define_method in ruby

So one more step close to meta programming define_method.

The usual way of defining new methods is to put your intended logic between def & end. And that works perfectly fine for the most part.

But consider a situation where you have to create a series of methods all of which have the same basic structure save for one string, and which can only be one of a certain set of strings. Now, you might say we'll just define one method and accept that one string as a parameter, which we can then use wherever we want. And you'd be right. But, the problem with such an approach is it's not particularly declarative: you don't know exactly what values that optional parameter can accept.

  class DefineMethod
     define_method("perform_chunk") do |argument|
         "performing chunk on #{argument}"
     end
     define_method("perform_chomp") do |argument|
         "performing chomp on #{argument}"
     end
 end

 d= DefineMethod.new
 puts d.perform_chunk("nose")
 puts d.perform_chomp("string")

The real power of define_method is realised when we realise we call it like any other code. Here's an example where we call it inside a loop and it achieves the same result as above.

 class Doctor
   ["assoc", "rassoc", "reduce"].each do |action|
           define_method("perform_#{action}") do |argument|
                         "performing #{action.gsub('_', ' ')} on #{argument}"
           end
   end
 end

   doctor = Doctor.new
   puts doctor.perform_assoc("nose")
   puts doctor.perform_rassoc("throat")
   puts doctor.perform_reduce("in da club")

Isn't that nice? We have code that's generating the code for us. Pretty meta, isn't it?
In fact, we can now extend this by adding any number of elements to that array -- or replacing the array entirely with an external data source. Many Ruby libraries use exactly this technique to define methods dynamically based on, say, database schemas. Rails, for example, will create methods such as Customer#find_by_first_name_and_last_name