Handle dynamic argument in dynamic language

In dynamic language, most language doesn’t provide the function overload mechanism like static language does, which means you cannot define functions with same name but different arguments, and the language itself won’t help you to dispatch the call according to the arguments.
So you have to deal with the overload by yourself in dynamic languages.

In dynamic language, since you have to dispatch the call by yourself, so you can play some tricks during process the arguments. The most common trick is grammar sugar, which can help you simplify the code and increase the readability. And it is a very important feature when you’re building DSL.

Syntax sugar in argument means you can omit something unnecessary or unimportant parameters in specific context, then the function will try to infer and complete the parameters. For example, developer should be able to omit the password parameter if the system enable authentication mechanism. These kind of tricks are very common in dynamic language frameworks, such as jQuery.

Here is a list of some common grammar sugar cases:

• Set default value for omit parameter, such as for jQuery animation function jQuery.fadeIn([duration] [, callback] ), you can omit the parameter duration, which is equivalent to provide duration as 400
• Provide different type of value for a same parameter, still the jQuery animation function jQuery.fadeIn([duration] [, callback] ), you can provide number for duration, or you can provide string “fast” and “slow” as the value of duration.
• Provide a single value rather than a complex hash, such as function jQuery ajax function jQuery.ajax(settings), you can provide a url string, which is equivalent to provide a hash{url: <url string>}
• Pass a single element instead of a array of the element.

After we analyze the cases, we will find that all the grammar sugar is to allow user to provide exactly same information but in different formats. Since all the data are same piece of information, so it should be possible to unify all the information into one format! Which means to support grammar sugar, the major problem is to unify the type of parameter.
Besides unify the type, another important problem is to handle the null values, such asnull and undefined in javascript, nil in ruby, etc.

Here is a piece of ruby code that I parse the argument by unifying the parameter type:

code

  def apply_to_items(options = nil)
    options = unify_type(options, Hash) { |items| {:only => items} }
    options[:only] = unify_type(options[:only], Array) { |item| item.nil? ? list_all_items : [item]  }
    options[:except] = unify_type(options[:except], Array) { |item| item.nil? ? [] : [item] }

    options[:only] = unify_array_item_type(options[:only], String) { |symbol| symbol.to_s }
    options[:except] = unify_array_item_type(options[:except], String) { |symbol| symbol.to_s }

    target_items = options[:only].select { |item| options[:except].exclude? item }

    target_items.each do |item|
      yield item
    end
  end

  private

  def list_all_items
	# return all items fetched from database or API
	# ...
  end

  def unify_type(input, type)
    if input.is_a?(type)
      input
    else
      yield input
    end
  end
end

And here is the test for the code:

title

require 'spec_helper'

describe Module  do
  before do
    extend Moudle
  end

  it "should populate all items" do
    visited = []
    apply_to_items do |item|
      visited << item
    end

    visited.should =~ %w(public another_item)
  end

  describe "should populate the provided items" do
    it "provide as string array" do
      visited = []
      apply_to_items(%w(another_item)) do |item|
        visited << item
      end

      visited.should =~ %w(another_item)
    end

    it "provide as symbol array" do
      visited = []
      apply_to_items([:another_item]) do |item|
        visited << item
      end

      visited.should =~ %w(another_item)
    end

    it "provide as string item" do
      visited = []
      apply_to_items('another_item') do |item|
        visited << item
      end

      visited.should =~ %w(another_item)
    end

    it "provide as symbol item" do
      visited = []
      apply_to_items(:another_item) do |item|
        visited << item
      end

      visited.should =~ %w(another_item)
    end

    it "provide as string array in hash" do
      visited = []
      apply_to_items(:only => %w(another_item)) do |item|
        visited << item
      end

      visited.should =~ %w(another_item)
    end

    it "provide as symbol array in hash" do
      visited = []
      apply_to_items(:only => [:another_item]) do |item|
        visited << item
      end

      visited.should =~ %w(another_item)
    end

    it "provide as string item in hash" do
      visited = []
      apply_to_items(:only => 'public') do |item|
        visited << item
      end

      visited.should =~ %w(public)
    end

    it "provide as symbol item in hash" do
      visited = []
      apply_to_items(:only => :public) do |item|
        visited << item
      end

      visited.should =~ %w(public)
    end
  end

  describe "should except the not used items" do

    it "except as string item in hash" do
      visited = []
      apply_to_items(:except => 'public') do |item|
        visited << item
      end

      visited.should =~ %w(another_item)
    end

    it "except as symbol item in hash" do
      visited = []
      apply_to_items(:except => :public) do |item|
        visited << item
      end

      visited.should =~ %w(another_item)
    end

    it "except as string array in hash" do
      visited = []
      apply_to_items(:except => %w(public)) do |item|
        visited << item
      end

      visited.should =~ %w(another_item)
    end

    it "except as symbol array in hash" do
      visited = []
      apply_to_items(:except => :public) do |item|
        visited << item
      end

      visited.should =~ %w(another_item)
    end
  end
end

The algorithm in previous code is language independent, so ideally, it could be reused in any language, such as java script or python.