params[field], options[:min]) if options[:min] and params[field] < options[:min]
      raise TooBig.new(field, params[field], options[:max]) if options[:max] and params[field] > options[:max]
      instance_variable_set("@#{field}", params[field])
    end
  end
  
  def self.included(klass)
    klass.instance_eval do
      error [RequiredFieldsMissing, TooSmall, TooBig] do
        status 422
        {:error => env['sinatra.error'].message}.to_json
      end
    end if klass.is_a? Sinatra::Application
  end
end

class RequiredFieldsMissing < Exception
  def initialize(*fields)
    super fields.map(&:to_s).join(', ') + ' are required for this request.'
  end
end

class TooSmall < Exception
  def initialize(name, actual, minimum)
    super "#{name} is too small. Minimum is #{minimum}, got '#{actual}'."
  end
end

class TooBig < Exception
  def initialize(name, actual, maximum)
    super "#{name} is too big. Maximum is #{maximum}, got '#{actual}'."
  end
end